https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77499
--- Comment #2 from Segher Boessenkool <segher at gcc dot gnu.org> --- (In reply to avieira from comment #0) > PS: I am not sure I completely understand the way the last_set_value stuff > works for pseudo's in combine, but it looks to me like each instruction is > visited in a top down order per basic block again in a top-down order. Yeah. > And > each pseudo will have its 'last_set_value' according to the last time that > register was seen being set, without any regards to loop or proper dataflow > analysis. Can anyone explain to me how this doesnt go horribly wrong? It only allows a last set value when either: - It is set in the same extended basic block, but before this insn; - The register is set exactly once, and is not a function parameter. This might also be why it optimises less well now for you?
