布尔值在编译器中为 8 位.对它们的操作效率低下吗?

可能的优化指南:将来自外部源(函数参数/内存)的 bool 与按位运算符(如 a&b)结合起来.MSVC 和 ICC 在这方面做得更好.IDK 如果本地 bool 更糟.请注意，对于 bool，a&b 仅等效于 a&&b，而不是整数类型.<代码>2 &&1 为真，但 2 &1 是 0 是假的.按位 OR 没有这个问题.

Possible optimization guideline: combine bools from external sources (function args / memory) with bitwise operators, like a&b. MSVC and ICC do better with this. IDK if it's ever worse for local bools. Beware that a&b is only equivalent to a&&b for bool, not integer types. 2 && 1 is true, but 2 & 1 is 0 which is false. Bitwise OR doesn't have this problem.

IDK，如果此指南会对通过函数内的比较(或内联的东西)设置的局部变量造成伤害.例如.它可能会导致编译器实际生成整数布尔值，而不是在可能的情况下直接使用比较结果.另请注意，它似乎对当前的 gcc 和 clang 没有帮助.

IDK if this guideline will ever hurt for locals that were set from a comparison within the function (or in something that inlined). E.g. it might lead the compiler to actually make integer booleans instead of just using comparison results directly when possible. Also note that it doesn't seem to help with current gcc and clang.

是的，x86 上的 C++ 实现将 bool 存储在一个始终为 0 或 1 的字节中(至少在编译器必须遵守 ABI/调用约定的函数调用边界中需要这样做.)

Yes, C++ implementations on x86 store bool in a byte that's always 0 or 1 (at least across function-call boundaries where the compiler has to respect the ABI / calling convention which requires this.)

编译器有时会利用这一点，例如对于 bool->int 转换，即使是 gcc 4.4 也只是零扩展到 32 位(movzx eax, dil).Clang 和 MSVC 也这样做.C 和 C++ 规则要求这种转换产生 0 或 1，因此这种行为只有在总是安全地假设 bool 函数 arg 或全局变量具有 0 时才是安全的或 1 个值.

Compilers do sometimes take advantage of this, e.g. for bool->int conversion even gcc 4.4 simply zero-extends to 32-bit (movzx eax, dil). Clang and MSVC do this, too. C and C++ rules require this conversion to produce 0 or 1, so this behaviour is only safe if it's always safe to assume that a bool function arg or global variable has a 0 or 1 value.

即使是旧的编译器通常也会在 bool->int 中利用它，但在其他情况下不会.因此，Agner 说的原因是错误的:

Even old compilers typically did take advantage of it for bool->int, but not in other cases. Thus, Agner is wrong about the reason when he says:

编译器之所以不做这样的假设是因为如果变量未初始化或来自未知来源，它们可能具有其他值.

The reason why the compiler doesn't make such an assumption is that the variables might have other values if they are uninitialized or come from unknown sources.

MSVC CL19 生成的代码假定 bool 函数参数为 0 或 1，因此 Windows x86-64 ABI 必须保证这一点.

MSVC CL19 does make code that assumes bool function args are 0 or 1, so the Windows x86-64 ABI must guarantee this.

在 x86-64 System V ABI(用于除 Windows 之外的所有内容)，修订版 0.98 的更改日志说指定 _Bool(又名 bool)在调用者处被布尔化."我认为甚至在这种变化之前，编译器就已经假设了它，但这只是记录了编译器已经依赖的东西.x86-64 SysV ABI 中的当前语言是:

In the x86-64 System V ABI (used by everything other than Windows), the changelog for revision 0.98 says "Specify that _Bool (aka bool) is booleanized at the caller." I think even before that change, compilers were assuming it, but this just documents what compilers were already relying on. The current language in the x86-64 SysV ABI is:

3.1.2 数据表示

布尔值，当存储在内存对象中时，存储为单字节对象，其值始终为 0(假)或 1(真).当存储在整数寄存器中时(除了作为参数传递)，寄存器的所有 8 个字节都是有效的；任何非零值都被认为是真的.

Booleans, when stored in a memory object, are stored as single byte objects the value of which is always 0 (false) or 1 (true). When stored in integer registers (except for passing as arguments), all 8 bytes of the register are significant; any nonzero value is considered true.

第二句是无稽之谈:ABI 没有告诉编译器如何在函数内的寄存器中存储东西，只在不同编译单元(内存/函数参数和返回值)之间的边界处.我不久前在维护它的 github 页面上报告了这个 ABI 缺陷.

The second sentence is nonsense: the ABI has no business telling compilers how to store things in registers inside a function, only at boundaries between different compilation units (memory / function args and return values). I reported this ABI defect a while ago on the github page where it's maintained.

3.2.3 参数传递:

当_Bool 类型的值在寄存器或堆栈中返回或传递时，位 0 包含真值，位 1 到 7 应为零¹⁶.

When a value of type _Bool is returned or passed in a register or on the stack, bit 0 contains the truth value and bits 1 to 7 shall be zero¹⁶.

(脚注 16):其他位未指定，因此这些值的消费者方面可以依赖它在截断为 8 位时是 0 或 1.

(footnote 16): Other bits are left unspecified, hence the consumer side of those values can rely on it being 0 or 1 when truncated to 8 bit.

i386 System V ABI 中的语言是相同的，IIRC.

The language in the i386 System V ABI is the same, IIRC.

