What techniques can be used to speed up C compilation times

Question

What techniques can be used to speed up C   compilation times   This question came up in some comments to Stack Overflow question C   programming style  and I m interested to hear what ideas there are   I ve seen a related question  Why does C   compilation take so long   but that doesn t provide many solutions

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Just for completeness  a build might be slow because the build system is being stupid as well as because the compiler is taking a long time to do its work    Read Recursive Make Considered Harmful  PDF  for a discussion of this topic in Unix environments

User · Answer

Once you have applied all the code tricks above  forward declarations  reducing header inclusion to the minimum in public headers  pushing most details inside the implementation file with Pimpl     and nothing else can be gained language-wise  consider your build system  If you use Linux  consider using distcc  distributed compiler  and ccache  cache compiler    The first one  distcc  executes the preprocessor step locally and then sends the output to the first available compiler in the network  It requires the same compiler and library versions in all the configured nodes in the network   The latter  ccache  is a compiler cache  It again executes the preprocessor and then check with an internal database  held in a local directory  if that preprocessor file has already been compiled with the same compiler parameters  If it does  it just pops up the binary and output from the first run of the compiler   Both can be used at the same time  so that if ccache does not have a local copy it can send it trough the net to another node with distcc  or else it can just inject the solution without further processing

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When I came out of college  the first real production-worthy C   code I saw had these arcane  ifndef      endif directives in between them where the headers were defined  I asked the guy who was writing the code about these overarching things in a very naive fashion and was introduced to world of large-scale programming   Coming back to the point  using directives to prevent duplicate header definitions was the first thing I learned when it came to reducing compiling times

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When I came out of college  the first real production-worthy C   code I saw had these arcane  ifndef      endif directives in between them where the headers were defined  I asked the guy who was writing the code about these overarching things in a very naive fashion and was introduced to world of large-scale programming   Coming back to the point  using directives to prevent duplicate header definitions was the first thing I learned when it came to reducing compiling times

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Although not a  technique   I couldn t figure out how Win32 projects with many source files compiled faster than my  Hello World  empty project   Thus  I hope this helps someone like it did me   In Visual Studio  one option to increase compile times is Incremental Linking   INCREMENTAL    It s incompatible with Link-time Code Generation   LTCG  so remember to disable incremental linking when doing release builds

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Language techniques  Pimpl Idiom  Take a look at the Pimpl idiom here  and here  also known as an opaque pointer or handle classes  Not only does it speed up compilation  it also increases exception safety when combined with a non-throwing swap function  The Pimpl idiom lets you reduce the dependencies between headers and reduces the amount of recompilation that needs to be done   Forward Declarations  Wherever possible  use forward declarations  If the compiler only needs to know that SomeIdentifier is a struct or a pointer or whatever  don t include the entire definition  forcing the compiler to do more work than it needs to  This can have a cascading effect  making this way slower than they need to be   The I O streams are particularly known for slowing down builds  If you need them in a header file  try  including  lt iosfwd gt  instead of  lt iostream gt  and  include the  lt iostream gt  header in the implementation file only   The  lt iosfwd gt  header holds forward declarations only  Unfortunately the other standard headers don t have a respective declarations header   Prefer pass-by-reference to pass-by-value in function signatures  This will eliminate the need to  include the respective type definitions in the header file and you will only need to forward-declare the type  Of course  prefer const references to non-const references to avoid obscure bugs  but this is an issue for another question   Guard Conditions  Use guard conditions to keep header files from being included more than once in a single translation unit    pragma once  ifndef filename h  define filename h     Header declarations   definitions   endif   By using both the pragma and the ifndef  you get the portability of the plain macro solution  as well as the compilation speed optimization that some compilers can do in the presence of the pragma once directive   Reduce interdependency  The more modular and less interdependent your code design is in general  the less often you will have to recompile everything  You can also end up reducing the amount of work the compiler has to do on any individual block at the same time  by virtue of the fact that it has less to keep track of   Compiler options  Precompiled Headers  These are used to compile a common section of included headers once for many translation units  The compiler compiles it once  and saves its internal state  That state can then be loaded quickly to get a head start in compiling another file with that same set of headers   Be careful that you only include rarely changed stuff in the precompiled headers  or you could end up doing full rebuilds more often than necessary  This is a good place for STL headers and other library include files   ccache is another utility that takes advantage of caching techniques to speed things up   Use Parallelism  Many compilers   IDEs support using multiple cores CPUs to do compilation simultaneously  In GNU Make  usually used with GCC   use the -j  N  option  In Visual Studio  there s an option under preferences to allow it to build multiple projects in parallel  You can also use the  MP option for file-level paralellism  instead of just project-level paralellism   Other parallel utilities    Incredibuild Unity Build distcc   Use a Lower Optimization Level  The more the compiler tries to optimize  the harder it has to work   Shared Libraries  Moving your less frequently modified code into libraries can reduce compile time  By using shared libraries   so or  dll   you can reduce linking time as well   Get a Faster Computer  More RAM  faster hard drives  including SSDs   and more CPUs cores will all make a difference in compilation speed

User · Answer

There s an entire book on this topic  which is titled Large-Scale C   Software Design  written by John Lakos    The book pre-dates templates  so to the contents of that book add  using templates  too  can make the compiler slower

User · Answer

I will just link to my other answer  How do YOU reduce compile time  and linking time for Visual C   projects  native C      Another point I want to add  but which causes often problems is to use precompiled headers  But please  only use them for parts which hardly ever change  like GUI toolkit headers   Otherwise  they will cost you more time than they save you in the end   Another option is  when you work with GNU make  to turn on -j lt N gt  option     -j  N   --jobs  N           Allow N jobs at once  infinite jobs with no arg    I usually have it at 3 since I ve got a dual core here  It will then run compilers in parallel for different translation units  provided there are no dependencies between them  Linking cannot be done in parallel  since there is only one linker process linking together all object files   But the linker itself can be threaded  and this is what the GNU gold ELF linker does  It s optimized threaded C   code which is said to link ELF object files a magnitude faster than the old ld  and was actually included into binutils

User · Answer

Use   pragma once   at the top of header files  so if they re included more than once in a translation unit  the text of the header will only get included and parsed once

User · Answer

Networks shares will drastically slow down your build  as the seek latency is high  For something like Boost  it made a huge difference for me  even though our network share drive is pretty fast    Time to compile a toy Boost program went from about 1 minute to 1 second when I switched from a network share to a local SSD

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Not about the compilation time  but about the build time    Use ccache if you have to rebuild the same files when you are working on your buildfiles Use ninja-build instead of make  I am currently compiling a project with  100 source files and everything is cached by ccache  make needs 5 minutes  ninja less than 1    You can generate your ninja files from cmake with -GNinja

