不同编译器中 C++ 和 C 之间的无符号位域整数表达式截断不一致

Question

编辑2：

当以前驻留在 C++ 源文件中但逐字移入 C 文件的函数开始返回不正确的结果时，我正在调试一个奇怪的测试失败。下面的 MVE 允许重现 GCC 的问题。然而，当我一时兴起用 Clang（后来用 VS）编译这个例子时，我得到了不同的结果！我不知道是将其视为其中一个编译器中的错误，还是将其视为 C 或 C++ 标准允许的未定义结果的表现。奇怪的是，没有一个编译器给我任何关于表达式的警告。

罪魁祸首是这个表达式：

ctl.b.p52 << 12;

在这里，p52输入为uint64_t; 它也是工会的一部分（见control_t下文）。移位操作不会丢失任何数据，因为结果仍然适合 64 位。但是，如果我使用 C 编译器，那么 GCC 决定将结果截断为 52 位！使用 C++ 编译器，所有 64 位结果都被保留。

为了说明这一点，下面的示例程序编译了两个具有相同主体的函数，然后比较它们的结果。c_behavior()放在 C 源文件和cpp_behavior()C++ 文件中，并main()进行比较。

带有示例代码的存储库：https : //github.com/grigory-rechistov/c-cpp-bitfields

头文件 common.h 定义了 64 位宽位域和整数的联合，并声明了两个函数：

#ifndef COMMON_H
#define COMMON_H

#include <stdint.h>

typedef union control {
        uint64_t q;
        struct {
                uint64_t a: 1;
                uint64_t b: 1;
                uint64_t c: 1;
                uint64_t d: 1;
                uint64_t e: 1;
                uint64_t f: 1;
                uint64_t g: 4;
                uint64_t h: 1;
                uint64_t i: 1;
                uint64_t p52: 52;
        } b;
} control_t;

#ifdef __cplusplus
extern "C" {
#endif

uint64_t cpp_behavior(control_t ctl);
uint64_t c_behavior(control_t ctl);

#ifdef __cplusplus
}
#endif

#endif // COMMON_H

这些函数具有相同的主体，只是一个被视为 C，另一个被视为 C++。

c-part.c：

#include <stdint.h>
#include "common.h"
uint64_t c_behavior(control_t ctl) {
    return ctl.b.p52 << 12;
}

cpp-part.cpp：

#include <stdint.h>
#include "common.h"
uint64_t cpp_behavior(control_t ctl) {
    return ctl.b.p52 << 12;
}

主文件：

#include <stdio.h>
#include "common.h"

int main() {
    control_t ctl;
    ctl.q = 0xfffffffd80236000ull;

    uint64_t c_res = c_behavior(ctl);
    uint64_t cpp_res = cpp_behavior(ctl);
    const char *announce = c_res == cpp_res? "C == C++" : "OMG C != C++";
    printf("%s\n", announce);

    return c_res == cpp_res? 0: 1;
}

GCC 显示了它们返回的结果之间的差异：

$ gcc -Wpedantic main.c c-part.c cpp-part.cpp

$ ./a.exe
OMG C != C++

但是，使用 Clang C 和 C++ 的行为与预期相同：

$ clang -Wpedantic main.c c-part.c cpp-part.cpp

$ ./a.exe
C == C++

使用 Visual Studio，我得到与使用 Clang 相同的结果：

C:\Users\user\Documents>cl main.c c-part.c cpp-part.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.00.24234.1 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

main.c
c-part.c
Generating Code...
Compiling...
cpp-part.cpp
Generating Code...
Microsoft (R) Incremental Linker Version 14.00.24234.1
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:main.exe
main.obj
c-part.obj
cpp-part.obj

C:\Users\user\Documents>main.exe
C == C++

我在 Windows 上尝试了这些示例，尽管 GCC 的原始问题是在 Linux 上发现的。

Answer 1

C and C++ treat the types of bit-field members differently.

C 2018 6.7.2.1 10 says:

A bit-field is interpreted as having a signed or unsigned integer type consisting of the specified number of bits…

Observe this is not specific about the type—it is some integer type—and it does not say the type is the type that was used to declare the bit-field, as in the uint64_t a : 1; shown in the question. This apparently leaves it open to the implementation to choose the type.

C++ 2017 draft n4659 12.2.4 [class.bit] 1 says, of a bit-field declaration:

… The bit-field attribute is not part of the type of the class member…

This implies that, in a declaration such as uint64_t a : 1;, the : 1 is not part of the type of the class member a, so the type is as if it were uint64_t a;, and thus the type of a is uint64_t.

