What does the restrict keyword mean in C

Question

I was always unsure  what does the restrict keyword mean in C     Does it mean the two or more pointer given to the function does not overlap  What else does it mean

User · Answer

There s no such keyword in C    List of C   keywords can be found in section 2 11 1 of C   language standard  restrict is a keyword in C99 version of C language and not in C

User · Answer

Nothing  It was added to the C99 standard

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Since header files from some C libraries use the keyword  the C   language will have to do something about it   at the minimum  ignoring the keyword  so we don t have to  define the keyword to a blank macro to suppress the keyword

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This is the original proposal to add this keyword  As dirkgently pointed out though  this is a C99 feature  it has nothing to do with C

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In his paper  Memory Optimization  Christer Ericson says that while restrict is not part of the C   standard yet  that it is supported by many compilers and he recommends it s usage when available      restrict   keyword         New to 1999 ANSI ISO C standard            Not in C   standard yet  but supported by many C   compilers            A hint only  so may do nothing and still be conforming      A restrict-qualified pointer  or reference                   is basically a   promise to the compiler that for the        scope of the pointer  the target of the pointer will only        be accessed through that pointer  and pointers copied        from it     In C   compilers that support it it should probably behave the same as in C   See this SO post for details   Realistic usage of the C99    restrict    keyword   Take half an hour to skim through Ericson s paper  it s interesting and worth the time   Edit  I also found that IBM s AIX C C   compiler supports the   restrict   keyword   g   also seems to support this as the following program compiles cleanly on g      include  lt stdio h gt   int foo int     restrict   a  int     restrict   b        return  a    b     int main void        int a   1  b   1  c       c   foo  amp a   amp b        printf  c     d n   c        return 0      I also found a nice article on the use of restrict   Demystifying The Restrict Keyword  Edit2  I ran across an article which specifically discusses the use of restrict in C   programs   Load-hit-stores and the   restrict keyword  Also  Microsoft Visual C   also supports the   restrict keyword

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As others said  if means nothing as of C  14  so let s consider the   restrict   GCC extension which does the same as the C99 restrict  C99 restrict says that two pointers cannot point to overlapping memory regions  The most common usage is for function arguments  This restricts how the function can be called  but allows for more compile optimizations  If the caller does not follow the restrict contract  undefined behavior  The C99 N1256 draft 6 7 3 7  quot Type qualifiers quot  says   The intended use of the restrict qualifier  like the register storage class  is to promote optimization  and deleting all instances of the qualifier from all preprocessing translation units composing a conforming program does not change its meaning  i e   observable behavior    and 6 7 3 1  quot Formal definition of restrict quot  gives the gory details  A possible optimization The Wikipedia example is very illuminating  It clearly shows how as it allows to save one assembly instruction  Without restrict  void f int  a  int  b  int  x       a     x     b     x     Pseudo assembly  load R1    x      Load the value of x pointer load R2    a      Load the value of a pointer add R2    R1      Perform Addition set R2    a       Update the value of a pointer   Similarly for b  note that x is loaded twice    because x may point to a  a aliased by x  thus    the value of x will change when the value of a   changes  load R1    x load R2    b add R2    R1 set R2    b  With restrict  void fr int  restrict a  int  restrict b  int  restrict x    Pseudo assembly  load R1    x load R2    a add R2    R1 set R2    a   Note that x is not reloaded    because the compiler knows it is unchanged    quot load R1    x quot  is no longer needed  load R2    b add R2    R1 set R2    b  Does GCC really do it  g   4 8 Linux x86-64  g   -g -std gnu  98 -O0 -c main cpp objdump -S main o  With -O0  they are the same  With -O3  void f int  a  int  b  int  x         a     x     0    8b 02                   mov      rdx   eax    2    01 07                   add     eax   rdi       b     x     4    8b 02                   mov      rdx   eax    6    01 06                   add     eax   rsi     void fr int    restrict   a  int    restrict   b  int    restrict   x         a     x    10    8b 02                   mov      rdx   eax   12    01 07                   add     eax   rdi       b     x    14    01 06                   add     eax   rsi    For the uninitiated  the calling convention is   rdi   first parameter rsi   second parameter rdx   third parameter  GCC output was even clearer than the wiki article  4 instructions vs 3 instructions  Arrays So far we have single instruction savings  but if pointer represent arrays to be looped over  a common use case  then a bunch of instructions could be saved  as mentioned by supercat and michael  Consider for example  void f char  restrict p1  char  restrict p2  size t size         for  size t i   0  i  lt  size  i               p1 i    4           p2 i    9             Because of restrict  a smart compiler  or human   could optimize that to  memset p1  4  size   memset p2  9  size    Which is potentially much more efficient as it may be assembly optimized on a decent libc implementation  like glibc  Is it better to use std  memcpy   or std  copy   in terms to performance   possibly with SIMD instructions  Without  restrict  this optimization could not be done  e g  consider  char p1 4   char  p2    amp p1 1   f p1  p2  3    Then for version makes  p1     4  4  4  9   while the memset version makes  p1     4  9  9  9   Does GCC really do it  GCC 5 2 1 Linux x86-64 Ubuntu 15 10  gcc -g -std c99 -O0 -c main c objdump -dr main o  With -O0  both are the same  With -O3   with restrict  3f0    48 85 d2                test    rdx  rdx 3f3    74 33                   je     428  lt fr 0x38 gt  3f5    55                      push    rbp 3f6    53                      push    rbx 3f7    48 89 f5                mov     rsi  rbp 3fa    be 04 00 00 00          mov     0x4  esi 3ff    48 89 d3                mov     rdx  rbx 402    48 83 ec 08             sub     0x8  rsp 406    e8 00 00 00 00          callq  40b  lt fr 0x1b gt                          407  R X86 64 PC32      memset-0x4 40b    48 83 c4 08             add     0x8  rsp 40f    48 89 da                mov     rbx  rdx 412    48 89 ef                mov     rbp  rdi 415    5b                      pop     rbx 416    5d                      pop     rbp 417    be 09 00 00 00          mov     0x9  esi 41c    e9 00 00 00 00          jmpq   421  lt fr 0x31 gt                          41d  R X86 64 PC32      memset-0x4 421    0f 1f 80 00 00 00 00    nopl   0x0  rax  428    f3 c3                   repz retq  Two memset calls as expected   without restrict  no stdlib calls  just a 16 iteration wide loop unrolling which I do not intend to reproduce here  -    I haven t had the patience to benchmark them  but I believe that the restrict version will be faster  Strict aliasing rule The restrict keyword only affects pointers of compatible types  e g  two int   because the strict aliasing rules says that aliasing incompatible types is undefined behavior by default  and so compilers can assume it does not happen and optimize away  See  What is the strict aliasing rule  Does it work for references  According to the GCC docs it does  https   gcc gnu org onlinedocs gcc-5 1 0 gcc Restricted-Pointers html with syntax  int  amp   restrict   rref  There is even a version for this of member functions  void T  fn      restrict

[c++] What does the restrict keyword mean in C++?

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