在取消引用指针之前对 NULL 检查的条件移动指令

Question

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Answer 1

这是一个无分支的版本:

inline long* select(long* p, long* q) {
    uintptr_t a = (uintptr_t)p;
    uintptr_t b = (uintptr_t)q;
    uintptr_t c = !a;
    uintptr_t r = (a & (c - 1)) | (b & (!c - 1));
    return (long*)r;
}

long cond(long* p) {
    long t = 0;
    return *select(p, &t);
}

gcc-8.2 的程序集:

cond(long*):
        mov     QWORD PTR [rsp-8], 0
        xor     eax, eax
        test    rdi, rdi
        sete    al
        lea     rdx, [rax-1]
        neg     rax
        and     rdi, rdx
        lea     rdx, [rsp-8]
        and     rax, rdx
        or      rdi, rax
        mov     rax, QWORD PTR [rdi]
        ret

clang-7 的汇编:

cond(long*):                              # @cond(long*)
        mov     qword ptr [rsp - 8], 0
        xor     eax, eax
        test    rdi, rdi
        lea     rcx, [rsp - 8]
        cmovne  rcx, rax
        or      rcx, rdi
        mov     rax, qword ptr [rcx]
        ret