So it appears GCC treats a bit-field in C as some integer type 32-bits or narrower if it fits and a bit-field in C++ as its declared type, and this does not appear to violate the standards.

Answer 2

Andrew Henle 建议对 C 标准进行严格的解释：位域的类型是具有完全指定宽度的有符号或无符号整数类型。

这是一个支持这种解释的测试：使用 C1x_Generic()结构，我试图确定不同宽度的位域的类型。我必须用类型定义它们long long int以避免在使用 clang 编译时出现警告。

这是来源：

#include <stdint.h>
#include <stdio.h>

#define typeof(X)  _Generic((X),                         \
                       long double: "long double",       \
                       double: "double",                 \
                       float: "float",                   \
                       unsigned long long int: "unsigned long long int",  \
                       long long int: "long long int",   \
                       unsigned long int: "unsigned long int",  \
                       long int: "long int",             \
                       unsigned int: "unsigned int",     \
                       int: "int",                       \
                       unsigned short: "unsigned short", \
                       short: "short",                   \
                       unsigned char: "unsigned char",   \
                       signed char: "signed char",       \
                       char: "char",                     \
                       _Bool: "_Bool",                   \
                       __int128_t: "__int128_t",         \
                       __uint128_t: "__uint128_t",       \
                       default: "other")

#define stype long long int
#define utype unsigned long long int

struct s {
    stype s1 : 1;
    stype s2 : 2;
    stype s3 : 3;
    stype s4 : 4;
    stype s5 : 5;
    stype s6 : 6;
    stype s7 : 7;
    stype s8 : 8;
    stype s9 : 9;
    stype s10 : 10;
    stype s11 : 11;
    stype s12 : 12;
    stype s13 : 13;
    stype s14 : 14;
    stype s15 : 15;
    stype s16 : 16;
    stype s17 : 17;
    stype s18 : 18;
    stype s19 : 19;
    stype s20 : 20;
    stype s21 : 21;
    stype s22 : 22;
    stype s23 : 23;
    stype s24 : 24;
    stype s25 : 25;
    stype s26 : 26;
    stype s27 : 27;
    stype s28 : 28;
    stype s29 : 29;
    stype s30 : 30;
    stype s31 : 31;
    stype s32 : 32;
    stype s33 : 33;
    stype s34 : 34;
    stype s35 : 35;
    stype s36 : 36;
    stype s37 : 37;
    stype s38 : 38;
    stype s39 : 39;
    stype s40 : 40;
    stype s41 : 41;
    stype s42 : 42;
    stype s43 : 43;
    stype s44 : 44;
    stype s45 : 45;
    stype s46 : 46;
    stype s47 : 47;
    stype s48 : 48;
    stype s49 : 49;
    stype s50 : 50;
    stype s51 : 51;
    stype s52 : 52;
    stype s53 : 53;
    stype s54 : 54;
    stype s55 : 55;
    stype s56 : 56;
    stype s57 : 57;
    stype s58 : 58;
    stype s59 : 59;
    stype s60 : 60;
    stype s61 : 61;
    stype s62 : 62;
    stype s63 : 63;
    stype s64 : 64;

    utype u1 : 1;
    utype u2 : 2;
    utype u3 : 3;
    utype u4 : 4;
    utype u5 : 5;
    utype u6 : 6;
    utype u7 : 7;
    utype u8 : 8;
    utype u9 : 9;
    utype u10 : 10;
    utype u11 : 11;
    utype u12 : 12;
    utype u13 : 13;
    utype u14 : 14;
    utype u15 : 15;
    utype u16 : 16;
    utype u17 : 17;
    utype u18 : 18;
    utype u19 : 19;
    utype u20 : 20;
    utype u21 : 21;
    utype u22 : 22;
    utype u23 : 23;
    utype u24 : 24;
    utype u25 : 25;
    utype u26 : 26;
    utype u27 : 27;
    utype u28 : 28;
    utype u29 : 29;
    utype u30 : 30;
    utype u31 : 31;
    utype u32 : 32;
    utype u33 : 33;
    utype u34 : 34;
    utype u35 : 35;
    utype u36 : 36;
    utype u37 : 37;
    utype u38 : 38;
    utype u39 : 39;
    utype u40 : 40;
    utype u41 : 41;
    utype u42 : 42;
    utype u43 : 43;
    utype u44 : 44;
    utype u45 : 45;
    utype u46 : 46;
    utype u47 : 47;
    utype u48 : 48;
    utype u49 : 49;
    utype u50 : 50;
    utype u51 : 51;
    utype u52 : 52;
    utype u53 : 53;
    utype u54 : 54;
    utype u55 : 55;
    utype u56 : 56;
    utype u57 : 57;
    utype u58 : 58;
    utype u59 : 59;
    utype u60 : 60;
    utype u61 : 61;
    utype u62 : 62;
    utype u63 : 63;
    utype u64 : 64;
} x;

