From a6fc94b71e2d8e4ab38a3d437fad659201d50627 Mon Sep 17 00:00:00 2001 From: Chris Lattner Date: Sat, 18 Feb 2006 07:57:38 +0000 Subject: [PATCH] improved support for branch folding, still not enabled. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@26289 91177308-0d34-0410-b5e6-96231b3b80d8 --- lib/Transforms/Scalar/LoopUnswitch.cpp | 412 ++++++++++++++++--------- 1 file changed, 269 insertions(+), 143 deletions(-) diff --git a/lib/Transforms/Scalar/LoopUnswitch.cpp b/lib/Transforms/Scalar/LoopUnswitch.cpp index 6b5a71cbdf3..77767f7abaf 100644 --- a/lib/Transforms/Scalar/LoopUnswitch.cpp +++ b/lib/Transforms/Scalar/LoopUnswitch.cpp @@ -36,6 +36,7 @@ #include "llvm/Transforms/Utils/Local.h" #include "llvm/Transforms/Utils/BasicBlockUtils.h" #include "llvm/ADT/Statistic.h" +#include "llvm/ADT/PostOrderIterator.h" #include "llvm/Support/Debug.h" #include "llvm/Support/CommandLine.h" #include @@ -57,6 +58,9 @@ namespace { class LoopUnswitch : public FunctionPass { LoopInfo *LI; // Loop information + + // LoopProcessWorklist - List of loops we need to process. + std::vector LoopProcessWorklist; public: virtual bool runOnFunction(Function &F); bool visitLoop(Loop *L); @@ -72,18 +76,30 @@ namespace { } private: + /// RemoveLoopFromWorklist - If the specified loop is on the loop worklist, + /// remove it. + void RemoveLoopFromWorklist(Loop *L) { + std::vector::iterator I = std::find(LoopProcessWorklist.begin(), + LoopProcessWorklist.end(), L); + if (I != LoopProcessWorklist.end()) + LoopProcessWorklist.erase(I); + } + bool UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L); unsigned getLoopUnswitchCost(Loop *L, Value *LIC); - void VersionLoop(Value *LIC, Constant *OnVal, - Loop *L, Loop *&Out1, Loop *&Out2); void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val, bool EntersWhenTrue, BasicBlock *ExitBlock); + void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L); BasicBlock *SplitEdge(BasicBlock *From, BasicBlock *To); BasicBlock *SplitBlock(BasicBlock *Old, Instruction *SplitPt); - void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,Constant *Val, - bool isEqual); + + void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, + Constant *Val, bool isEqual); + + void SimplifyCode(std::vector &Worklist); void RemoveBlockIfDead(BasicBlock *BB, std::vector &Worklist); + void RemoveLoopFromHierarchy(Loop *L); }; RegisterOpt X("loop-unswitch", "Unswitch loops"); } @@ -94,12 +110,95 @@ bool LoopUnswitch::runOnFunction(Function &F) { bool Changed = false; LI = &getAnalysis(); - // Transform all the top-level loops. Copy the loop list so that the child - // can update the loop tree if it needs to delete the loop. - std::vector SubLoops(LI->begin(), LI->end()); - for (unsigned i = 0, e = SubLoops.size(); i != e; ++i) - Changed |= visitLoop(SubLoops[i]); + // Populate the worklist of loops to process in post-order. + for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I) + for (po_iterator LI = po_begin(*I), E = po_end(*I); LI != E; ++LI) + LoopProcessWorklist.push_back(*LI); + // Process the loops in worklist order, this is a post-order visitation of + // the loops. We use a worklist of loops so that loops can be removed at any + // time if they are deleted (e.g. the backedge of a loop is removed). + while (!LoopProcessWorklist.empty()) { + Loop *L = LoopProcessWorklist.back(); + LoopProcessWorklist.pop_back(); + Changed |= visitLoop(L); + } + + return Changed; +} + +/// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is +/// invariant in the loop, or has an invariant piece, return the invariant. +/// Otherwise, return null. +static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) { + // Constants should be folded, not unswitched on! + if (isa(Cond)) return false; + + // TODO: Handle: br (VARIANT|INVARIANT). + // TODO: Hoist simple expressions out of loops. + if (L->isLoopInvariant(Cond)) return Cond; + + if (BinaryOperator *BO = dyn_cast(Cond)) + if (BO->getOpcode() == Instruction::And || + BO->getOpcode() == Instruction::Or) { + // If either the left or right side is invariant, we can unswitch on this, + // which will