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39f379d037
This patch just uses a peephole test for "add; compare; branch" sequences within a single block. The IR optimizers already convert loops to decrement-and-branch-on-nonzero form in some cases, so even this simplistic test triggers many times during a clang bootstrap and projects/test-suite run. It looks like there are still cases where we need to more strongly prefer branches on nonzero though. E.g. I saw a case where a loop that started out with a check for 0 ended up with a check for -1. I'll try to look at that sometime. I ended up adding the Reference class because MachineInstr::readsRegister() doesn't check for subregisters (by design, as far as I could tell). llvm-svn: 187723