From 4673f1056820347dfae0475a4005744b9756a0b5 Mon Sep 17 00:00:00 2001 From: Hyojin Sung Date: Tue, 29 Mar 2016 04:08:57 +0000 Subject: [SimlifyCFG] Prevent passes from destroying canonical loop structure, especially for nested loops When eliminating or merging almost empty basic blocks, the existence of non-trivial PHI nodes is currently used to recognize potential loops of which the block is the header and keep the block. However, the current algorithm fails if the loops' exit condition is evaluated only with volatile values hence no PHI nodes in the header. Especially when such a loop is an outer loop of a nested loop, the loop is collapsed into a single loop which prevent later optimizations from being applied (e.g., transforming nested loops into simplified forms and loop vectorization). The patch augments the existing PHI node-based check by adding a pre-test if the BB actually belongs to a set of loop headers and not eliminating it if yes. llvm-svn: 264697 --- llvm/lib/Transforms/Scalar/JumpThreading.cpp | 5 ++++- llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp | 10 +++++++++- llvm/lib/Transforms/Utils/SimplifyCFG.cpp | 24 +++++++++++++++++++----- 3 files changed, 32 insertions(+), 7 deletions(-) (limited to 'llvm/lib/Transforms') diff --git a/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/llvm/lib/Transforms/Scalar/JumpThreading.cpp index c79e756f572..2def0b54c6d 100644 --- a/llvm/lib/Transforms/Scalar/JumpThreading.cpp +++ b/llvm/lib/Transforms/Scalar/JumpThreading.cpp @@ -245,10 +245,13 @@ bool JumpThreading::runOnFunction(Function &F) { // Can't thread an unconditional jump, but if the block is "almost // empty", we can replace uses of it with uses of the successor and make // this dead. + // We should not eliminate the loop header either, because eliminating + // a loop header might later prevent LoopSimplify from transforming nested + // loops into simplified form. if (BI && BI->isUnconditional() && BB != &BB->getParent()->getEntryBlock() && // If the terminator is the only non-phi instruction, try to nuke it. - BB->getFirstNonPHIOrDbg()->isTerminator()) { + BB->getFirstNonPHIOrDbg()->isTerminator() && !LoopHeaders.count(BB)) { // Since TryToSimplifyUncondBranchFromEmptyBlock may delete the // block, we have to make sure it isn't in the LoopHeaders set. We // reinsert afterward if needed. diff --git a/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp b/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp index 687d388701e..30fbe047190 100644 --- a/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp +++ b/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp @@ -28,6 +28,7 @@ #include "llvm/Analysis/GlobalsModRef.h" #include "llvm/Analysis/AssumptionCache.h" #include "llvm/Analysis/TargetTransformInfo.h" +#include "llvm/Analysis/CFG.h" #include "llvm/IR/Attributes.h" #include "llvm/IR/CFG.h" #include "llvm/IR/Constants.h" @@ -131,12 +132,19 @@ static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI, unsigned BonusInstThreshold) { bool Changed = false; bool LocalChange = true; + + SmallVector, 32> Edges; + FindFunctionBackedges(F, Edges); + SmallPtrSet LoopHeaders; + for (unsigned i = 0, e = Edges.size(); i != e; ++i) + LoopHeaders.insert(const_cast(Edges[i].second)); + while (LocalChange) { LocalChange = false; // Loop over all of the basic blocks and remove them if they are unneeded. for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) { - if (SimplifyCFG(&*BBIt++, TTI, BonusInstThreshold, AC)) { + if (SimplifyCFG(&*BBIt++, TTI, BonusInstThreshold, AC, &LoopHeaders)) { LocalChange = true; ++NumSimpl; } diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp index 75d42617a6a..f750509a5e8 100644 --- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp +++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp @@ -135,6 +135,7 @@ class SimplifyCFGOpt { const DataLayout &DL; unsigned BonusInstThreshold; AssumptionCache *AC; + SmallPtrSetImpl *LoopHeaders; Value *isValueEqualityComparison(TerminatorInst *TI); BasicBlock *GetValueEqualityComparisonCases(TerminatorInst *TI, std::vector &Cases); @@ -157,8 +158,10 @@ class SimplifyCFGOpt { public: SimplifyCFGOpt(const TargetTransformInfo &TTI, const DataLayout &DL, - unsigned BonusInstThreshold, AssumptionCache *AC) - : TTI(TTI), DL(DL), BonusInstThreshold(BonusInstThreshold), AC(AC) {} + unsigned BonusInstThreshold, AssumptionCache *AC, + SmallPtrSetImpl *LoopHeaders) + : TTI(TTI), DL(DL), BonusInstThreshold(BonusInstThreshold), AC(AC), + LoopHeaders(LoopHeaders) {} bool run(BasicBlock *BB); }; } @@ -3362,6 +3365,7 @@ bool SimplifyCFGOpt::SimplifySingleResume(ResumeInst *RI) { // The landingpad is now unreachable. Zap it. BB->eraseFromParent(); + if (LoopHeaders) LoopHeaders->erase(BB); return true; } @@ -3560,9 +3564,11 @@ bool SimplifyCFGOpt::SimplifyReturn(ReturnInst *RI, IRBuilder<> &Builder) { } // If we eliminated all predecessors of the block, delete the block now. - if (pred_empty(BB)) + if (pred_empty(BB)) { // We know there are no successors, so just nuke the block. BB->eraseFromParent(); + if (LoopHeaders) LoopHeaders->erase(BB); + } return true; } @@ -3719,6 +3725,7 @@ bool SimplifyCFGOpt::SimplifyUnreachable(UnreachableInst *UI) { BB != &BB->getParent()->getEntryBlock()) { // We know there are no successors, so just nuke the block. BB->eraseFromParent(); + if (LoopHeaders) LoopHeaders->erase(BB); return true; } @@ -5062,8 +5069,14 @@ bool SimplifyCFGOpt::SimplifyUncondBranch(BranchInst *BI, IRBuilder<> &Builder){ return true; // If the Terminator is the only non-phi instruction, simplify the block. + // if LoopHeader is provided, check if the block is a loop header + // (This is for early invocations before loop simplify and vectorization + // to keep canonical loop forms for nested loops. + // These blocks can be eliminated when the pass is invoked later + // in the back-end.) BasicBlock::iterator I = BB->getFirstNonPHIOrDbg()->getIterator(); if (I->isTerminator() && BB != &BB->getParent()->getEntryBlock() && + (!LoopHeaders || (LoopHeaders && !LoopHeaders->count(BB))) && TryToSimplifyUncondBranchFromEmptyBlock(BB)) return true; @@ -5343,7 +5356,8 @@ bool SimplifyCFGOpt::run(BasicBlock *BB) { /// of the CFG. It returns true if a modification was made. /// bool llvm::SimplifyCFG(BasicBlock *BB, const TargetTransformInfo &TTI, - unsigned BonusInstThreshold, AssumptionCache *AC) { + unsigned BonusInstThreshold, AssumptionCache *AC, + SmallPtrSetImpl *LoopHeaders) { return SimplifyCFGOpt(TTI, BB->getModule()->getDataLayout(), - BonusInstThreshold, AC).run(BB); + BonusInstThreshold, AC, LoopHeaders).run(BB); } -- cgit v1.2.3