int main(void) {
#define X(v)  printf("typeof(" #v "): %s\n", typeof(v))
    X(x.s1);
    X(x.s2);
    X(x.s3);
    X(x.s4);
    X(x.s5);
    X(x.s6);
    X(x.s7);
    X(x.s8);
    X(x.s9);
    X(x.s10);
    X(x.s11);
    X(x.s12);
    X(x.s13);
    X(x.s14);
    X(x.s15);
    X(x.s16);
    X(x.s17);
    X(x.s18);
    X(x.s19);
    X(x.s20);
    X(x.s21);
    X(x.s22);
    X(x.s23);
    X(x.s24);
    X(x.s25);
    X(x.s26);
    X(x.s27);
    X(x.s28);
    X(x.s29);
    X(x.s30);
    X(x.s31);
    X(x.s32);
    X(x.s33);
    X(x.s34);
    X(x.s35);
    X(x.s36);
    X(x.s37);
    X(x.s38);
    X(x.s39);
    X(x.s40);
    X(x.s41);
    X(x.s42);
    X(x.s43);
    X(x.s44);
    X(x.s45);
    X(x.s46);
    X(x.s47);
    X(x.s48);
    X(x.s49);
    X(x.s50);
    X(x.s51);
    X(x.s52);
    X(x.s53);
    X(x.s54);
    X(x.s55);
    X(x.s56);
    X(x.s57);
    X(x.s58);
    X(x.s59);
    X(x.s60);
    X(x.s61);
    X(x.s62);
    X(x.s63);
    X(x.s64);

    X(x.u1);
    X(x.u2);
    X(x.u3);
    X(x.u4);
    X(x.u5);
    X(x.u6);
    X(x.u7);
    X(x.u8);
    X(x.u9);
    X(x.u10);
    X(x.u11);
    X(x.u12);
    X(x.u13);
    X(x.u14);
    X(x.u15);
    X(x.u16);
    X(x.u17);
    X(x.u18);
    X(x.u19);
    X(x.u20);
    X(x.u21);
    X(x.u22);
    X(x.u23);
    X(x.u24);
    X(x.u25);
    X(x.u26);
    X(x.u27);
    X(x.u28);
    X(x.u29);
    X(x.u30);
    X(x.u31);
    X(x.u32);
    X(x.u33);
    X(x.u34);
    X(x.u35);
    X(x.u36);
    X(x.u37);
    X(x.u38);
    X(x.u39);
    X(x.u40);
    X(x.u41);
    X(x.u42);
    X(x.u43);
    X(x.u44);
    X(x.u45);
    X(x.u46);
    X(x.u47);
    X(x.u48);
    X(x.u49);
    X(x.u50);
    X(x.u51);
    X(x.u52);
    X(x.u53);
    X(x.u54);
    X(x.u55);
    X(x.u56);
    X(x.u57);
    X(x.u58);
    X(x.u59);
    X(x.u60);
    X(x.u61);
    X(x.u62);
    X(x.u63);
    X(x.u64);