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I will just link to my other answer  How do YOU reduce compile time  and linking time for Visual C   projects  native C      Another point I want to add  but which causes often problems is to use precompiled headers  But please  only use them for parts which hardly ever change  like GUI toolkit headers   Otherwise  they will cost you more time than they save you in the end   Another option is  when you work with GNU make  to turn on -j lt N gt  option     -j  N   --jobs  N           Allow N jobs at once  infinite jobs with no arg    I usually have it at 3 since I ve got a dual core here  It will then run compilers in parallel for different translation units  provided there are no dependencies between them  Linking cannot be done in parallel  since there is only one linker process linking together all object files   But the linker itself can be threaded  and this is what the GNU gold ELF linker does  It s optimized threaded C   code which is said to link ELF object files a magnitude faster than the old ld  and was actually included into binutils

User · Answer

Use forward declarations where you can  If a class declaration only uses a pointer or reference to a type  you can just forward declare it and include the header for the type in the implementation file   For example      T h class Class2     Forward declaration  class T   public      void doSomething Class2  amp c2   private      Class2  m Class2Ptr         T cpp  include  Class2 h  void Class2  doSomething Class2  amp c2           Whatever you want here     Fewer includes means far less work for the preprocessor if you do it enough

User · Answer

Language techniques  Pimpl Idiom  Take a look at the Pimpl idiom here  and here  also known as an opaque pointer or handle classes  Not only does it speed up compilation  it also increases exception safety when combined with a non-throwing swap function  The Pimpl idiom lets you reduce the dependencies between headers and reduces the amount of recompilation that needs to be done   Forward Declarations  Wherever possible  use forward declarations  If the compiler only needs to know that SomeIdentifier is a struct or a pointer or whatever  don t include the entire definition  forcing the compiler to do more work than it needs to  This can have a cascading effect  making this way slower than they need to be   The I O streams are particularly known for slowing down builds  If you need them in a header file  try  including  lt iosfwd gt  instead of  lt iostream gt  and  include the  lt iostream gt  header in the implementation file only   The  lt iosfwd gt  header holds forward declarations only  Unfortunately the other standard headers don t have a respective declarations header   Prefer pass-by-reference to pass-by-value in function signatures  This will eliminate the need to  include the respective type definitions in the header file and you will only need to forward-declare the type  Of course  prefer const references to non-const references to avoid obscure bugs  but this is an issue for another question   Guard Conditions  Use guard conditions to keep header files from being included more than once in a single translation unit    pragma once  ifndef filename h  define filename h     Header declarations   definitions   endif   By using both the pragma and the ifndef  you get the portability of the plain macro solution  as well as the compilation speed optimization that some compilers can do in the presence of the pragma once directive   Reduce interdependency  The more modular and less interdependent your code design is in general  the less often you will have to recompile everything  You can also end up reducing the amount of work the compiler has to do on any individual block at the same time  by virtue of the fact that it has less to keep track of   Compiler options  Precompiled Headers  These are used to compile a common section of included headers once for many translation units  The compiler compiles it once  and saves its internal state  That state can then be loaded quickly to get a head start in compiling another file with that same set of headers   Be careful that you only include rarely changed stuff in the precompiled headers  or you could end up doing full rebuilds more often than necessary  This is a good place for STL headers and other library include files   ccache is another utility that takes advantage of caching techniques to speed things up   Use Parallelism  Many compilers   IDEs support using multiple cores CPUs to do compilation simultaneously  In GNU Make  usually used with GCC   use the -j  N  option  In Visual Studio  there s an option under preferences to allow it to build multiple projects in parallel  You can also use the  MP option for file-level paralellism  instead of just project-level paralellism   Other parallel utilities    Incredibuild Unity Build distcc   Use a Lower Optimization Level  The more the compiler tries to optimize  the harder it has to work   Shared Libraries  Moving your less frequently modified code into libraries can reduce compile time  By using shared libraries   so or  dll   you can reduce linking time as well   Get a Faster Computer  More RAM  faster hard drives  including SSDs   and more CPUs cores will all make a difference in compilation speed

User · Answer

More RAM   Someone talked about RAM drives in another answer  I did this with a 80286 and Turbo C    shows age  and the results were phenomenal  As was the loss of data when the machine crashed

User · Answer

More RAM   Someone talked about RAM drives in another answer  I did this with a 80286 and Turbo C    shows age  and the results were phenomenal  As was the loss of data when the machine crashed

User · Answer

If you have a multicore processor  both Visual Studio  2005 and later  as well as GCC support multi-processor compiles  It is something to enable if you have the hardware  for sure

User · Answer

Networks shares will drastically slow down your build  as the seek latency is high  For something like Boost  it made a huge difference for me  even though our network share drive is pretty fast    Time to compile a toy Boost program went from about 1 minute to 1 second when I switched from a network share to a local SSD

User · Answer

If you have a multicore processor  both Visual Studio  2005 and later  as well as GCC support multi-processor compiles  It is something to enable if you have the hardware  for sure

User · Answer

More RAM   Someone talked about RAM drives in another answer  I did this with a 80286 and Turbo C    shows age  and the results were phenomenal  As was the loss of data when the machine crashed

User · Answer

More RAM   Someone talked about RAM drives in another answer  I did this with a 80286 and Turbo C    shows age  and the results were phenomenal  As was the loss of data when the machine crashed

User · Answer

There s an entire book on this topic  which is titled Large-Scale C   Software Design  written by John Lakos    The book pre-dates templates  so to the contents of that book add  using templates  too  can make the compiler slower

User · Answer

Use forward declarations where you can  If a class declaration only uses a pointer or reference to a type  you can just forward declare it and include the header for the type in the implementation file   For example      T h class Class2     Forward declaration  class T   public      void doSomething Class2  amp c2   private      Class2  m Class2Ptr         T cpp  include  Class2 h  void Class2  doSomething Class2  amp c2           Whatever you want here     Fewer includes means far less work for the preprocessor if you do it enough

User · Answer

First of all  we have to understand what so different about C   that sets it apart from other languages  Some people say it s that C   has many too features  But hey  there are languages that have a lot more features and they are nowhere near that slow  Some people say it s the size of a file that matters  Nope  source lines of code don t correlate with compile times  But wait  how can it be  More lines of code should mean longer compile times  what s the sorcery  The trick is that a lot of lines of code is hidden in preprocessor directives  Yes  Just one  include can ruin your module s compilation performance  You see  C   doesn t have a module system  All   cpp files are compiled from scratch  So having 1000   cpp files means compiling your project a thousand times  You have more than that  Too bad  That s why C   developers hesitate to split classes into multiple files  All those headers are tedious to maintain  So what can we do other than using precompiled headers  merging all the cpp files into one  and keeping the number of headers minimal  C  20 brings us preliminary support of modules  Eventually  you ll be able to forget about  include and the horrible compile performance that header files bring with them  Touched one file  Recompile only that file  Need to compile a fresh checkout  Compile in seconds rather than minutes and hours  The C   community should move to C  20 as soon as possible  C   compiler developers should put more focus on this  C   developers should start testing preliminary support in various compilers and use those compilers that support modules  This is the most important moment in C   history

