------- Comment #4 from hjl dot tools at gmail dot com 2008-09-19 18:03 ------- (In reply to comment #1) > Root cause is that instruction length of fused jcc is set to 16, which prevent > the block from merging and copying. For some reason Core2 runs poorly with a > unmerged branch block under certain circonstances. >
I looked at the generated code without TARGET_FUSE_CMP_AND_BRANCH. In most cases, gcc doesn't put any instructions between TEST/CMP and JCC and we get macro-fusion optimization automatically even when TARGET_FUSE_CMP_AND_BRANCH is off. Since TARGET_FUSE_CMP_AND_BRANCH generates patterns with incorrect instruction length, it prevents the block from merging and copying, which hurt performance. We have 2 choices: 1. Correct insn length for *jcc_fused_X patterns, which what Joey's patch does. 2. Remove *jcc_fused_X patterns and optimize macro-fusion in Core 2 scheduling. I think we should do #2. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37571
