Thanks!

Turns out struct size with bitfield is implementation defined.

The following on GCC yields different values.

#include <stdio.h>

struct S1 {
    unsigned char b1 : 1;
    //unsigned :0; // #1. start a new byte
    unsigned char b2 : 1;
};

struct S2 {
    unsigned int b1 : 1;
    unsigned int b2 : 1;
};

int main() {
    printf("%d\n", sizeof(char)); 
    printf("%d\n", sizeof(int)); 
    printf("%d\n", sizeof(struct S1)); // usually prints 1
    printf("%d\n", sizeof(struct S2)); // usually prints 4
}

Turns out struct size with bitfield is implementation defined.

Yup, this is all correctly implemented by https://github.com/Vexu/arocc and the plan is to make translate-c use it instead of Clang.

Page 14: https://www.uclibc.org/docs/psABI-x86_64.pdf says:

• bit-fields are allocated from right to left
• bit-fields must be contained in a storage unit appropriate for its declared type
• bit-fields may share a storage unit with other struct / union members
• Unnamed bit-fields’ types do not affect the alignment of a structure or union. (used to separate storage unit only)

From experimenting with GCC:

• char:1 + char:1 = 1 byte (struct S1 above)
• char:1 + int:1 = 4 bytes
• int:1 + char:1 = 4 bytes
• int:1 + int:1 = 4 bytes (struct S2 above)