cause the branch to go away in one loop and the condition to + // simplify in the other one. + if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed)) + return LHS; + if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed)) + return RHS; + } + + return 0; +} + +bool LoopUnswitch::visitLoop(Loop *L) { + bool Changed = false; + + // Loop over all of the basic blocks in the loop. If we find an interior + // block that is branching on a loop-invariant condition, we can unswitch this + // loop. + for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); + I != E; ++I) { + TerminatorInst *TI = (*I)->getTerminator(); + if (BranchInst *BI = dyn_cast(TI)) { + // If this isn't branching on an invariant condition, we can't unswitch + // it. + if (BI->isConditional()) { + // See if this, or some part of it, is loop invariant. If so, we can + // unswitch on it if we desire. + Value *LoopCond = FindLIVLoopCondition(BI->getCondition(), L, Changed); + if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantBool::True, L)) { + ++NumBranches; + return true; + } + } + } else if (SwitchInst *SI = dyn_cast(TI)) { + Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed); + if (LoopCond && SI->getNumCases() > 1) { + // Find a value to unswitch on: + // FIXME: this should chose the most expensive case! + Constant *UnswitchVal = SI->getCaseValue(1); + if (UnswitchIfProfitable(LoopCond, UnswitchVal, L)) { + ++NumSwitches; + return true; + } + } + } + + // Scan the instructions to check for unswitchable values. + for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end(); + BBI != E; ++BBI) + if (SelectInst *SI = dyn_cast(BBI)) { + Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed); + if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantBool::True, L)) { + ++NumSelects; + return true; + } + } + } + return Changed; } @@ -165,6 +264,9 @@ static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB, return true; } +/// isTrivialLoopExitBlock - Return true if the specified block unconditionally +/// leads to an exit from the specified loop, and has no side-effects in the +/// process. If so, return the block that is exited to, otherwise return null. static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) { std::set Visited; Visited.insert(L->getHeader()); // Branches to header are ok. @@ -266,88 +368,6 @@ unsigned LoopUnswitch::getLoopUnswitchCost(Loop *L, Value *LIC) { return Cost; } -/// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is -/// invariant in the loop, or has an invariant piece, return the invariant. -/// Otherwise, return null. -static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) { - // Constants should be folded, not unswitched on! - if (isa(Cond)) return false; - - // TODO: Handle: br (VARIANT|INVARIANT). - // TODO: Hoist simple expressions out of loops. - if (L->isLoopInvariant(Cond)) return Cond; - - if (BinaryOperator *BO = dyn_cast(Cond)) - if (BO->getOpcode() == Instruction::And || - BO->getOpcode() == Instruction::Or) { - // If either the left or right side is invariant, we can unswitch on this, - // which will cause the branch to go away in one loop and the condition to - // simplify in the other one. - if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed)) - return LHS; - if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed)) - return RHS; - } - - return 0; -} - -bool LoopUnswitch::visitLoop(Loop *L) { - bool Changed = false; - - // Recurse through all subloops before we process this loop. Copy the loop - // list so that the child can update the loop tree if it needs to delete the - // loop. - std::vector SubLoops(L->begin(), L->end()); - for (unsigned i = 0, e = SubLoops.size(); i != e; ++i) - Changed |= visitLoop(SubLoops[i]); - - // Loop over all of the basic blocks in the loop. If we find an interior - // block that is branching on a loop-invariant condition, we can unswitch this - // loop. - for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); - I != E; ++I) { - TerminatorInst *TI = (*I)->getTerminator(); - if (BranchInst *BI = dyn_cast(TI)) { - // If this isn't branching on an invariant