GCC Aliasing Checks w/Restrict pointers

Question

Consider the following two snippets:

#define ALIGN_BYTES 32 #define ASSUME_ALIGNED(x) x = __builtin_assume_aligned(x, ALIGN_BYTES)  void fn0(const float *restrict a0, const float *restrict a1,          float *restrict b, int n) {     ASSUME_ALIGNED(a0); ASSUME_ALIGNED(a1); ASSUME_ALIGNED(b);      for (int i = 0; i < n; ++i)         b[i] = a0[i] + a1[i]; }  void fn1(const float *restrict *restrict a, float *restrict b, int n) {     ASSUME_ALIGNED(a[0]); ASSUME_ALIGNED(a[1]); ASSUME_ALIGNED(b);      for (int i = 0; i < n; ++i)         b[i] = a[0][i] + a[1][i]; } 

When I compile the function as gcc-4.7.2 -Ofast -march=native -std=c99 -ftree-vectorizer-verbose=5 -S test.c -Wall I find that GCC inserts aliasing checks for the second function.

How can I prevent this such that the resulting assembly for fn1 is the same as that for fn0? (When the number of parameters increases from three to, say, 30 the argument-passing approach (fn0) becomes cumbersome and the number of aliasing checks in the fn1 approach becomes ridiculous .)

Assembly (x86-64, AVX capable chip); aliasing cruft at .LFB10

fn0: .LFB9:     .cfi_startproc     testl   %ecx, %ecx     jle .L1     movl    %ecx, %r10d     shrl    $3, %r10d     leal    0(,%r10,8), %r9d     testl   %r9d, %r9d     je  .L8     cmpl    $7, %ecx     jbe .L8     xorl    %eax, %eax     xorl    %r8d, %r8d     .p2align 4,,10     .p2align 3 .L4:     vmovaps (%rsi,%rax), %ymm0     addl    $1, %r8d     vaddps  (%rdi,%rax), %ymm0, %ymm0     vmovaps %ymm0, (%rdx,%rax)     addq    $32, %rax     cmpl    %r8d, %r10d     ja  .L4     cmpl    %r9d, %ecx     je  .L1 .L3:     movslq  %r9d, %rax     salq    $2, %rax     addq    %rax, %rdi     addq    %rax, %rsi     addq    %rax, %rdx     xorl    %eax, %eax     .p2align 4,,10     .p2align 3 .L6:     vmovss  (%rsi,%rax,4), %xmm0     vaddss  (%rdi,%rax,4), %xmm0, %xmm0     vmovss  %xmm0, (%rdx,%rax,4)     addq    $1, %rax     leal    (%r9,%rax), %r8d     cmpl    %r8d, %ecx     jg  .L6 .L1:     vzeroupper     ret .L8:     xorl    %r9d, %r9d     jmp .L3     .cfi_endproc .LFE9:     .size   fn0, .-fn0     .p2align 4,,15     .globl  fn1     .type   fn1, @function fn1: .LFB10:     .cfi_startproc     testq   %rdx, %rdx     movq    (%rdi), %r8     movq    8(%rdi), %r9     je  .L12     leaq    32(%rsi), %rdi     movq    %rdx, %r10     leaq    32(%r8), %r11     shrq    $3, %r10     cmpq    %rdi, %r8     leaq    0(,%r10,8), %rax     setae   %cl     cmpq    %r11, %rsi     setae   %r11b     orl %r11d, %ecx     cmpq    %rdi, %r9     leaq    32(%r9), %r11     setae   %dil     cmpq    %r11, %rsi     setae   %r11b     orl %r11d, %edi     andl    %edi, %ecx     cmpq    $7, %rdx     seta    %dil     testb   %dil, %cl     je  .L19     testq   %rax, %rax     je  .L19     xorl    %ecx, %ecx     xorl    %edi, %edi     .p2align 4,,10     .p2align 3 .L15:     vmovaps (%r9,%rcx), %ymm0     addq    $1, %rdi     vaddps  (%r8,%rcx), %ymm0, %ymm0     vmovaps %ymm0, (%rsi,%rcx)     addq    $32, %rcx     cmpq    %rdi, %r10     ja  .L15     cmpq    %rax, %rdx     je  .L12     .p2align 4,,10     .p2align 3 .L20:     vmovss  (%r9,%rax,4), %xmm0     vaddss  (%r8,%rax,4), %xmm0, %xmm0     vmovss  %xmm0, (%rsi,%rax,4)     addq    $1, %rax     cmpq    %rax, %rdx     ja  .L20 .L12:     vzeroupper     ret .L19:     xorl    %eax, %eax     jmp .L20     .cfi_endproc 

Answer 1

There is away to tell compiler to stop checking aliasing:

please add line:

#pragma GCC ivdep 

right in front of the loop you want to vectorize, if you need more information please read:

https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gcc/Loop-Specific-Pragmas.html