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I d recommend these articles from  Games from Within  Indie Game Design And Programming     Physical Structure and C       Part 1  A First Look Physical Structure and C       Part 2  Build Times Even More Experiments with Includes How Incredible Is Incredibuild  The Care and Feeding of Pre-Compiled Headers The Quest for the Perfect Build System The Quest for the Perfect Build System  Part 2    Granted  they are pretty old - you ll have to re-test everything with the latest versions  or versions available to you   to get realistic results  Either way  it is a good source for ideas

User · Answer

Here are some    Use all processor cores by starting a multiple-compile job  make -j2 is a good example   Turn off or lower optimizations  for example  GCC is much faster with -O1 than -O2 or -O3   Use precompiled headers

User · Answer

Using dynamic linking instead of static one make you compiler faster that can feel   If you use t Cmake  active the property   set BUILD SHARED LIBS ON    Build Release  using static linking can get more optimize

User · Answer

If you have a multicore processor  both Visual Studio  2005 and later  as well as GCC support multi-processor compiles  It is something to enable if you have the hardware  for sure

User · Answer

Language techniques  Pimpl Idiom  Take a look at the Pimpl idiom here  and here  also known as an opaque pointer or handle classes  Not only does it speed up compilation  it also increases exception safety when combined with a non-throwing swap function  The Pimpl idiom lets you reduce the dependencies between headers and reduces the amount of recompilation that needs to be done   Forward Declarations  Wherever possible  use forward declarations  If the compiler only needs to know that SomeIdentifier is a struct or a pointer or whatever  don t include the entire definition  forcing the compiler to do more work than it needs to  This can have a cascading effect  making this way slower than they need to be   The I O streams are particularly known for slowing down builds  If you need them in a header file  try  including  lt iosfwd gt  instead of  lt iostream gt  and  include the  lt iostream gt  header in the implementation file only   The  lt iosfwd gt  header holds forward declarations only  Unfortunately the other standard headers don t have a respective declarations header   Prefer pass-by-reference to pass-by-value in function signatures  This will eliminate the need to  include the respective type definitions in the header file and you will only need to forward-declare the type  Of course  prefer const references to non-const references to avoid obscure bugs  but this is an issue for another question   Guard Conditions  Use guard conditions to keep header files from being included more than once in a single translation unit    pragma once  ifndef filename h  define filename h     Header declarations   definitions   endif   By using both the pragma and the ifndef  you get the portability of the plain macro solution  as well as the compilation speed optimization that some compilers can do in the presence of the pragma once directive   Reduce interdependency  The more modular and less interdependent your code design is in general  the less often you will have to recompile everything  You can also end up reducing the amount of work the compiler has to do on any individual block at the same time  by virtue of the fact that it has less to keep track of   Compiler options  Precompiled Headers  These are used to compile a common section of included headers once for many translation units  The compiler compiles it once  and saves its internal state  That state can then be loaded quickly to get a head start in compiling another file with that same set of headers   Be careful that you only include rarely changed stuff in the precompiled headers  or you could end up doing full rebuilds more often than necessary  This is a good place for STL headers and other library include files   ccache is another utility that takes advantage of caching techniques to speed things up   Use Parallelism  Many compilers   IDEs support using multiple cores CPUs to do compilation simultaneously  In GNU Make  usually used with GCC   use the -j  N  option  In Visual Studio  there s an option under preferences to allow it to build multiple projects in parallel  You can also use the  MP option for file-level paralellism  instead of just project-level paralellism   Other parallel utilities    Incredibuild Unity Build distcc   Use a Lower Optimization Level  The more the compiler tries to optimize  the harder it has to work   Shared Libraries  Moving your less frequently modified code into libraries can reduce compile time  By using shared libraries   so or  dll   you can reduce linking time as well   Get a Faster Computer  More RAM  faster hard drives  including SSDs   and more CPUs cores will all make a difference in compilation speed

User · Answer

Once you have applied all the code tricks above  forward declarations  reducing header inclusion to the minimum in public headers  pushing most details inside the implementation file with Pimpl     and nothing else can be gained language-wise  consider your build system  If you use Linux  consider using distcc  distributed compiler  and ccache  cache compiler    The first one  distcc  executes the preprocessor step locally and then sends the output to the first available compiler in the network  It requires the same compiler and library versions in all the configured nodes in the network   The latter  ccache  is a compiler cache  It again executes the preprocessor and then check with an internal database  held in a local directory  if that preprocessor file has already been compiled with the same compiler parameters  If it does  it just pops up the binary and output from the first run of the compiler   Both can be used at the same time  so that if ccache does not have a local copy it can send it trough the net to another node with distcc  or else it can just inject the solution without further processing

User · Answer

I had an idea about using a RAM drive  It turned out that for my projects it doesn t make that much of a difference after all  But then they are pretty small still  Try it  I d be interested in hearing how much it helped

User · Answer

Just for completeness  a build might be slow because the build system is being stupid as well as because the compiler is taking a long time to do its work    Read Recursive Make Considered Harmful  PDF  for a discussion of this topic in Unix environments

User · Answer

Just for completeness  a build might be slow because the build system is being stupid as well as because the compiler is taking a long time to do its work    Read Recursive Make Considered Harmful  PDF  for a discussion of this topic in Unix environments

User · Answer

Once you have applied all the code tricks above  forward declarations  reducing header inclusion to the minimum in public headers  pushing most details inside the implementation file with Pimpl     and nothing else can be gained language-wise  consider your build system  If you use Linux  consider using distcc  distributed compiler  and ccache  cache compiler    The first one  distcc  executes the preprocessor step locally and then sends the output to the first available compiler in the network  It requires the same compiler and library versions in all the configured nodes in the network   The latter  ccache  is a compiler cache  It again executes the preprocessor and then check with an internal database  held in a local directory  if that preprocessor file has already been compiled with the same compiler parameters  If it does  it just pops up the binary and output from the first run of the compiler   Both can be used at the same time  so that if ccache does not have a local copy it can send it trough the net to another node with distcc  or else it can just inject the solution without further processing

User · Answer

Just for completeness  a build might be slow because the build system is being stupid as well as because the compiler is taking a long time to do its work    Read Recursive Make Considered Harmful  PDF  for a discussion of this topic in Unix environments

User · Answer

Starting with Visual Studio 2017 you have the capability to have some compiler metrics about what takes time   Add those parameters to C C   -  Command line  Additional Options  in the project properties window   Bt   d2cgsummary  d1reportTime  You can have more informations in this post