    return 0;
}

这是使用 64 位 clang 编译的程序输出：

typeof(x.s1): long long int
typeof(x.s2): long long int
typeof(x.s3): long long int
typeof(x.s4): long long int
typeof(x.s5): long long int
typeof(x.s6): long long int
typeof(x.s7): long long int
typeof(x.s8): long long int
typeof(x.s9): long long int
typeof(x.s10): long long int
typeof(x.s11): long long int
typeof(x.s12): long long int
typeof(x.s13): long long int
typeof(x.s14): long long int
typeof(x.s15): long long int
typeof(x.s16): long long int
typeof(x.s17): long long int
typeof(x.s18): long long int
typeof(x.s19): long long int
typeof(x.s20): long long int
typeof(x.s21): long long int
typeof(x.s22): long long int
typeof(x.s23): long long int
typeof(x.s24): long long int
typeof(x.s25): long long int
typeof(x.s26): long long int
typeof(x.s27): long long int
typeof(x.s28): long long int
typeof(x.s29): long long int
typeof(x.s30): long long int
typeof(x.s31): long long int
typeof(x.s32): long long int
typeof(x.s33): long long int
typeof(x.s34): long long int
typeof(x.s35): long long int
typeof(x.s36): long long int
typeof(x.s37): long long int
typeof(x.s38): long long int
typeof(x.s39): long long int
typeof(x.s40): long long int
typeof(x.s41): long long int
typeof(x.s42): long long int
typeof(x.s43): long long int
typeof(x.s44): long long int
typeof(x.s45): long long int
typeof(x.s46): long long int
typeof(x.s47): long long int
typeof(x.s48): long long int
typeof(x.s49): long long int
typeof(x.s50): long long int
typeof(x.s51): long long int
typeof(x.s52): long long int
typeof(x.s53): long long int
typeof(x.s54): long long int
typeof(x.s55): long long int
typeof(x.s56): long long int
typeof(x.s57): long long int
typeof(x.s58): long long int
typeof(x.s59): long long int
typeof(x.s60): long long int
typeof(x.s61): long long int
typeof(x.s62): long long int
typeof(x.s63): long long int
typeof(x.s64): long long int
typeof(x.u1): unsigned long long int
typeof(x.u2): unsigned long long int
typeof(x.u3): unsigned long long int
typeof(x.u4): unsigned long long int
typeof(x.u5): unsigned long long int
typeof(x.u6): unsigned long long int
typeof(x.u7): unsigned long long int
typeof(x.u8): unsigned long long int
typeof(x.u9): unsigned long long int
typeof(x.u10): unsigned long long int
typeof(x.u11): unsigned long long int
typeof(x.u12): unsigned long long int
typeof(x.u13): unsigned long long int
typeof(x.u14): unsigned long long int
typeof(x.u15): unsigned long long int
typeof(x.u16): unsigned long long int
typeof(x.u17): unsigned long long int
typeof(x.u18): unsigned long long int
typeof(x.u19): unsigned long long int
typeof(x.u20): unsigned long long int
typeof(x.u21): unsigned long long int
typeof(x.u22): unsigned long long int
typeof(x.u23): unsigned long long int
typeof(x.u24): unsigned long long int
typeof(x.u25): unsigned long long int
typeof(x.u26): unsigned long long int
typeof(x.u27): unsigned long long int
typeof(x.u28): unsigned long long int
typeof(x.u29): unsigned long long int
typeof(x.u30): unsigned long long int
typeof(x.u31): unsigned long long int
typeof(x.u32): unsigned long long int
typeof(x.u33): unsigned long long int
typeof(x.u34): unsigned long long int
typeof(x.u35): unsigned long long int
typeof(x.u36): unsigned long long int
typeof(x.u37): unsigned long long int
typeof(x.u38): unsigned long long int
typeof(x.u39): unsigned long long int
typeof(x.u40): unsigned long long int
typeof(x.u41): unsigned long long int
typeof(x.u42): unsigned long long int
typeof(x.u43): unsigned long long int
typeof(x.u44): unsigned long long int
typeof(x.u45): unsigned long long int
typeof(x.u45): unsigned long long int
typeof(x.u46): unsigned long long int
typeof(x.u47): unsigned long long int
typeof(x.u48): unsigned long long int
typeof(x.u49): unsigned long long int
typeof(x.u50): unsigned long long int
typeof(x.u51): unsigned long long int
typeof(x.u52): unsigned long long int
typeof(x.u53): unsigned long long int
typeof(x.u54): unsigned long long int
typeof(x.u55): unsigned long long int
typeof(x.u56): unsigned long long int
typeof(x.u57): unsigned long long int
typeof(x.u58): unsigned long long int
typeof(x.u59): unsigned long long int
typeof(x.u60): unsigned long long int
typeof(x.u61): unsigned long long int
typeof(x.u62): unsigned long long int
typeof(x.u63): unsigned long long int
typeof(x.u64): unsigned long long int

