merge bit fields across anonymous structs in C

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.

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