merge bit fields across anonymous structs in C

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I have the following code :

typedef unsigned short u16;

struct S {
    struct {
        u16 a: 9;
        u16 b: 1;
        u16 c: 1;
        u16 d: 1;
    } __attribute__((packed));

    u16 e: 4;

} __attribute__((packed));

when I check sizeof(S), it returns 3. is it possible to somehow instruct gcc to merge bitfields across anonymous structs so that sizeof(S) returns 2.

anonymous-struct compiler-optimization bit-fields gcc c
1 Answers

The result you are looking for can be obtained by making the structure union instead, with two bit-fields overlapping. The bits "used" by the first bitfield will be marked as "reserved" in the second one:

union S {
    struct {
        u16 a: 9;
        u16 b: 1;
        u16 c: 1;
        u16 d: 1;
    } ;
    
    struct {
        u16 reserved: 12; // Number of bits used in the first struct
        u16 e: 4;
    };
};

Demo

user_42532290
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