所有位域似乎都具有定义的类型，而不是特定于定义宽度的类型。

这是使用 64 位 gcc 编译的程序输出：

typestr(x.s1): other
typestr(x.s2): other
typestr(x.s3): other
typestr(x.s4): other
typestr(x.s5): other
typestr(x.s6): other
typestr(x.s7): other
typestr(x.s8): signed char
typestr(x.s9): other
typestr(x.s10): other
typestr(x.s11): other
typestr(x.s12): other
typestr(x.s13): other
typestr(x.s14): other
typestr(x.s15): other
typestr(x.s16): short
typestr(x.s17): other
typestr(x.s18): other
typestr(x.s19): other
typestr(x.s20): other
typestr(x.s21): other
typestr(x.s22): other
typestr(x.s23): other
typestr(x.s24): other
typestr(x.s25): other
typestr(x.s26): other
typestr(x.s27): other
typestr(x.s28): other
typestr(x.s29): other
typestr(x.s30): other
typestr(x.s31): other
typestr(x.s32): int
typestr(x.s33): other
typestr(x.s34): other
typestr(x.s35): other
typestr(x.s36): other
typestr(x.s37): other
typestr(x.s38): other
typestr(x.s39): other
typestr(x.s40): other
typestr(x.s41): other
typestr(x.s42): other
typestr(x.s43): other
typestr(x.s44): other
typestr(x.s45): other
typestr(x.s46): other
typestr(x.s47): other
typestr(x.s48): other
typestr(x.s49): other
typestr(x.s50): other
typestr(x.s51): other
typestr(x.s52): other
typestr(x.s53): other
typestr(x.s54): other
typestr(x.s55): other
typestr(x.s56): other
typestr(x.s57): other
typestr(x.s58): other
typestr(x.s59): other
typestr(x.s60): other
typestr(x.s61): other
typestr(x.s62): other
typestr(x.s63): other
typestr(x.s64): long long int
typestr(x.u1): other
typestr(x.u2): other
typestr(x.u3): other
typestr(x.u4): other
typestr(x.u5): other
typestr(x.u6): other
typestr(x.u7): other
typestr(x.u8): unsigned char
typestr(x.u9): other
typestr(x.u10): other
typestr(x.u11): other
typestr(x.u12): other
typestr(x.u13): other
typestr(x.u14): other
typestr(x.u15): other
typestr(x.u16): unsigned short
typestr(x.u17): other
typestr(x.u18): other
typestr(x.u19): other
typestr(x.u20): other
typestr(x.u21): other
typestr(x.u22): other
typestr(x.u23): other
typestr(x.u24): other
typestr(x.u25): other
typestr(x.u26): other
typestr(x.u27): other
typestr(x.u28): other
typestr(x.u29): other
typestr(x.u30): other
typestr(x.u31): other
typestr(x.u32): unsigned int
typestr(x.u33): other
typestr(x.u34): other
typestr(x.u35): other
typestr(x.u36): other
typestr(x.u37): other
typestr(x.u38): other
typestr(x.u39): other
typestr(x.u40): other
typestr(x.u41): other
typestr(x.u42): other
typestr(x.u43): other
typestr(x.u44): other
typestr(x.u45): other
typestr(x.u46): other
typestr(x.u47): other
typestr(x.u48): other
typestr(x.u49): other
typestr(x.u50): other
typestr(x.u51): other
typestr(x.u52): other
typestr(x.u53): other
typestr(x.u54): other
typestr(x.u55): other
typestr(x.u56): other
typestr(x.u57): other
typestr(x.u58): other
typestr(x.u59): other
typestr(x.u60): other
typestr(x.u61): other
typestr(x.u62): other
typestr(x.u63): other
typestr(x.u64): unsigned long long int

这与具有不同类型的每个宽度一致。

该表达式E1 << E2具有提升的左操作数的类型，因此任何小于的宽度INT_WIDTH都int通过整数提升提升，而大于的任何宽度INT_WIDTH单独保留。如果该宽度大于 ，则表达式的结果确实应该被截断为位域的宽度INT_WIDTH。更准确地说，它应该为无符号类型截断，它可能是为有符号类型定义的实现。

同样应当发生对E1 + E2其它的算术运算符，如果和E1或E2是位字段的宽度比的大int。宽度较小的操作数转换为宽度较大的类型，结果也具有类型类型。这种非常违反直觉的行为会导致许多意想不到的结果，这可能是人们普遍认为位字段是虚假的并且应该避免的原因。

许多编译器似乎并不遵循对 C 标准的这种解释，从当前的措辞来看，这种解释也不明显。在 C 标准的未来版本中阐明涉及位域操作数的算术运算的语义会很有用。