User · Answer

I will just link to my other answer  How do YOU reduce compile time  and linking time for Visual C   projects  native C      Another point I want to add  but which causes often problems is to use precompiled headers  But please  only use them for parts which hardly ever change  like GUI toolkit headers   Otherwise  they will cost you more time than they save you in the end   Another option is  when you work with GNU make  to turn on -j lt N gt  option     -j  N   --jobs  N           Allow N jobs at once  infinite jobs with no arg    I usually have it at 3 since I ve got a dual core here  It will then run compilers in parallel for different translation units  provided there are no dependencies between them  Linking cannot be done in parallel  since there is only one linker process linking together all object files   But the linker itself can be threaded  and this is what the GNU gold ELF linker does  It s optimized threaded C   code which is said to link ELF object files a magnitude faster than the old ld  and was actually included into binutils

User · Answer

Upgrade your computer   Get a quad core  or a dual-quad system  Get LOTS of RAM  Use a RAM drive to drastically reduce file I O delays   There are companies that make IDE and SATA RAM drives that act like hard drives    Then you have all your other typical suggestions   Use precompiled headers if available  Reduce the amount of coupling between parts of your project   Changing one header file usually shouldn t require recompiling your entire project

User · Answer

Upgrade your computer   Get a quad core  or a dual-quad system  Get LOTS of RAM  Use a RAM drive to drastically reduce file I O delays   There are companies that make IDE and SATA RAM drives that act like hard drives    Then you have all your other typical suggestions   Use precompiled headers if available  Reduce the amount of coupling between parts of your project   Changing one header file usually shouldn t require recompiling your entire project

User · Answer

Using dynamic linking instead of static one make you compiler faster that can feel   If you use t Cmake  active the property   set BUILD SHARED LIBS ON    Build Release  using static linking can get more optimize

User · Answer

I had an idea about using a RAM drive  It turned out that for my projects it doesn t make that much of a difference after all  But then they are pretty small still  Try it  I d be interested in hearing how much it helped

User · Answer

Dynamic linking   so  can be much much faster than static linking   a    Especially when you have a slow network drive   This is since you have all of the code in the  a file which needs to be processed and written out   In addition  a much larger executable file needs to be written out to the disk

User · Answer

Use forward declarations where you can  If a class declaration only uses a pointer or reference to a type  you can just forward declare it and include the header for the type in the implementation file   For example      T h class Class2     Forward declaration  class T   public      void doSomething Class2  amp c2   private      Class2  m Class2Ptr         T cpp  include  Class2 h  void Class2  doSomething Class2  amp c2           Whatever you want here     Fewer includes means far less work for the preprocessor if you do it enough

User · Answer

Faster hard disks   Compilers write many  and possibly huge  files to disk  Work with SSD instead of typical hard disk and compilation times are much lower

User · Answer

Dynamic linking   so  can be much much faster than static linking   a    Especially when you have a slow network drive   This is since you have all of the code in the  a file which needs to be processed and written out   In addition  a much larger executable file needs to be written out to the disk

User · Answer

You could use Unity Builds

User · Answer

Once you have applied all the code tricks above  forward declarations  reducing header inclusion to the minimum in public headers  pushing most details inside the implementation file with Pimpl     and nothing else can be gained language-wise  consider your build system  If you use Linux  consider using distcc  distributed compiler  and ccache  cache compiler    The first one  distcc  executes the preprocessor step locally and then sends the output to the first available compiler in the network  It requires the same compiler and library versions in all the configured nodes in the network   The latter  ccache  is a compiler cache  It again executes the preprocessor and then check with an internal database  held in a local directory  if that preprocessor file has already been compiled with the same compiler parameters  If it does  it just pops up the binary and output from the first run of the compiler   Both can be used at the same time  so that if ccache does not have a local copy it can send it trough the net to another node with distcc  or else it can just inject the solution without further processing

User · Answer

Where are you spending your time  Are you CPU bound  Memory bound  Disk bound  Can you use more cores  More RAM  Do you need RAID  Do you simply want to improve the efficiency of your current system   Under gcc g    have you looked at ccache  It can be helpful if you are doing make clean  make a lot

User · Answer

Use   pragma once   at the top of header files  so if they re included more than once in a translation unit  the text of the header will only get included and parsed once

User · Answer

You could use Unity Builds

User · Answer

Here are some    Use all processor cores by starting a multiple-compile job  make -j2 is a good example   Turn off or lower optimizations  for example  GCC is much faster with -O1 than -O2 or -O3   Use precompiled headers

User · Answer

I d recommend these articles from  Games from Within  Indie Game Design And Programming     Physical Structure and C       Part 1  A First Look Physical Structure and C       Part 2  Build Times Even More Experiments with Includes How Incredible Is Incredibuild  The Care and Feeding of Pre-Compiled Headers The Quest for the Perfect Build System The Quest for the Perfect Build System  Part 2    Granted  they are pretty old - you ll have to re-test everything with the latest versions  or versions available to you   to get realistic results  Either way  it is a good source for ideas

User · Answer

Use   pragma once   at the top of header files  so if they re included more than once in a translation unit  the text of the header will only get included and parsed once

User · Answer

First of all  we have to understand what so different about C   that sets it apart from other languages  Some people say it s that C   has many too features  But hey  there are languages that have a lot more features and they are nowhere near that slow  Some people say it s the size of a file that matters  Nope  source lines of code don t correlate with compile times  But wait  how can it be  More lines of code should mean longer compile times  what s the sorcery  The trick is that a lot of lines of code is hidden in preprocessor directives  Yes  Just one  include can ruin your module s compilation performance  You see  C   doesn t have a module system  All   cpp files are compiled from scratch  So having 1000   cpp files means compiling your project a thousand times  You have more than that  Too bad  That s why C   developers hesitate to split classes into multiple files  All those headers are tedious to maintain  So what can we do other than using precompiled headers  merging all the cpp files into one  and keeping the number of headers minimal  C  20 brings us preliminary support of modules  Eventually  you ll be able to forget about  include and the horrible compile performance that header files bring with them  Touched one file  Recompile only that file  Need to compile a fresh checkout  Compile in seconds rather than minutes and hours  The C   community should move to C  20 as soon as possible  C   compiler developers should put more focus on this  C   developers should start testing preliminary support in various compilers and use those compilers that support modules  This is the most important moment in C   history

User · Answer

Where are you spending your time  Are you CPU bound  Memory bound  Disk bound  Can you use more cores  More RAM  Do you need RAID  Do you simply want to improve the efficiency of your current system   Under gcc g    have you looked at ccache  It can be helpful if you are doing make clean  make a lot

User · Answer

I had an idea about using a RAM drive  It turned out that for my projects it doesn t make that much of a difference after all  But then they are pretty small still  Try it  I d be interested in hearing how much it helped

User · Answer

If you have a multicore processor  both Visual Studio  2005 and later  as well as GCC support multi-processor compiles  It is something to enable if you have the hardware  for sure

