GCC别名检查w /限制指针

Question

请考虑以下两个片段:

#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];
}

当我编译函数时,gcc-4.7.2 -Ofast -march=native -std=c99 -ftree-vectorizer-verbose=5 -S test.c -Wall我发现GCC插入第二个函数的别名检查.

如何防止这种情况导致生成的程序集fn1与fn0？(当参数数量从3增加到30时,参数传递方法(fn0)会变得很麻烦,并且fn1方法中的别名检查数量变得荒谬.)

组装(x86-64,支持AVX的芯片); 别名在.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

这有帮助吗？

void fn1(const float **restrict a, float *restrict b, int n)
{
    const float * restrict a0 = a[0];
    const float * restrict a1 = a[1];

    ASSUME_ALIGNED(a0); ASSUME_ALIGNED(a1); ASSUME_ALIGNED(b);

    for (int i = 0; i < n; ++i)
        b[i] = a0[i] + a1[i];
}

编辑：第二次尝试:)。信息来自http://locklessinc.com/articles/vectorize/

gcc --fast-math ...