https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101523
--- Comment #51 from Segher Boessenkool <segher at gcc dot gnu.org> --- (In reply to Richard Biener from comment #46) > Maybe combine already knows that it just "keeps i2" rather than replacing it? It never does that. Instead, it thinks it is making a new I2, but it ends up to be exactly the same instruction. This is not a good thing to do, combine can change the whole thing back to the previous shape for example, when it feels like it (combine does not make canonical forms ever!) > When !newi2pat we seem to delete i2. Anyway, somebody more familiar with > combine should produce a good(TM) patch. Yes, the most common combinations delete I2, they combine 2->1 or 3->1 or 4->1. When this isn't possible combine tries to combine to two instructions, it has various strategies for this: the backend can do it explicitly (via a define_split), or it can break apart the expression that was the src in the one set that was the ->1 result, hoping that the two instructions it gets that way are valid insns. It tries only one way to do this, and it isn't very smart about it, just very heuristic.