User · Answer

Upgrade your computer   Get a quad core  or a dual-quad system  Get LOTS of RAM  Use a RAM drive to drastically reduce file I O delays   There are companies that make IDE and SATA RAM drives that act like hard drives    Then you have all your other typical suggestions   Use precompiled headers if available  Reduce the amount of coupling between parts of your project   Changing one header file usually shouldn t require recompiling your entire project

User · Answer

I d recommend these articles from  Games from Within  Indie Game Design And Programming     Physical Structure and C       Part 1  A First Look Physical Structure and C       Part 2  Build Times Even More Experiments with Includes How Incredible Is Incredibuild  The Care and Feeding of Pre-Compiled Headers The Quest for the Perfect Build System The Quest for the Perfect Build System  Part 2    Granted  they are pretty old - you ll have to re-test everything with the latest versions  or versions available to you   to get realistic results  Either way  it is a good source for ideas

User · Answer

Use   pragma once   at the top of header files  so if they re included more than once in a translation unit  the text of the header will only get included and parsed once

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There s an entire book on this topic  which is titled Large-Scale C   Software Design  written by John Lakos    The book pre-dates templates  so to the contents of that book add  using templates  too  can make the compiler slower

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Language techniques  Pimpl Idiom  Take a look at the Pimpl idiom here  and here  also known as an opaque pointer or handle classes  Not only does it speed up compilation  it also increases exception safety when combined with a non-throwing swap function  The Pimpl idiom lets you reduce the dependencies between headers and reduces the amount of recompilation that needs to be done   Forward Declarations  Wherever possible  use forward declarations  If the compiler only needs to know that SomeIdentifier is a struct or a pointer or whatever  don t include the entire definition  forcing the compiler to do more work than it needs to  This can have a cascading effect  making this way slower than they need to be   The I O streams are particularly known for slowing down builds  If you need them in a header file  try  including  lt iosfwd gt  instead of  lt iostream gt  and  include the  lt iostream gt  header in the implementation file only   The  lt iosfwd gt  header holds forward declarations only  Unfortunately the other standard headers don t have a respective declarations header   Prefer pass-by-reference to pass-by-value in function signatures  This will eliminate the need to  include the respective type definitions in the header file and you will only need to forward-declare the type  Of course  prefer const references to non-const references to avoid obscure bugs  but this is an issue for another question   Guard Conditions  Use guard conditions to keep header files from being included more than once in a single translation unit    pragma once  ifndef filename h  define filename h     Header declarations   definitions   endif   By using both the pragma and the ifndef  you get the portability of the plain macro solution  as well as the compilation speed optimization that some compilers can do in the presence of the pragma once directive   Reduce interdependency  The more modular and less interdependent your code design is in general  the less often you will have to recompile everything  You can also end up reducing the amount of work the compiler has to do on any individual block at the same time  by virtue of the fact that it has less to keep track of   Compiler options  Precompiled Headers  These are used to compile a common section of included headers once for many translation units  The compiler compiles it once  and saves its internal state  That state can then be loaded quickly to get a head start in compiling another file with that same set of headers   Be careful that you only include rarely changed stuff in the precompiled headers  or you could end up doing full rebuilds more often than necessary  This is a good place for STL headers and other library include files   ccache is another utility that takes advantage of caching techniques to speed things up   Use Parallelism  Many compilers   IDEs support using multiple cores CPUs to do compilation simultaneously  In GNU Make  usually used with GCC   use the -j  N  option  In Visual Studio  there s an option under preferences to allow it to build multiple projects in parallel  You can also use the  MP option for file-level paralellism  instead of just project-level paralellism   Other parallel utilities    Incredibuild Unity Build distcc   Use a Lower Optimization Level  The more the compiler tries to optimize  the harder it has to work   Shared Libraries  Moving your less frequently modified code into libraries can reduce compile time  By using shared libraries   so or  dll   you can reduce linking time as well   Get a Faster Computer  More RAM  faster hard drives  including SSDs   and more CPUs cores will all make a difference in compilation speed

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Upgrade your computer   Get a quad core  or a dual-quad system  Get LOTS of RAM  Use a RAM drive to drastically reduce file I O delays   There are companies that make IDE and SATA RAM drives that act like hard drives    Then you have all your other typical suggestions   Use precompiled headers if available  Reduce the amount of coupling between parts of your project   Changing one header file usually shouldn t require recompiling your entire project

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There s an entire book on this topic  which is titled Large-Scale C   Software Design  written by John Lakos    The book pre-dates templates  so to the contents of that book add  using templates  too  can make the compiler slower

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Where are you spending your time  Are you CPU bound  Memory bound  Disk bound  Can you use more cores  More RAM  Do you need RAID  Do you simply want to improve the efficiency of your current system   Under gcc g    have you looked at ccache  It can be helpful if you are doing make clean  make a lot

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On Linux  and maybe some other  NIXes   you can really speed the compilation by NOT STARING at the output and changing to another TTY   Here is the experiment  printf slows down my program

