https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93802
Segher Boessenkool <segher at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |segher at gcc dot gnu.org Ever confirmed|0 |1 Status|UNCONFIRMED |NEW Last reconfirmed| |2020-04-30 --- Comment #1 from Segher Boessenkool <segher at gcc dot gnu.org> --- Confirmed. Needs -m32. There are complications from parameter passing. Making x a global doesn't improve things: the zero-extend becomes part of the memory access, which prevents us from optimising it well later. Adding something like x *= 42; as extra first line doesn't work, either: Failed to match this instruction: (set (reg:SI 124) (ior:SI (and:SI (reg:SI 123) (const_int 65534 [0xfffe])) (ashift:SI (reg:SI 123) (const_int 16 [0x10])))) (Note combine has figured out the low bit is always zero, How helpful ;-) ) But with x *= 421; we get f: mulli 3,3,421 # 6 [c=8 l=4] mulsi3/1 rlwimi 3,3,16,0,31-16 # 14 [c=4 l=4] *rotlsi3_insert_3 blr # 22 [c=4 l=4] simple_return which is exactly what we wanted. Both the original case and the "even" case should work better if we had better known bits tracking (the "even" one could maybe be made to work already, even, in target code).