https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82621
--- Comment #8 from Segher Boessenkool <segher at gcc dot gnu.org> --- Author: segher Date: Fri Nov 24 17:00:57 2017 New Revision: 255142 URL: https://gcc.gnu.org/viewcvs?rev=255142&root=gcc&view=rev Log: combine: Don't split insns if half is unused (PR82621) If we have a PARALLEL of two SETs, and one half is unused, we currently happily split that into two instructions (albeit the unused one is useless). Worse, as PR82621 shows, combine will happily merge this insn into I3 even if some intervening insn sets the same register again, which is wrong. This fixes it by not splitting PARALLELs with REG_UNUSED notes. It all is handled fine by combine in that case; just the "single set that is unused" case isn't handled properly. This also results in better code: combine will now actually throw away the unused SET. (It still won't do that in an I3). PR rtl-optimization/82621 * combine.c (try_combine): Do not split PARALLELs of two SETs if the dest of one of those SETs is unused. Modified: branches/gcc-7-branch/gcc/ChangeLog branches/gcc-7-branch/gcc/combine.c