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I work on the STAPL project which is a heavily-templated C   library  Once in a while  we have to revisit all the techniques to reduce compilation time  In here  I have summarized the techniques we use  Some of these techniques are already listed above  Finding the most time-consuming sections Although there is no proven correlation between the symbol lengths and compilation time  we have observed that smaller average symbol sizes can improve compilation time on all compilers  So your first goals it to find the largest symbols in your code  Method 1 - Sort symbols based on size You can use the nm command to list the symbols based on their sizes  nm --print-size --size-sort --radix d YOUR BINARY  In this command the --radix d lets you see the sizes in decimal numbers  default is hex   Now by looking at the largest symbol  identify if you can break the corresponding class and try to redesign it by factoring the non-templated parts in a base class  or by splitting the class into multiple classes  Method 2 - Sort symbols based on length You can run the regular nm command and pipe it to your favorite script  AWK  Python  etc   to sort the symbols based on their length  Based on our experience  this method identifies the largest trouble making candidates better than method 1  Method 3 - Use Templight  quot Templight is a Clang-based tool to profile the time and memory consumption of template instantiations and to perform interactive debugging sessions to gain introspection into the template instantiation process quot   You can install Templight by checking out LLVM and Clang  instructions  and applying the Templight patch on it  The default setting for LLVM and Clang is on debug and assertions  and these can impact your compilation time significantly  It does seem like Templight needs both  so you have to use the default settings  The process of installing LLVM and Clang should take about an hour or so  After applying the patch you can use templight   located in the build folder you specified upon installation to compile your code  Make sure that templight   is in your PATH  Now to compile add the following switches to your CXXFLAGS in your Makefile or to your command line options  CXXFLAGS  -Xtemplight -profiler -Xtemplight -memory -Xtemplight -ignore-system  Or templight   -Xtemplight -profiler -Xtemplight -memory -Xtemplight -ignore-system  After compilation is done  you will have a  trace memory pbf and  trace pbf generated in the same folder  To visualize these traces  you can use the Templight Tools that can convert these to other formats  Follow these instructions to install templight-convert  We usually use the callgrind output  You can also use the GraphViz output if your project is small    templight-convert --format callgrind YOUR BINARY --output YOUR BINARY trace    templight-convert --format graphviz YOUR BINARY --output YOUR BINARY dot  The callgrind file generated can be opened using kcachegrind in which you can trace the most time memory consuming instantiation  Reducing the number of template instantiations Although there are no exact solution for reducing the number of template instantiations  there are a few guidelines that can help  Refactor classes with more than one template arguments For example  if you have a class  template  lt typename T  typename U gt  struct foo       and both of T and U can have 10 different options  you have increased the possible template instantiations of this class to 100  One way to resolve this is to abstract the common part of the code to a different class  The other method is to use inheritance inversion  reversing the class hierarchy   but make sure that your design goals are not compromised before using this technique  Refactor non-templated code to individual translation units Using this technique  you can compile the common section once and link it with your other TUs  translation units  later on  Use extern template instantiations  since C  11  If you know all the possible instantiations of a class you can use this technique to compile all cases in a different translation unit  For example  in  enum class PossibleChoices    Option1  Option2  Option3   template  lt PossibleChoices pc gt  struct foo       We know that this class can have three possible instantiations  template class foo lt PossibleChoices  Option1 gt   template class foo lt PossibleChoices  Option2 gt   template class foo lt PossibleChoices  Option3 gt    Put the above in a translation unit and use the extern keyword in your header file  below the class definition  extern template class foo lt PossibleChoices  Option1 gt   extern template class foo lt PossibleChoices  Option2 gt   extern template class foo lt PossibleChoices  Option3 gt    This technique can save you time if you are compiling different tests with a common set of instantiations   NOTE   MPICH2 ignores the explicit instantiation at this point and always compiles the instantiated classes in all compilation units   Use unity builds The whole idea behind unity builds is to include all the  cc files that you use in one file and compile that file only once  Using this method  you can avoid reinstantiating common sections of different files and if your project includes a lot of common files  you probably would save on disk accesses as well  As an example  let s assume you have three files foo1 cc  foo2 cc  foo3 cc and they all include tuple from STL  You can create a foo-all cc that looks like   include  quot foo1 cc quot   include  quot foo2 cc quot   include  quot foo3 cc quot   You compile this file only once and potentially reduce the common instantiations among the three files  It is hard to generally predict if the improvement can be significant or not  But one evident fact is that you would lose parallelism in your builds  you can no longer compile the three files at the same time   Further  if any of these files happen to take a lot of memory  you might actually run out of memory before the compilation is over  On some compilers  such as GCC  this might ICE  Internal Compiler Error  your compiler for lack of memory  So don t use this technique unless you know all the pros and cons  Precompiled headers Precompiled headers  PCHs  can save you a lot of time in compilation by compiling your header files to an intermediate representation recognizable by a compiler  To generate precompiled header files  you only need to compile your header file with your regular compilation command  For example  on GCC    g   YOUR HEADER hpp  This will generate a YOUR HEADER hpp gch file   gch is the extension for PCH files in GCC  in the same folder  This means that if you include YOUR HEADER hpp in some other file  the compiler will use your YOUR HEADER hpp gch instead of YOUR HEADER hpp in the same folder before  There are two issues with this technique   You have to make sure that the header files being precompiled is stable and is not going to change  you can always change your makefile  You can only include one PCH per compilation unit  on most of compilers   This means that if you have more than one header file to be precompiled  you have to include them in one file  e g   all-my-headers hpp   But that means that you have to include the new file in all places  Fortunately  GCC has a solution for this problem  Use -include and give it the new header file  You can comma separate different files using this technique   For example  g   foo cc -include all-my-headers hpp  Use unnamed or anonymous namespaces Unnamed namespaces  a k a  anonymous namespaces  can reduce the generated binary sizes significantly  Unnamed namespaces use internal linkage  meaning that the symbols generated in those namespaces will not be visible to other TU  translation or compilation units   Compilers usually generate unique names for unnamed namespaces  This means that if you have a file foo hpp  namespace    template  lt typename T gt  struct foo           Anonymous namespace using A   foo lt int gt    And you happen to include this file in two TUs  two  cc files and compile them separately   The two foo template instances will not be the same  This violates the One Definition Rule  ODR   For the same reason  using unnamed namespaces is discouraged in the header files  Feel free to use them in your  cc files to avoid symbols showing up in your binary files  In some cases  changing all the internal details for a  cc file showed a 10  reduction in the generated binary sizes  Changing visibility options In newer compilers you can select your symbols to be either visible or invisible in the Dynamic Shared Objects   DSOs   Ideally  changing the visibility can improve compiler performance  link time optimizations  LTOs   and generated binary sizes  If you look at the STL header files in GCC you can see that it is widely used  To enable visibility choices  you need to change your code per function  per class  per variable and more importantly per compiler  With the help of visibility you can hide the symbols that you consider them private from the generated shared objects  On GCC you can control the visibility of symbols by passing default or hidden to the -visibility option of your compiler  This is in some sense similar to the unnamed namespace but in a more elaborate and intrusive way  If you would like to specify the visibilities per case  you have to add the following attributes to your functions  variables  and classes    attribute    visibility  quot default quot     void  foo1         attribute    visibility  quot hidden quot      void  foo2         attribute    visibility  quot hidden quot      class foo3        void foo4        The default visibility in GCC is default  public   meaning that if you compile the above as a shared library  -shared  method  foo2 and class foo3 will not be visible in other TUs  foo1 and foo4 will be visible   If you compile with -visibility hidden then only foo1 will be visible  Even foo4 would be hidden  You can read more about visibility on GCC wiki

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Use forward declarations where you can  If a class declaration only uses a pointer or reference to a type  you can just forward declare it and include the header for the type in the implementation file   For example      T h class Class2     Forward declaration  class T   public      void doSomething Class2  amp c2   private      Class2  m Class2Ptr         T cpp  include  Class2 h  void Class2  doSomething Class2  amp c2           Whatever you want here     Fewer includes means far less work for the preprocessor if you do it enough

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I will just link to my other answer  How do YOU reduce compile time  and linking time for Visual C   projects  native C      Another point I want to add  but which causes often problems is to use precompiled headers  But please  only use them for parts which hardly ever change  like GUI toolkit headers   Otherwise  they will cost you more time than they save you in the end   Another option is  when you work with GNU make  to turn on -j lt N gt  option     -j  N   --jobs  N           Allow N jobs at once  infinite jobs with no arg    I usually have it at 3 since I ve got a dual core here  It will then run compilers in parallel for different translation units  provided there are no dependencies between them  Linking cannot be done in parallel  since there is only one linker process linking together all object files   But the linker itself can be threaded  and this is what the GNU gold ELF linker does  It s optimized threaded C   code which is said to link ELF object files a magnitude faster than the old ld  and was actually included into binutils

