Multiple inconsistent behavior of signed bit-fields

问题

I have come across a strange behavior on signed bit-fields:

#include <stdio.h>

struct S {
    long long a31 : 31;
    long long a32 : 32;
    long long a33 : 33;
    long long : 0;
    unsigned long long b31 : 31;
    unsigned long long b32 : 32;
    unsigned long long b33 : 33;
};

long long f31(struct S *p) { return p->a31 + p->b31; }
long long f32(struct S *p) { return p->a32 + p->b32; }
long long f33(struct S *p) { return p->a33 + p->b33; }

int main() {
    struct S s = { -2, -2, -2, 1, 1, 1 };
    long long a32 = -2;
    unsigned long long b32 = 1;
    printf("f31(&s)       => %lld\n", f31(&s));
    printf("f32(&s)       => %lld\n", f32(&s));
    printf("f33(&s)       => %lld\n", f33(&s));
    printf("s.a31 + s.b31 => %lld\n", s.a31 + s.b31);
    printf("s.a32 + s.b32 => %lld\n", s.a32 + s.b32);
    printf("s.a33 + s.b33 => %lld\n", s.a33 + s.b33);
    printf("  a32 +   b32 => %lld\n",   a32 +   b32);
    return 0;
}

Using Clang on OS/X, I get this output:

f31(&s)       => -1
f32(&s)       => 4294967295
f33(&s)       => -1
s.a31 + s.b31 => 4294967295
s.a32 + s.b32 => 4294967295
s.a33 + s.b33 => -1
  a32 +   b32 => -1

Using GCC on Linux, I get this:

f31(&s)       => -1
f32(&s)       => 4294967295
f33(&s)       => 8589934591
s.a31 + s.b31 => 4294967295
s.a32 + s.b32 => 4294967295
s.a33 + s.b33 => 8589934591
  a32 +   b32 => -1

The above output shows 3 types of inconsistencies:

• different behavior for different compilers;
• different behavior for different bit-field widths;
• different behavior for inline expressions and equivalent expressions wrapped in a function.

The C Standard has this language:

6.7.2 Type specifiers

...

Each of the comma-separated multisets designates the same type, except that for bit-fields, it is implementation-defined whether the specifier int designates the same type as signed int or the same type as unsigned int.

Bit-fields are notoriously broken in many older compilers...
Is the behavior of Clang and GCC conformant or are these inconsistencies the result of one or more bugs?

来源：https://stackoverflow.com/questions/58846584/multiple-inconsistent-behavior-of-signed-bit-fields