condition, we can't unswitch - // it. - if (BI->isConditional()) { - // See if this, or some part of it, is loop invariant. If so, we can - // unswitch on it if we desire. - Value *LoopCond = FindLIVLoopCondition(BI->getCondition(), L, Changed); - if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantBool::True, L)) { - ++NumBranches; - return true; - } - } - } else if (SwitchInst *SI = dyn_cast(TI)) { - Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed); - if (LoopCond && SI->getNumCases() > 1) { - // Find a value to unswitch on: - // FIXME: this should chose the most expensive case! - Constant *UnswitchVal = SI->getCaseValue(1); - if (UnswitchIfProfitable(LoopCond, UnswitchVal, L)) { - ++NumSwitches; - return true; - } - } - } - - // Scan the instructions to check for unswitchable values. - for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end(); - BBI != E; ++BBI) - if (SelectInst *SI = dyn_cast(BBI)) { - Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed); - if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantBool::True, L)) { - ++NumSelects; - return true; - } - } - } - - return Changed; -} - /// UnswitchIfProfitable - We have found that we can unswitch L when /// LoopCond == Val to simplify the loop. If we decide that this is profitable, /// unswitch the loop, reprocess the pieces, then return true. @@ -372,9 +392,6 @@ bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L){ return false; } - //std::cerr << "BEFORE:\n"; LI->dump(); - Loop *NewLoop1 = 0, *NewLoop2 = 0; - // If this is a trivial condition to unswitch (which results in no code // duplication), do it now. Constant *CondVal; @@ -383,16 +400,10 @@ bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L){ if (IsTrivialUnswitchCondition(L, LoopCond, &CondVal, &EntersWhenTrue, &ExitBlock)) { UnswitchTrivialCondition(L, LoopCond, CondVal, EntersWhenTrue, ExitBlock); - NewLoop1 = L; } else { - VersionLoop(LoopCond, Val, L, NewLoop1, NewLoop2); + UnswitchNontrivialCondition(LoopCond, Val, L); } - - //std::cerr << "AFTER:\n"; LI->dump(); - - // Try to unswitch each of our new loops now! - if (NewLoop1) visitLoop(NewLoop1); - if (NewLoop2) visitLoop(NewLoop2); + return true; } @@ -549,6 +560,9 @@ void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond, } OrigPH->getTerminator()->eraseFromParent(); + // We need to reprocess this loop, it could be unswitched again. + LoopProcessWorklist.push_back(L); + // Now that we know that the loop is never entered when this condition is a // particular value, rewrite the loop with this info. We know that this will // at least eliminate the old branch. @@ -560,10 +574,9 @@ void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond, /// VersionLoop - We determined that the loop is profitable to unswitch when LIC /// equal Val. Split it into loop versions and test the condition outside of /// either loop. Return the loops created as Out1/Out2. -void LoopUnswitch::VersionLoop(Value *LIC, Constant *Val, Loop *L, - Loop *&Out1, Loop *&Out2) { +void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val, + Loop *L) { Function *F = L->getHeader()->getParent(); - DEBUG(std::cerr << "loop-unswitch: Unswitching loop %" << L->getHeader()->getName() << " [" << L->getBlocks().size() << " blocks] in Function " << F->getName() @@ -675,13 +688,18 @@ void LoopUnswitch::VersionLoop(Value *LIC, Constant *Val, Loop *L, // Emit the new branch that selects between the two versions of this loop. EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR); OldBR->eraseFromParent(); + + LoopProcessWorklist.push_back(L); + LoopProcessWorklist.push_back(NewLoop); // Now we rewrite the original code to know that the condition is true and the // new code to know that the condition is false. - RewriteLoopBodyWithConditionConstant(L, LIC, Val, false); - RewriteLoopBodyWithConditionConstant(NewLoop, LIC, Val, true); - Out1 = L; - Out2 = NewLoop; + RewriteLoopBodyWithConditionConstant(L , LIC, Val, false); + + // It's possible that simplifying one loop could cause the other to be + // deleted. If so, don't simplify