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You could use Unity Builds

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Although not a  technique   I couldn t figure out how Win32 projects with many source files compiled faster than my  Hello World  empty project   Thus  I hope this helps someone like it did me   In Visual Studio  one option to increase compile times is Incremental Linking   INCREMENTAL    It s incompatible with Link-time Code Generation   LTCG  so remember to disable incremental linking when doing release builds

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When I came out of college  the first real production-worthy C   code I saw had these arcane  ifndef      endif directives in between them where the headers were defined  I asked the guy who was writing the code about these overarching things in a very naive fashion and was introduced to world of large-scale programming   Coming back to the point  using directives to prevent duplicate header definitions was the first thing I learned when it came to reducing compiling times

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Here are some    Use all processor cores by starting a multiple-compile job  make -j2 is a good example   Turn off or lower optimizations  for example  GCC is much faster with -O1 than -O2 or -O3   Use precompiled headers

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I d recommend these articles from  Games from Within  Indie Game Design And Programming     Physical Structure and C       Part 1  A First Look Physical Structure and C       Part 2  Build Times Even More Experiments with Includes How Incredible Is Incredibuild  The Care and Feeding of Pre-Compiled Headers The Quest for the Perfect Build System The Quest for the Perfect Build System  Part 2    Granted  they are pretty old - you ll have to re-test everything with the latest versions  or versions available to you   to get realistic results  Either way  it is a good source for ideas

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Dynamic linking   so  can be much much faster than static linking   a    Especially when you have a slow network drive   This is since you have all of the code in the  a file which needs to be processed and written out   In addition  a much larger executable file needs to be written out to the disk

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Here are some    Use all processor cores by starting a multiple-compile job  make -j2 is a good example   Turn off or lower optimizations  for example  GCC is much faster with -O1 than -O2 or -O3   Use precompiled headers

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Dynamic linking   so  can be much much faster than static linking   a    Especially when you have a slow network drive   This is since you have all of the code in the  a file which needs to be processed and written out   In addition  a much larger executable file needs to be written out to the disk

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On Linux  and maybe some other  NIXes   you can really speed the compilation by NOT STARING at the output and changing to another TTY   Here is the experiment  printf slows down my program

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One technique which worked quite well for me in the past  don t compile multiple C   source files independently  but rather generate one C   file which includes all the other files  like this      myproject all cpp    Automatically generated file - don t edit this by hand   include  main cpp   include  mainwindow cpp   include  filterdialog cpp   include  database cpp    Of course this means you have to recompile all of the included source code in case any of the sources changes  so the dependency tree gets worse  However  compiling multiple source files as one translation unit is faster  at least in my experiments with MSVC and GCC  and generates smaller binaries  I also suspect that the compiler is given more potential for optimizations  since it can see more code at once    This technique breaks in various cases  for instance  the compiler will bail out in case two or more source files declare a global function with the same name  I couldn t find this technique described in any of the other answers though  that s why I m mentioning it here   For what it s worth  the KDE Project used this exact same technique since 1999 to build optimized binaries  possibly for a release   The switch to the build configure script was called --enable-final  Out of archaeological interest I dug up the posting which announced this feature  http   lists kde org  l kde-devel amp m 92722836009368 amp w 2

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Where are you spending your time  Are you CPU bound  Memory bound  Disk bound  Can you use more cores  More RAM  Do you need RAID  Do you simply want to improve the efficiency of your current system   Under gcc g    have you looked at ccache  It can be helpful if you are doing make clean  make a lot

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One technique which worked quite well for me in the past  don t compile multiple C   source files independently  but rather generate one C   file which includes all the other files  like this      myproject all cpp    Automatically generated file - don t edit this by hand   include  main cpp   include  mainwindow cpp   include  filterdialog cpp   include  database cpp    Of course this means you have to recompile all of the included source code in case any of the sources changes  so the dependency tree gets worse  However  compiling multiple source files as one translation unit is faster  at least in my experiments with MSVC and GCC  and generates smaller binaries  I also suspect that the compiler is given more potential for optimizations  since it can see more code at once    This technique breaks in various cases  for instance  the compiler will bail out in case two or more source files declare a global function with the same name  I couldn t find this technique described in any of the other answers though  that s why I m mentioning it here   For what it s worth  the KDE Project used this exact same technique since 1999 to build optimized binaries  possibly for a release   The switch to the build configure script was called --enable-final  Out of archaeological interest I dug up the posting which announced this feature  http   lists kde org  l kde-devel amp m 92722836009368 amp w 2

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Starting with Visual Studio 2017 you have the capability to have some compiler metrics about what takes time   Add those parameters to C C   -  Command line  Additional Options  in the project properties window   Bt   d2cgsummary  d1reportTime  You can have more informations in this post

