Clang vs GCC - which produces faster binaries

Question

I m currently using GCC  but I discovered Clang recently and I m pondering switching  There is one deciding factor though - quality  speed  memory footprint  reliability  of binaries it produces - if gcc -O3can produce a binary that runs 1  faster  or Clang binaries take up more memory or just fail due to compiler bugs  it s a deal-breaker  Clang boasts better compile speeds and lower compile-time memory footprint than GCC  but I m really interested in benchmarks comparisons of resulting compiled software - could you point me to some or describe your experiences

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Phoronix did some benchmarks about this  but it is about a snapshot version of Clang LLVM from a few months back  The results being that things were more-or-less a push  neither GCC nor Clang is definitively better in all cases   Since you d use the latest Clang  it s maybe a little less relevant  Then again  GCC 4 6 is slated to have some major optimizations for Core 2 and i7  apparently   I figure Clang s faster compilation speed will be nicer for original developers  and then when you push the code out into the world  Linux distro BSD etc  end-users will  use GCC for the faster binaries

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There is very little overall difference between GCC 4 8 and clang 3 3 in terms of speed of the resulting binary  In most cases code generated by both compilers performs similarly  Neither of these two compilers dominates the other one   Benchmarks telling that there is a significant performance gap between GCC and clang are coincidental   Program performance is affected by the choice of the compiler  If a developer or a group of developers is exclusively using GCC then the program can be expected to run slightly faster with GCC than with clang  and vice versa   From developer viewpoint  a notable difference between GCC 4 8  and clang 3 3 is that GCC has the -Og command line option  This option enables optimizations that do not interfere with debugging  so for example it is always possible to get accurate stack traces  The absence of this option in clang makes clang harder to use as an optimizing compiler for some developers

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A peculiar difference I have noted on gcc 5 2 1 and clang 3 6 2 is that if you have a critical loop like  for            if   visited                           node        if    node  break       Then gcc will  when compiling with -O3 or -O2  speculatively unroll the loop eight times  Clang will not unroll it at all  Through trial and error I found that in my specific case with my program data  the right amount of unrolling is five so gcc overshot and clang undershot  However  overshooting was more detrimental to performance  so gcc performed much worse here  I have no idea if the unrolling difference is a general trend or just something that was specific to my scenario  A while back I wrote a few garbage collectors to teach myself more about performance optimization in C  And the results I got is in my mind enough to slightly favor clang  Especially since garbage collection is mostly about pointer chasing and copying memory  The results are  numbers in seconds    --------------------- ----- -----   Type                  GCC   Clang   --------------------- ----- -----   Copying GC            22 46 22 55   Copying GC  optimized 22 01 20 22   Mark  amp  Sweep           8 72  8 38   Ref Counting Cycles   15 14 14 49   Ref Counting Plain     9 94  9 32   --------------------- ----- -----   This is all pure C code  and I make no claim about either compiler s performance when compiling C   code  On Ubuntu 15 10  x86 64  and an AMD Phenom tm  II X6 1090T processor

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The fact that Clang compiles code faster may not be as important as the speed of the resulting binary  However  here is a series of benchmarks

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Basically speaking  the answer is  it depends  There are many many benchmarks focusing on different kinds of application   My benchmark on my app is   gcc   icc   clang   There are rare IO  but many CPU float and data structure operations   compile flags is -Wall -g -DNDEBUG -O3   https   github com zhangyafeikimi ml-pack blob master gbdt profile benchmark