任何为一件事假定 0/1(例如转换为 int)但在其他情况下未能利用它的编译器都会错过优化.不幸的是，这种错过的优化仍然存在，尽管它们比 Agner 写那段关于编译器总是重新布尔化的情况少得多.

Any compiler that assumes 0/1 for one thing (e.g. conversion to int) but fails to take advantage of it in other cases has a missed optimization. Unfortunately such missed-optimizations still exist, although they are rarer than when Agner wrote that paragraph about compilers always re-booleanizing.

(在 Godbolt 编译器资源管理器，用于 gcc4.6/4.7 和 cl昂/MSVC.另请参阅 Matt Godbolt 的 CppCon2017 演讲 我的编译器最近为我做了什么?打开编译器的盖子)

bool logical_or(bool a, bool b) { return a||b; }

 # gcc4.6.4 -O3 for the x86-64 System V ABI
    test    dil, dil            # test a against itself (for non-zero)
    mov     eax, 1
    cmove   eax, esi            # return   a ? 1 : b;
    ret

因此，即使 gcc4.6 也没有重新布尔化 b，但它确实错过了 gcc4.7 所做的优化:(以及其他答案中显示的 clang 和更高版本的编译器):

So even gcc4.6 didn't re-booleanize b, but it did miss the optimization that gcc4.7 makes: (and clang and later compilers as shown in other answers):

    # gcc4.7 -O3 to present: looks ideal to me.
    mov     eax, esi
    or      eax, edi
    ret

(Clang 的 或 dil, sil/mov eax, edi 很愚蠢:在阅读 时，它肯定会导致 Nehalem 或更早的 Intel 上的部分寄存器停顿edi 在编写 dil 之后，由于需要一个 REX 前缀来使用 edi 的低 8 部分，它的代码大小更糟.更好的选择可能是 或 dil,sil/movzx eax, dil 如果您想避免读取任何 32 位寄存器，以防您的调用者留下一些带有脏"部分寄存器的 arg-passing 寄存器.)

(Clang's or dil, sil / mov eax, edi is silly: it's guaranteed to cause a partial-register stall on Nehalem or earlier Intel when reading edi after writing dil, and it has worse code size from needing a REX prefix to use the low-8 part of edi. A better choice might be or dil,sil / movzx eax, dil if you want to avoid reading any 32-bit registers in case your caller left some arg-passing registers with "dirty" partial registers.)

MSVC 发出这段代码，分别检查 a 和 b，完全没有利用任何东西，甚至使用 xoral,al 而不是 xor eax,eax.因此，它对大多数 CPU 上的 eax 旧值具有错误的依赖性(包括 Haswell/Skylake，它们不会将低 8 部分 regs 与整个寄存器分开重命名，只有 AH/BH/...).这只是愚蠢的.使用 xor al,al 的唯一原因是您明确希望保留高位字节.

MSVC emits this code that checks a then b separately, completely failing to take advantage of anything, and even using xor al,al instead of xor eax,eax. So it has a false dependency on the old value of eax on most CPUs (including Haswell/Skylake, which don't rename low-8 partial regs separately from the whole register, only AH/BH/...). This is just dumb. The only reason to ever use xor al,al is when you explicitly want to preserve the upper bytes.

logical_or PROC                     ; x86-64 MSVC CL19
    test     cl, cl                 ; Windows ABI passes args in ecx, edx
    jne      SHORT $LN3@logical_or
    test     dl, dl
    jne      SHORT $LN3@logical_or
    xor      al, al                 ; missed peephole: xor eax,eax is strictly better
    ret      0
$LN3@logical_or:
    mov      al, 1
    ret      0
logical_or ENDP

ICC18 也没有利用输入的已知 0/1 性质，它只是使用 or 指令根据两个输入的按位或来设置标志，并且 setcc 产生一个 0/​​1.

ICC18 also doesn't take advantage of the known 0/1 nature of the inputs, it just uses an or instruction to set flags according to the bitwise OR of the two inputs, and setcc to produce a 0/1.

logical_or(bool, bool):             # ICC18
    xor       eax, eax                                      #4.42
    movzx     edi, dil                                      #4.33
    movzx     esi, sil                                      #4.33
    or        edi, esi                                      #4.42
    setne     al                                            #4.42
    ret                                                     #4.42

ICC 发出相同的代码，即使对于 bool bitwise_or(bool a, bool b) { return a|b;}.它提升为 int(使用 movzx)，并使用 or 根据按位 OR 设置标志.与 或 dil,sil/setne al 相比，这是愚蠢的.

ICC emits the same code even for bool bitwise_or(bool a, bool b) { return a|b; }. It promotes to int (with movzx), and uses or to set flags according to the bitwise OR. This is dumb compared to or dil,sil / setne al.

对于bitwise_or，MSVC 只使用or 指令(在每个输入的movzx 之后)，但无论如何都不会重新布尔化.

For bitwise_or, MSVC does just use an or instruction (after movzx on each input), but anyway doesn't re-booleanize.

只有 ICC/MSVC 用上面的简单函数制作了愚蠢的代码，但是这个函数仍然给 gcc 和 clang 带来麻烦:

Only ICC/MSVC were making dumb code with the simple function above, but this function still gives gcc and clang trouble:

int select(bool a, bool b, int x, int y) {
    return (a&&b) ? x : y;
}