Answer by Alex Lop. for How can I get GCC to optimize this bit-shifting instruction into a move?

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.