it. + if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop) + RewriteLoopBodyWithConditionConstant(NewLoop, LIC, Val, true); } /// RemoveFromWorklist - Remove all instances of I from the worklist vector @@ -723,39 +741,60 @@ static void ReplaceUsesOfWith(Instruction *I, Value *V, /// void LoopUnswitch::RemoveBlockIfDead(BasicBlock *BB, std::vector &Worklist) { - if (pred_begin(BB) != pred_end(BB)) return; // not dead. + if (pred_begin(BB) != pred_end(BB)) { + // This block isn't dead, since an edge to BB was just removed, see if there + // are any easy simplifications we can do now. + if (BasicBlock *Pred = BB->getSinglePredecessor()) { + // If it has one pred, fold phi nodes in BB. + while (isa(BB->begin())) + ReplaceUsesOfWith(BB->begin(), + cast(BB->begin())->getIncomingValue(0), + Worklist); + + // If this is the header of a loop and the only pred is the latch, we now + // have an unreachable loop. + if (Loop *L = LI->getLoopFor(BB)) + if (L->getHeader() == BB && L->contains(Pred)) { + // Remove the branch from the latch to the header block, this makes + // the header dead, which will make the latch dead (because the header + // dominates the latch). + Pred->getTerminator()->eraseFromParent(); + new UnreachableInst(Pred); + + // The loop is now broken, remove it from LI. + RemoveLoopFromHierarchy(L); + + // Reprocess the header, which now IS dead. + RemoveBlockIfDead(BB, Worklist); + return; + } + + // If pred ends in a uncond branch, add uncond branch to worklist so that + // the two blocks will get merged. + if (BranchInst *BI = dyn_cast(Pred->getTerminator())) + if (BI->isUnconditional()) + Worklist.push_back(BI); + } + return; + } DEBUG(std::cerr << "Nuking dead block: " << *BB); // Remove the instructions in the basic block from the worklist. - for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) + for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) { RemoveFromWorklist(I, Worklist); + + // Anything that uses the instructions in this basic block should have their + // uses replaced with undefs. + if (!I->use_empty()) + I->replaceAllUsesWith(UndefValue::get(I->getType())); + } - // If this is the header block for a loop, remove the loop and all subloops. - // We leave all of the blocks that used to be part of this loop in the parent - // loop if any. + // If this is the edge to the header block for a loop, remove the loop and + // promote all subloops. if (Loop *BBLoop = LI->getLoopFor(BB)) { - if (BBLoop->getHeader() == BB) { - if (Loop *ParentLoop = BBLoop->getParentLoop()) { // not a top-level loop. - for (Loop::iterator I = ParentLoop->begin(), E = ParentLoop->end();; - ++I) { - assert(I != E && "Couldn't find loop"); - if (*I == BBLoop) { - ParentLoop->removeChildLoop(I); - break; - } - } - } else { - for (LoopInfo::iterator I = LI->begin(), E = LI->end();; ++I) { - assert(I != E && "Couldn't find loop"); - if (*I == BBLoop) { - LI->removeLoop(I); - break; - } - } - } - delete BBLoop; - } + if (BBLoop->getLoopLatch() == BB) + RemoveLoopFromHierarchy(BBLoop); } // Remove the block from the loop info, which removes it from any loops it @@ -771,17 +810,94 @@ void LoopUnswitch::RemoveBlockIfDead(BasicBlock *BB, TI->getSuccessor(i)->removePredecessor(BB); } - // Unique the successors. + // Unique the successors, remove anything with multiple uses. std::sort(Succs.begin(), Succs.end()); Succs.erase(std::unique(Succs.begin(), Succs.end()), Succs.end()); // Remove the basic block, including all of the instructions contained in it. BB->eraseFromParent(); + // Remove successor blocks here that are not dead, so that we know we only + // have dead blocks in this list. Nondead blocks have a way of becoming dead, + // then getting removed before we revisit them, which is badness. + // + for (unsigned i = 0; i != Succs.size(); ++i) + if (pred_begin(Succs[i]) != pred_end(Succs[i])) { + // One exception is loop headers. If this block was the preheader for a + // loop, then we DO want to visit the loop so the loop gets deleted. + // We know that if the successor is a loop header, that this loop had to + // be the preheader: the case where this was the latch block was