https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62103
--- Comment #10 from Thomas Preud'homme <thopre01 at gcc dot gnu.org> --- (In reply to Jiong Wang from comment #8) > looks like this fix is too conservative. it will disable const fold for > bit-field completely. for bitfld-6/little-endian, previously, we can > generated > > main: > mov w0, 0 > ret > > while after this patch, we always load from memory. any specific reason for > this? > > shouldn't we fix the code to let big-endian get correct folded constant? We only disable const folding for initialization of bitfields in a union. The rationale was that the fix was much simpler (and could thus be backported) and accessing a bitfield in a union that was initialized with a constant rare enough that it wasn't worth the extra effort anyway. If this latter assumption proves to be wrong I have the beginning of a patch that make double_int::from_buffer and wi::from_buffer work at the bit level. It would need to be extended to do the same to ::to_buffer.
