https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105231
--- Comment #26 from Segher Boessenkool <segher at gcc dot gnu.org> --- (In reply to Richard Biener from comment #25) > (In reply to Segher Boessenkool from comment #24) > > Wrt keeping REG_EQUAL notes... If you want to keep them you need to make > > sure > > they still are valid. GCC keeps those on i3, it is much too hard in general > > to > > validate other such notes. > > It seems to be that the REG_EQUAL notes of the SET dests in the final > instruction we combine into, so 24 for > > Trying 78, 17 -> 24: > > are better always valid even after the combination - at least when they > are not refering to pseudos set by insns combined (78, 17 here) and always > when the REG_EQUAL is constant. When there are multiple sets in the final > insn (24) then which note ends up where is determined by the SET dests. In the *resulting* final insn. And some of those notes may have to be moved to i2 -- when there are multiple sets in the *resulting* insn, and it is split. > For this particular case preserving _constant_ REG_EQUAL notes would be > important. And for this case only handling single-set would be good enough. You can *never* require REG_EQUAL notes for correctness. It is always correct to delete REG_EQUAL notes. They exist to make optimisation easier, and for nothing else. > Of course if i3 is not always '24' (in this case) then we have to find > a better place to preserve these than distribute_notes. Or fix the bug: that which requires REG_EQUAL notes to not be deleted. This is a problem whether or not we hide the problem here (by making things optimised better).