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I work on the STAPL project which is a heavily-templated C   library  Once in a while  we have to revisit all the techniques to reduce compilation time  In here  I have summarized the techniques we use  Some of these techniques are already listed above  Finding the most time-consuming sections Although there is no proven correlation between the symbol lengths and compilation time  we have observed that smaller average symbol sizes can improve compilation time on all compilers  So your first goals it to find the largest symbols in your code  Method 1 - Sort symbols based on size You can use the nm command to list the symbols based on their sizes  nm --print-size --size-sort --radix d YOUR BINARY  In this command the --radix d lets you see the sizes in decimal numbers  default is hex   Now by looking at the largest symbol  identify if you can break the corresponding class and try to redesign it by factoring the non-templated parts in a base class  or by splitting the class into multiple classes  Method 2 - Sort symbols based on length You can run the regular nm command and pipe it to your favorite script  AWK  Python  etc   to sort the symbols based on their length  Based on our experience  this method identifies the largest trouble making candidates better than method 1  Method 3 - Use Templight  quot Templight is a Clang-based tool to profile the time and memory consumption of template instantiations and to perform interactive debugging sessions to gain introspection into the template instantiation process quot   You can install Templight by checking out LLVM and Clang  instructions  and applying the Templight patch on it  The default setting for LLVM and Clang is on debug and assertions  and these can impact your compilation time significantly  It does seem like Templight needs both  so you have to use the default settings  The process of installing LLVM and Clang should take about an hour or so  After applying the patch you can use templight   located in the build folder you specified upon installation to compile your code  Make sure that templight   is in your PATH  Now to compile add the following switches to your CXXFLAGS in your Makefile or to your command line options  CXXFLAGS  -Xtemplight -profiler -Xtemplight -memory -Xtemplight -ignore-system  Or templight   -Xtemplight -profiler -Xtemplight -memory -Xtemplight -ignore-system  After compilation is done  you will have a  trace memory pbf and  trace pbf generated in the same folder  To visualize these traces  you can use the Templight Tools that can convert these to other formats  Follow these instructions to install templight-convert  We usually use the callgrind output  You can also use the GraphViz output if your project is small    templight-convert --format callgrind YOUR BINARY --output YOUR BINARY trace    templight-convert --format graphviz YOUR BINARY --output YOUR BINARY dot  The callgrind file generated can be opened using kcachegrind in which you can trace the most time memory consuming instantiation  Reducing the number of template instantiations Although there are no exact solution for reducing the number of template instantiations  there are a few guidelines that can help  Refactor classes with more than one template arguments For example  if you have a class  template  lt typename T  typename U gt  struct foo       and both of T and U can have 10 different options  you have increased the possible template instantiations of this class to 100  One way to resolve this is to abstract the common part of the code to a different class  The other method is to use inheritance inversion  reversing the class hierarchy   but make sure that your design goals are not compromised before using this technique  Refactor non-templated code to individual translation units Using this technique  you can compile the common section once and link it with your other TUs  translation units  later on  Use extern template instantiations  since C  11  If you know all the possible instantiations of a class you can use this technique to compile all cases in a different translation unit  For example  in  enum class PossibleChoices    Option1  Option2  Option3   template  lt PossibleChoices pc gt  struct foo       We know that this class can have three possible instantiations  template class foo lt PossibleChoices  Option1 gt   template class foo lt PossibleChoices  Option2 gt   template class foo lt PossibleChoices  Option3 gt    Put the above in a translation unit and use the extern keyword in your header file  below the class definition  extern template class foo lt PossibleChoices  Option1 gt   extern template class foo lt PossibleChoices  Option2 gt   extern template class foo lt PossibleChoices  Option3 gt    This technique can save you time if you are compiling different tests with a common set of instantiations   NOTE   MPICH2 ignores the explicit instantiation at this point and always compiles the instantiated classes in all compilation units   Use unity builds The whole idea behind unity builds is to include all the  cc files that you use in one file and compile that file only once  Using this method  you can avoid reinstantiating common sections of different files and if your project includes a lot of common files  you probably would save on disk accesses as well  As an example  let s assume you have three files foo1 cc  foo2 cc  foo3 cc and they all include tuple from STL  You can create a foo-all cc that looks like   include  quot foo1 cc quot   include  quot foo2 cc quot   include  quot foo3 cc quot   You compile this file only once and potentially reduce the common instantiations among the three files  It is hard to generally predict if the improvement can be significant or not  But one evident fact is that you would lose parallelism in your builds  you can no longer compile the three files at the same time   Further  if any of these files happen to take a lot of memory  you might actually run out of memory before the compilation is over  On some compilers  such as GCC  this might ICE  Internal Compiler Error  your compiler for lack of memory  So don t use this technique unless you know all the pros and cons  Precompiled headers Precompiled headers  PCHs  can save you a lot of time in compilation by compiling your header files to an intermediate representation recognizable by a compiler  To generate precompiled header files  you only need to compile your header file with your regular compilation command  For example  on GCC    g   YOUR HEADER hpp  This will generate a YOUR HEADER hpp gch file   gch is the extension for PCH files in GCC  in the same folder  This means that if you include YOUR HEADER hpp in some other file  the compiler will use your YOUR HEADER hpp gch instead of YOUR HEADER hpp in the same folder before  There are two issues with this technique   You have to make sure that the header files being precompiled is stable and is not going to change  you can always change your makefile  You can only include one PCH per compilation unit  on most of compilers   This means that if you have more than one header file to be precompiled  you have to include them in one file  e g   all-my-headers hpp   But that means that you have to include the new file in all places  Fortunately  GCC has a solution for this problem  Use -include and give it the new header file  You can comma separate different files using this technique   For example  g   foo cc -include all-my-headers hpp  Use unnamed or anonymous namespaces Unnamed namespaces  a k a  anonymous namespaces  can reduce the generated binary sizes significantly  Unnamed namespaces use internal linkage  meaning that the symbols generated in those namespaces will not be visible to other TU  translation or compilation units   Compilers usually generate unique names for unnamed namespaces  This means that if you have a file foo hpp  namespace    template  lt typename T gt  struct foo           Anonymous namespace using A   foo lt int gt    And you happen to include this file in two TUs  two  cc files and compile them separately   The two foo template instances will not be the same  This violates the One Definition Rule  ODR   For the same reason  using unnamed namespaces is discouraged in the header files  Feel free to use them in your  cc files to avoid symbols showing up in your binary files  In some cases  changing all the internal details for a  cc file showed a 10  reduction in the generated binary sizes  Changing visibility options In newer compilers you can select your symbols to be either visible or invisible in the Dynamic Shared Objects   DSOs   Ideally  changing the visibility can improve compiler performance  link time optimizations  LTOs   and generated binary sizes  If you look at the STL header files in GCC you can see that it is widely used  To enable visibility choices  you need to change your code per function  per class  per variable and more importantly per compiler  With the help of visibility you can hide the symbols that you consider them private from the generated shared objects  On GCC you can control the visibility of symbols by passing default or hidden to the -visibility option of your compiler  This is in some sense similar to the unnamed namespace but in a more elaborate and intrusive way  If you would like to specify the visibilities per case  you have to add the following attributes to your functions  variables  and classes    attribute    visibility  quot default quot     void  foo1         attribute    visibility  quot hidden quot      void  foo2         attribute    visibility  quot hidden quot      class foo3        void foo4        The default visibility in GCC is default  public   meaning that if you compile the above as a shared library  -shared  method  foo2 and class foo3 will not be visible in other TUs  foo1 and foo4 will be visible   If you compile with -visibility hidden then only foo1 will be visible  Even foo4 would be hidden  You can read more about visibility on GCC wiki

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You could use Unity Builds

User · Answer

When I came out of college  the first real production-worthy C   code I saw had these arcane  ifndef      endif directives in between them where the headers were defined  I asked the guy who was writing the code about these overarching things in a very naive fashion and was introduced to world of large-scale programming   Coming back to the point  using directives to prevent duplicate header definitions was the first thing I learned when it came to reducing compiling times

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Not about the compilation time  but about the build time    Use ccache if you have to rebuild the same files when you are working on your buildfiles Use ninja-build instead of make  I am currently compiling a project with  100 source files and everything is cached by ccache  make needs 5 minutes  ninja less than 1    You can generate your ninja files from cmake with -GNinja

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Faster hard disks   Compilers write many  and possibly huge  files to disk  Work with SSD instead of typical hard disk and compilation times are much lower

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I had an idea about using a RAM drive  It turned out that for my projects it doesn t make that much of a difference after all  But then they are pretty small still  Try it  I d be interested in hearing how much it helped

[c++] What techniques can be used to speed up C++ compilation times?

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