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Here are some up-to-date albeit narrow findings of mine with GCC 4 7 2  and Clang 3 2 for C     UPDATE  GCC 4 8 1 v clang 3 3 comparison appended below   UPDATE  GCC 4 8 2 v clang 3 4 comparison is appended to that   I maintain an OSS tool that is built for Linux with both GCC and Clang  and with Microsoft s compiler for Windows  The tool  coan  is a preprocessor  and analyser of C C   source files and codelines of such  its  computational profile majors on recursive-descent parsing and file-handling  The development branch  to which these results pertain  comprises at present around 11K LOC in about 90 files  It is coded  now  in C   that is rich in polymorphism and templates and but is still mired in many patches by its not-so-distant past in hacked-together C  Move semantics are not expressly exploited  It is single-threaded  I have devoted no serious effort to optimizing it  while the  architecture  remains so largely ToDo   I employed Clang prior to 3 2 only as an experimental compiler because  despite its superior compilation speed and diagnostics  its C  11 standard support lagged the contemporary GCC version in the respects exercised by coan  With 3 2  this gap has been closed   My Linux test harness for current coan development processes roughly  70K sources files in a mixture of one-file parser test-cases  stress tests consuming 1000s of files and scenario tests consuming  lt  1K files  As well as reporting the test results  the harness accumulates and  displays the totals of files consumed and the run time consumed in coan   it just passes each coan command line to the Linux time command and  captures and adds up the reported numbers   The timings are flattered  by the fact that any number of tests which take 0 measurable time will  all add up to 0  but the contribution of such tests is negligible  The  timing stats are displayed at the end of make check like this   coan test timer  info  coan processed 70844 input files  coan test timer  info  run time in coan  16 4 secs  coan test timer  info  Average processing time per input file  0 000231 secs    I compared the test harness performance as between GCC 4 7 2 and Clang 3 2  all things being equal except the compilers  As of Clang 3 2  I no longer require any preprocessor differentiation between code tracts that GCC will compile and Clang alternatives  I built to the  same C   library  GCC s  in each case and ran all the comparisons consecutively in the same terminal session   The default optimization level for my release build is -O2  I also successfully tested builds at -O3  I tested each configuration 3 times back-to-back and averaged the 3 outcomes  with the following results  The number in a data-cell is the average number of  microseconds consumed by the coan executable to process each of the  70K input files  read  parse and write output and diagnostics                -O2   -O3  O2 O3  ---------- ----- ----- -----  GCC-4 7 2   231   237  0 97   ---------- ----- ----- -----  Clang-3 2   234   186  1 25   ---------- ----- ----- ------ GCC Clang  0 99   1 27    Any particular application is very likely to have traits that play  unfairly to a compiler s strengths or weaknesses  Rigorous benchmarking employs diverse applications  With that well in mind  the noteworthy  features of these data are    -O3 optimization was marginally detrimental to GCC -O3 optimization was importantly beneficial to Clang At -O2 optimization  GCC was faster than Clang by just a whisker At -O3 optimization  Clang was importantly faster than GCC    A further interesting comparison of the two compilers emerged by accident shortly after those findings  Coan liberally employs smart pointers and one such is heavily exercised in the file handling  This particular  smart-pointer type had been typedef d in prior releases for the sake of  compiler-differentiation  to be an std  unique ptr lt X gt  if the  configured compiler had sufficiently mature support for its usage as  that  and otherwise an std  shared ptr lt X gt   The bias to std  unique ptr was foolish  since these pointers were in fact transferred around  but std  unique ptr looked like the fitter option for replacing std  auto ptr at a point when the C  11 variants were novel to me     In the course of experimental builds to gauge Clang 3 2 s continued need for this and similar differentiation  I inadvertently built  std  shared ptr lt X gt  when I had intended to build std  unique ptr lt X gt    and was surprised to observe that the resulting executable  with default -O2 optimization  was the fastest I had seen  sometimes achieving 184 msecs  per input file  With this one change to the source code  the corresponding results were these               -O2   -O3  O2 O3  ---------- ----- ----- -----  GCC-4 7 2   234   234  1 00   ---------- ----- ----- -----  Clang-3 2   188   187  1 00   ---------- ----- ----- ------ GCC Clang  1 24  1 25     The points of note here are    Neither compiler now benefits at all from -O3 optimization  Clang beats GCC just as importantly at each level of optimization  GCC s performance is only marginally affected by the smart-pointer type change   Clang s -O2 performance is importantly affected by the smart-pointer type change    Before and after the smart-pointer type change  Clang is able to build a  substantially faster coan executable at -O3 optimisation  and it can build an equally faster executable at -O2 and -O3 when that  pointer-type is the best one - std  shared ptr lt X gt  - for the job   An obvious question that I am not competent to comment upon is why Clang should be able to find a 25  -O2 speed-up in my application when a heavily used smart-pointer-type is changed from unique to shared  while GCC is indifferent to the same change  Nor do I know whether I should cheer or boo the discovery that Clang s -O2 optimization harbours such huge sensitivity to the wisdom of my smart-pointer choices   UPDATE  GCC 4 8 1 v clang 3 3  The corresponding results now are                -O2   -O3  O2 O3  ---------- ----- ----- -----  GCC-4 8 1   442   443  1 00   ---------- ----- ----- -----  Clang-3 3   374   370  1 01   ---------- ----- ----- ------ GCC Clang  1 18  1 20     The fact that all four executables now take a much greater average time than previously to process  1 file does not reflect on the latest compilers  performance  It is due to the fact that the later development branch of the test application has taken on lot of parsing sophistication in the meantime and pays for it in speed  Only the ratios are significant   The points of note now are not arrestingly novel    GCC is indifferent to -O3 optimization clang benefits very marginally from -O3 optimization clang beats GCC by a similarly important margin at each level of optimization    Comparing these results with those for GCC 4 7 2 and clang 3 2  it stands out that GCC has clawed back about a quarter of clang s lead at each optimization level  But since the test application has been heavily developed in the meantime one cannot confidently attribute this to a catch-up in GCC s code-generation    This time  I have noted the application snapshot from which the timings were obtained and can use it again    UPDATE  GCC 4 8 2 v clang 3 4  I finished the update for GCC 4 8 1 v Clang 3 3 saying that I would stick to the same coan snaphot for further updates  But I decided instead to test on that snapshot  rev  301  and on the latest development  snapshot I have that passes its test suite  rev  619   This gives the results a  bit of longitude  and I had another motive   My original posting noted that I had devoted no effort to optimizing coan for speed  This was still the case as of rev  301  However  after I had built the timing apparatus into the coan test harness  every time I ran the test suite the performance impact of the latest changes stared me in the face  I saw that  it was often surprisingly big and that the trend was more steeply negative than  I felt to be merited by gains in functionality   By rev  308 the average processing time per input file in the test suite had  well more than doubled since the first posting here  At that point I made a  U-turn on my 10 year policy of not bothering about performance  In the intensive spate of revisions up to 619 performance was always a consideration and a large number of them went purely to rewriting key load-bearers on fundamentally faster lines  though without using any non-standard compiler features to do so   It would be interesting to see each compiler s reaction to this U-turn   Here is the now familiar timings matrix for the latest two compilers  builds of rev 301   coan - rev 301 results              -O2   -O3  O2 O3  ---------- ----- ----- -----  GCC-4 8 2   428   428  1 00   ---------- ----- ----- -----  Clang-3 4   390   365  1 07   ---------- ----- ----- ------ GCC Clang   1 1   1 17    The story here is only marginally changed from GCC-4 8 1 and Clang-3 3  GCC s showing is a trifle better  Clang s is a trifle worse  Noise could well account for this  Clang still comes out ahead by -O2 and -O3 margins that wouldn t matter in most applications but would matter to quite a few   And here is the matrix for rev  619   coan - rev 619 results              -O2   -O3  O2 O3  ---------- ----- ----- -----  GCC-4 8 2   210   208  1 01   ---------- ----- ----- -----  Clang-3 4   252   250  1 01   ---------- ----- ----- ------ GCC Clang  0 83   0 83    Taking the 301 and the 619 figures side by side  several points speak out    I was aiming to write faster code  and both compilers emphatically vindicate my efforts  But  GCC repays those efforts far more generously than Clang  At -O2  optimization Clang s 619 build is 46  faster than its 301 build  at -O3 Clang s improvement is 31   Good  but at each optimization level GCC s 619 build is more than twice as fast as its 301  GCC more than reverses Clang s former superiority  And at each optimization  level GCC now beats Clang by 17   Clang s ability in the 301 build to get more leverage than GCC from -O3 optimization is gone in the 619 build  Neither compiler gains meaningfully from -O3    I was sufficiently surprised by this reversal of fortunes that I suspected I might have accidentally made a sluggish build of clang 3 4 itself  since I built it from source   So I re-ran the 619 test with my distro s stock Clang 3 3  The results were practically the same as for 3 4   So as regards reaction to the U-turn  On the numbers here  Clang has done much  better than GCC at at wringing speed out of my C   code when I was giving it no  help  When I put my mind to helping  GCC did a much better job than Clang   I don t elevate that observation into a principle  but I take the lesson that  Which compiler produces the better binaries   is a question that  even if you specify the test suite to which the answer shall be relative  still is not a clear-cut matter of just timing the binaries    Is your better binary the fastest binary  or is it the one that best  compensates for cheaply crafted code  Or best compensates for expensively  crafted code that prioritizes maintainability and reuse over speed  It depends on the  nature and relative weights of your motives for producing the binary  and of the constraints under which you do so    And in any case  if you deeply care about building  the best  binaries then you  had better keep checking how successive iterations of compilers deliver on your  idea of  the best  over successive iterations of your code

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The only way to determine this is to try it  FWIW I have seen some really good improvements using Apple s LLVM gcc 4 2 compared to the regular gcc 4 2  for x86-64 code with quite a lot of SSE   but YMMV for different code bases  Assuming you re working with x86 x86-64 and that you really do care about the last few percent then you ought to try Intel s ICC too  as this can often beat gcc - you can get a 30 day evaluation license from intel com and try it

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