handled + // above and headers can only have two predecessors. + if (!LI->isLoopHeader(Succs[i])) { + Succs.erase(Succs.begin()+i); + --i; + } + } + for (unsigned i = 0, e = Succs.size(); i != e; ++i) RemoveBlockIfDead(Succs[i], Worklist); } +/// RemoveLoopFromHierarchy - We have discovered that the specified loop has +/// become unwrapped, either because the backedge was deleted, or because the +/// edge into the header was removed. If the edge into the header from the +/// latch block was removed, the loop is unwrapped but subloops are still alive, +/// so they just reparent loops. If the loops are actually dead, they will be +/// removed later. +void LoopUnswitch::RemoveLoopFromHierarchy(Loop *L) { + if (Loop *ParentLoop = L->getParentLoop()) { // Not a top-level loop. + // Reparent all of the blocks in this loop. Since BBLoop had a parent, + // they are now all in it. + for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); + I != E; ++I) + if (LI->getLoopFor(*I) == L) // Don't change blocks in subloops. + LI->changeLoopFor(*I, ParentLoop); + + // Remove the loop from its parent loop. + for (Loop::iterator I = ParentLoop->begin(), E = ParentLoop->end();; + ++I) { + assert(I != E && "Couldn't find loop"); + if (*I == L) { + ParentLoop->removeChildLoop(I); + break; + } + } + + // Move all subloops into the parent loop. + while (L->begin() != L->end()) + ParentLoop->addChildLoop(L->removeChildLoop(L->end()-1)); + } else { + // Reparent all of the blocks in this loop. Since BBLoop had no parent, + // they no longer in a loop at all. + + for (unsigned i = 0; i != L->getBlocks().size(); ++i) { + // Don't change blocks in subloops. + if (LI->getLoopFor(L->getBlocks()[i]) == L) { + LI->removeBlock(L->getBlocks()[i]); + --i; + } + } + + // Remove the loop from the top-level LoopInfo object. + for (LoopInfo::iterator I = LI->begin(), E = LI->end();; ++I) { + assert(I != E && "Couldn't find loop"); + if (*I == L) { + LI->removeLoop(I); + break; + } + } + + // Move all of the subloops to the top-level. + while (L->begin() != L->end()) + LI->addTopLevelLoop(L->removeChildLoop(L->end()-1)); + } + + delete L; + RemoveLoopFromWorklist(L); +} + + + // RewriteLoopBodyWithConditionConstant - We know either that the value LIC has // the value specified by Val in the specified loop, or we know it does NOT have // that value. Rewrite any uses of LIC or of properties correlated to it. @@ -805,9 +921,8 @@ void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches, // selects, switches. std::vector Users(LIC->use_begin(), LIC->use_end()); - std::vector Worklist; - + // If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC // in the loop with the appropriate one directly. if (IsEqual || NotVal) { @@ -849,10 +964,21 @@ void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, // LIC == Val -> false. } } - - // Okay, now that we have simplified some instructions in the loop, walk over - // it and constant prop, dce, and fold control flow where possible. Note that - // this is effectively a very simple loop-structure-aware optimizer. + + SimplifyCode(Worklist); +} + +/// SimplifyCode - Okay, now that we have simplified some instructions in the +/// loop, walk over it and constant prop, dce, and fold control flow where +/// possible. Note that this is effectively a very simple loop-structure-aware +/// optimizer. During processing of this loop, L could very well be deleted, so +/// it must not be used. +/// +/// FIXME: When the loop optimizer is more mature, separate this out to a new +/// pass. +/// +void LoopUnswitch::SimplifyCode(std::vector &Worklist) { + Worklist.back()->getParent()->getParent()->viewCFG(); while (!Worklist.empty()) { Instruction *I = Worklist.back(); Worklist.pop_back(); @@ -940,7 +1066,7 @@ void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, Succ->eraseFromParent(); ++NumSimplify; } else if (ConstantBool *CB = dyn_cast(BI->getCondition())){ - break; // FIXME: disabled + break; // FIXME: Enable. // Conditional branch. Turn it into an unconditional branch, then // remove dead blocks. DEBUG(std::cerr << "Folded branch: " << *BI);