How can I get GCC to optimize this bit-shifting instruction into a move?

Question

I'm trying to use the following code to emulate a 16-bit half-float in software:

typedef struct half
{
    unsigned short mantissa:10;
    unsigned short exponent:5;
    unsigned short sign:1;
} half;

unsigned short from_half(half h)
{
    return h.mantissa | h.exponent << 10 | h.sign << 15;
}

half to_half(unsigned short s)
{
    half result = { s, s >> 10, s >> 15 };
    return result;
}

I set this up so that it could easily be optimized into a move instruction, but lo and behold, in from_half, GCC does the bit-shifting anyway (even at -O3):

from_half:
        mov     edx, edi
        mov     eax, edi
        and     di, 1023
        shr     dx, 15
        and     eax, 31744
        movzx   edx, dl
        sal     edx, 15
        or      eax, edx
        or      eax, edi
        ret

while to_half is optimized nicely:

to_half:
        mov     eax, edi
        ret

Godbolt

I've tried different optimization levels (-O1, -O2, -Os) but none optimize it into what I was hoping.

Clang does this how I would expect even at -O1:

from_half:                              # @from_half
        mov     eax, edi
        ret
to_half:                                # @to_half
        mov     eax, edi
        ret

Godbolt

How can I get GCC to optimize this into a move? Why isn't it optimized that way already?

Answer 1

In addition to Booboo's answer, you can try the following which answers your question

How can I get GCC to optimize this into a move?

Just cast each shifted bit-field expression to unsigned short

unsigned short from_half(half h)
{
    return (unsigned short)h.mantissa | (unsigned short)(h.exponent << 10) | (unsigned short)(h.sign << 15);
}

https://godbolt.org/z/CfZSgC

It results in:

from_half:
        mov     eax, edi
        ret

Why isn't it optimized that way already?

I am not sure I have a solid answer on this one. Apparently the intermediate promotion of the bit-fields to int confuses the optimizer... But this is just a guess.