From 9bfa5ab3d1982a7cef60ee00b935f4ddc89fc98e Mon Sep 17 00:00:00 2001 From: Philip Reames Date: Wed, 6 Nov 2019 12:36:28 -0800 Subject: [LoopPred] Fix two subtle issues found by inspection This patch fixes two issues noticed by inspection when going to enable the loop predication code in IndVarSimplify. Issue 1 - Both the LoopPredication transform, and the already on by default optimizeLoopExits transform, modify the exit count of the exits they modify. (either to 0 or Infinity) Looking at the code more closely, this was not reflected into SCEV and we were instead running later transforms with incorrect SCEVs. Fixing this requires forgetting the loop, weakening a too strong assert, and updating SCEV to not pessimize results when a loop is provable untaken. I haven't been able to find a test case to demonstrate the miscompile. Issue 2 - For modules without a data layout, we can end up with unsized pointer typed exit counts. Just bail out of this case. I think these are the last two issues which need addressed before we enable this by default. The code has already survived a decent amount of fuzzing without revealing either of the above. Differential Revision: https://reviews.llvm.org/D69695 --- llvm/lib/Analysis/ScalarEvolution.cpp | 11 +++++++++++ 1 file changed, 11 insertions(+) (limited to 'llvm/lib/Analysis/ScalarEvolution.cpp') diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp index e4295f64dca..ebdaf4eda07 100644 --- a/llvm/lib/Analysis/ScalarEvolution.cpp +++ b/llvm/lib/Analysis/ScalarEvolution.cpp @@ -7075,6 +7075,17 @@ ScalarEvolution::computeBackedgeTakenCount(const Loop *L, // Do a union of all the predicates here. for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) { BasicBlock *ExitBB = ExitingBlocks[i]; + + // We canonicalize untaken exits to br (constant), ignore them so that + // proving an exit untaken doesn't negatively impact our ability to reason + // about the loop as whole. + if (auto *BI = dyn_cast(ExitBB->getTerminator())) + if (auto *CI = dyn_cast(BI->getCondition())) { + bool ExitIfTrue = !L->contains(BI->getSuccessor(0)); + if ((ExitIfTrue && CI->isZero()) || (!ExitIfTrue && CI->isOne())) + continue; + } + ExitLimit EL = computeExitLimit(L, ExitBB, AllowPredicates); assert((AllowPredicates || EL.Predicates.empty()) && -- cgit v1.2.3