使gcc使用条件移动 [英] Make gcc use conditional moves

问题描述

是否存在gcc编译指示或可用于强制gcc在特定代码段上生成无分支指令的东西?

Is there a gcc pragma or something I can use to force gcc to generate branch-free instructions on a specific section of code?

我有一段代码，我希望gcc使用cmov指令将其编译为无分支代码:

I have a piece of code that I want gcc to compile to branch-free code using cmov instructions:

int foo(int *a, int n, int x) {
    int i = 0, j = n;

    while (i < n) {
#ifdef PREFETCH
        __builtin_prefetch(a+16*i + 15);
#endif /* PREFETCH */
        j = (x <= a[i]) ? i : j;
        i = (x <= a[i]) ? 2*i + 1 : 2*i + 2;
    }
    return j;
}

并且确实如此:

morin@soprano$ gcc -O4 -S -c test.c -o -    
    .file   "test.c"
    .text
    .p2align 4,,15
    .globl  foo
    .type   foo, @function
foo:
.LFB0:
    .cfi_startproc
    testl   %esi, %esi
    movl    %esi, %eax
    jle .L2
    xorl    %r8d, %r8d
    jmp .L3
    .p2align 4,,10
    .p2align 3
.L6:
    movl    %ecx, %r8d
.L3:
    movslq  %r8d, %rcx
    movl    (%rdi,%rcx,4), %r9d
    leal    (%r8,%r8), %ecx      # put 2*i in ecx
    leal    1(%rcx), %r10d       # put 2*i+1 in r10d
    addl    $2, %ecx             # put 2*i+2 in ecx
    cmpl    %edx, %r9d
    cmovge  %r10d, %ecx          # put 2*i+1 in ecx if appropriate
    cmovge  %r8d, %eax           # set j = i if appropriate
    cmpl    %esi, %ecx
    jl  .L6
.L2:
    rep ret
    .cfi_endproc
.LFE0:
    .size   foo, .-foo
    .ident  "GCC: (Ubuntu 4.8.2-19ubuntu1) 4.8.2"
    .section    .note.GNU-stack,"",@progbits

(是的，我意识到循环是一个分支，但是我在谈论循环内的选择运算符.)

(Yes, I realize the loop is a branch, but I'm talking about the choice operators inside the loop.)

不幸的是，当我启用 __ builtin_prefetch 调用时，gcc会生成分支代码:

Unfortunately, when I enable the __builtin_prefetch call, gcc generates branchy code:

morin@soprano$ gcc -DPREFETCH -O4 -S -c test.c -o -
    .file   "test.c"
    .text
    .p2align 4,,15
    .globl  foo
    .type   foo, @function
foo:
.LFB0:
    .cfi_startproc
    testl   %esi, %esi
    movl    %esi, %eax
    jle .L7
    xorl    %ecx, %ecx
    jmp .L5
    .p2align 4,,10
    .p2align 3
.L3:
    movl    %ecx, %eax           # this is the x <= a[i] branch
    leal    1(%rcx,%rcx), %ecx
    cmpl    %esi, %ecx
    jge .L11
.L5:
    movl    %ecx, %r8d           # this is the main branch
    sall    $4, %r8d             # setup the prefetch
    movslq  %r8d, %r8            # setup the prefetch
    prefetcht0  60(%rdi,%r8,4)   # do the prefetch
    movslq  %ecx, %r8
    cmpl    %edx, (%rdi,%r8,4)   # compare x with a[i]
    jge .L3
    leal    2(%rcx,%rcx), %ecx   # this is the x > a[i] branch
    cmpl    %esi, %ecx
    jl  .L5
.L11:
    rep ret
.L7:
    .p2align 4,,5
    rep ret
    .cfi_endproc
.LFE0:
    .size   foo, .-foo
    .ident  "GCC: (Ubuntu 4.8.2-19ubuntu1) 4.8.2"
    .section    .note.GNU-stack,"",@progbits

我已经尝试在此功能上使用 __ attribute __((optimize("if-conversion2")))，但这无效.

I've tried using __attribute__((optimize("if-conversion2"))) on this function, but that has no effect.

我非常关心的原因是，我手工编辑了由编译器生成的无分支代码(来自第一个示例)，以包含prefetcht0指令，并且它的运行速度比gcc生成的两个版本都快得多.

The reason I care so much is that I haved hand-edited compiler-generated branch-free code (from the first example) to include the prefetcht0 instructions and it runs considerably faster than both of the versions gcc produces.

推荐答案

看起来gcc可能难以在循环条件和后置条件中使用的变量上生成无分支代码，并且保持临时寄存器有效跨伪函数内部调用.

It looks like gcc might have troubles to generate branch-free code on variables used in loop conditions and post-conditions, together with the constraints of keeping temporary registers alive across a pseudo-function intrinsic call.

有些可疑之处，使用-funroll-all-loops和-fguess-branch-probability时，函数生成的代码不同.我生成许多退货说明.闻起来像gcc中的一个小错误，编译器的rtl遍历或简化的代码块.

There is something suspicious, the generated code from your function is different when using -funroll-all-loops and -fguess-branch-probability. I generates many return instructions. It smells like a little bug in gcc, around the rtl pass of the compiler, or simplifications of blocks of codes.

下面的代码在两种情况下都是无分支的.这是将错误提交给GCC的好理由.在-O3级别，GCC应该始终生成相同的代码.

The following code is branch-less in both cases. This would be a good reason to submit a bug to GCC. At level -O3, GCC should always generate the same code.

int foo( int *a,  int n,  int x) {
     int c, i = 0, j = n;

    while (i < n) {
#ifdef PREFETCH
        __builtin_prefetch(a+16*i + 15);
#endif /* PREFETCH */
        c = (x > a[i]);
        j = c ? j : i;
        i = 2*i + 1 + c;
    }
    return j;
}

生成此

        .cfi_startproc
        testl   %esi, %esi
        movl    %esi, %eax
        jle     .L4
        xorl    %ecx, %ecx
        .p2align 4,,10
        .p2align 3
.L3:
        movslq  %ecx, %r8
        cmpl    %edx, (%rdi,%r8,4)
        setl    %r8b
        cmovge  %ecx, %eax
        movzbl  %r8b, %r8d
        leal    1(%r8,%rcx,2), %ecx
        cmpl    %ecx, %esi
        jg      .L3
.L4:
        rep ret
        .cfi_endproc

还有这个

        .cfi_startproc
        testl   %esi, %esi
        movl    %esi, %eax
        jle     .L5
        xorl    %ecx, %ecx
        .p2align 4,,10
        .p2align 3
.L4:
        movl    %ecx, %r8d
        sall    $4, %r8d
        movslq  %r8d, %r8
        prefetcht0      60(%rdi,%r8,4)
        movslq  %ecx, %r8
        cmpl    %edx, (%rdi,%r8,4)
        setl    %r8b
        testb   %r8b, %r8b
        movzbl  %r8b, %r9d
        cmove   %ecx, %eax
        leal    1(%r9,%rcx,2), %ecx
        cmpl    %ecx, %esi
        jg      .L4
.L5:
        rep ret
        .cfi_endproc

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