forked from OSchip/llvm-project
c7cc87922e
In r312664 (D36404), JumpThreading stopped threading edges into loop headers. Unfortunately, I observed a significant performance regression as a result of this change. Upon further investigation, the problematic pattern looked something like this (after many high level optimizations): while (true) { bool cond = ...; if (!cond) { <body> } if (cond) break; } Now, naturally we want jump threading to essentially eliminate the second if check and hook up the edges appropriately. However, the above mentioned change, prevented it from doing this because it would have to thread an edge into the loop header. Upon further investigation, what is happening is that since both branches are threadable, JumpThreading picks one of them at arbitrarily. In my case, because of the way that the IR ended up, it tended to pick the one to the loop header, bailing out immediately after. However, if it had picked the one to the exit block, everything would have worked out fine (because the only remaining branch would then be folded, not thraded which is acceptable). Thus, to fix this problem, we can simply eliminate loop headers from consideration as possible threading targets earlier, to make sure that if there are multiple eligible branches, we can still thread one of the ones that don't target a loop header. Patch by Keno Fischer! Differential Revision: https://reviews.llvm.org/D42260 llvm-svn: 328798 |
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AggressiveInstCombine | ||
Coroutines | ||
Hello | ||
IPO | ||
InstCombine | ||
Instrumentation | ||
ObjCARC | ||
Scalar | ||
Utils | ||
Vectorize | ||
CMakeLists.txt | ||
LLVMBuild.txt |