X-Git-Url: http://plrg.eecs.uci.edu/git/?a=blobdiff_plain;f=lib%2FTransforms%2FScalar%2FLoopRotation.cpp;h=7eeb1527ad401c9b187832224a18f944d3061890;hb=a94d6e87c4c49f2e81b01d66d8bfb591277f8f96;hp=199c42009d29d8c076e1fe59ef0c6488863f172a;hpb=64c24db959815c6b7edbebd32e5a16936d75b2e1;p=oota-llvm.git diff --git a/lib/Transforms/Scalar/LoopRotation.cpp b/lib/Transforms/Scalar/LoopRotation.cpp index 199c42009d2..7eeb1527ad4 100644 --- a/lib/Transforms/Scalar/LoopRotation.cpp +++ b/lib/Transforms/Scalar/LoopRotation.cpp @@ -14,10 +14,12 @@ #define DEBUG_TYPE "loop-rotate" #include "llvm/Transforms/Scalar.h" #include "llvm/Function.h" +#include "llvm/IntrinsicInst.h" #include "llvm/Analysis/CodeMetrics.h" #include "llvm/Analysis/LoopPass.h" #include "llvm/Analysis/InstructionSimplify.h" #include "llvm/Analysis/ScalarEvolution.h" +#include "llvm/Analysis/ValueTracking.h" #include "llvm/Transforms/Utils/Local.h" #include "llvm/Transforms/Utils/BasicBlockUtils.h" #include "llvm/Transforms/Utils/SSAUpdater.h" @@ -51,13 +53,14 @@ namespace { } bool runOnLoop(Loop *L, LPPassManager &LPM); + void simplifyLoopLatch(Loop *L); bool rotateLoop(Loop *L); - + private: LoopInfo *LI; }; } - + char LoopRotate::ID = 0; INITIALIZE_PASS_BEGIN(LoopRotate, "loop-rotate", "Rotate Loops", false, false) INITIALIZE_PASS_DEPENDENCY(LoopInfo) @@ -72,6 +75,11 @@ Pass *llvm::createLoopRotatePass() { return new LoopRotate(); } bool LoopRotate::runOnLoop(Loop *L, LPPassManager &LPM) { LI = &getAnalysis(); + // Simplify the loop latch before attempting to rotate the header + // upward. Rotation may not be needed if the loop tail can be folded into the + // loop exit. + simplifyLoopLatch(L); + // One loop can be rotated multiple times. bool MadeChange = false; while (rotateLoop(L)) @@ -91,18 +99,18 @@ static void RewriteUsesOfClonedInstructions(BasicBlock *OrigHeader, BasicBlock::iterator I, E = OrigHeader->end(); for (I = OrigHeader->begin(); PHINode *PN = dyn_cast(I); ++I) PN->removeIncomingValue(PN->getBasicBlockIndex(OrigPreheader)); - + // Now fix up users of the instructions in OrigHeader, inserting PHI nodes // as necessary. SSAUpdater SSA; for (I = OrigHeader->begin(); I != E; ++I) { Value *OrigHeaderVal = I; - + // If there are no uses of the value (e.g. because it returns void), there // is nothing to rewrite. if (OrigHeaderVal->use_empty()) continue; - + Value *OrigPreHeaderVal = ValueMap[OrigHeaderVal]; // The value now exits in two versions: the initial value in the preheader @@ -110,27 +118,27 @@ static void RewriteUsesOfClonedInstructions(BasicBlock *OrigHeader, SSA.Initialize(OrigHeaderVal->getType(), OrigHeaderVal->getName()); SSA.AddAvailableValue(OrigHeader, OrigHeaderVal); SSA.AddAvailableValue(OrigPreheader, OrigPreHeaderVal); - + // Visit each use of the OrigHeader instruction. for (Value::use_iterator UI = OrigHeaderVal->use_begin(), UE = OrigHeaderVal->use_end(); UI != UE; ) { // Grab the use before incrementing the iterator. Use &U = UI.getUse(); - + // Increment the iterator before removing the use from the list. ++UI; - + // SSAUpdater can't handle a non-PHI use in the same block as an // earlier def. We can easily handle those cases manually. Instruction *UserInst = cast(U.getUser()); if (!isa(UserInst)) { BasicBlock *UserBB = UserInst->getParent(); - + // The original users in the OrigHeader are already using the // original definitions. if (UserBB == OrigHeader) continue; - + // Users in the OrigPreHeader need to use the value to which the // original definitions are mapped. if (UserBB == OrigPreheader) { @@ -138,32 +146,128 @@ static void RewriteUsesOfClonedInstructions(BasicBlock *OrigHeader, continue; } } - + // Anything else can be handled by SSAUpdater. SSA.RewriteUse(U); } } -} +} + +/// Determine whether the instructions in this range my be safely and cheaply +/// speculated. This is not an important enough situation to develop complex +/// heuristics. We handle a single arithmetic instruction along with any type +/// conversions. +static bool shouldSpeculateInstrs(BasicBlock::iterator Begin, + BasicBlock::iterator End) { + bool seenIncrement = false; + for (BasicBlock::iterator I = Begin; I != End; ++I) { + + if (!isSafeToSpeculativelyExecute(I)) + return false; + + if (isa(I)) + continue; + + switch (I->getOpcode()) { + default: + return false; + case Instruction::GetElementPtr: + // GEPs are cheap if all indices are constant. + if (!cast(I)->hasAllConstantIndices()) + return false; + // fall-thru to increment case + case Instruction::Add: + case Instruction::Sub: + case Instruction::And: + case Instruction::Or: + case Instruction::Xor: + case Instruction::Shl: + case Instruction::LShr: + case Instruction::AShr: + if (seenIncrement) + return false; + seenIncrement = true; + break; + case Instruction::Trunc: + case Instruction::ZExt: + case Instruction::SExt: + // ignore type conversions + break; + } + } + return true; +} + +/// Fold the loop tail into the loop exit by speculating the loop tail +/// instructions. Typically, this is a single post-increment. In the case of a +/// simple 2-block loop, hoisting the increment can be much better than +/// duplicating the entire loop header. In the cast of loops with early exits, +/// rotation will not work anyway, but simplifyLoopLatch will put the loop in +/// canonical form so downstream passes can handle it. +/// +/// I don't believe this invalidates SCEV. +void LoopRotate::simplifyLoopLatch(Loop *L) { + BasicBlock *Latch = L->getLoopLatch(); + if (!Latch || Latch->hasAddressTaken()) + return; + + BranchInst *Jmp = dyn_cast(Latch->getTerminator()); + if (!Jmp || !Jmp->isUnconditional()) + return; + + BasicBlock *LastExit = Latch->getSinglePredecessor(); + if (!LastExit || !L->isLoopExiting(LastExit)) + return; + + BranchInst *BI = dyn_cast(LastExit->getTerminator()); + if (!BI) + return; + + if (!shouldSpeculateInstrs(Latch->begin(), Jmp)) + return; + + DEBUG(dbgs() << "Folding loop latch " << Latch->getName() << " into " + << LastExit->getName() << "\n"); + + // Hoist the instructions from Latch into LastExit. + LastExit->getInstList().splice(BI, Latch->getInstList(), Latch->begin(), Jmp); + + unsigned FallThruPath = BI->getSuccessor(0) == Latch ? 0 : 1; + BasicBlock *Header = Jmp->getSuccessor(0); + assert(Header == L->getHeader() && "expected a backward branch"); + + // Remove Latch from the CFG so that LastExit becomes the new Latch. + BI->setSuccessor(FallThruPath, Header); + Latch->replaceSuccessorsPhiUsesWith(LastExit); + Jmp->eraseFromParent(); + + // Nuke the Latch block. + assert(Latch->empty() && "unable to evacuate Latch"); + LI->removeBlock(Latch); + if (DominatorTree *DT = getAnalysisIfAvailable()) + DT->eraseNode(Latch); + Latch->eraseFromParent(); +} /// Rotate loop LP. Return true if the loop is rotated. bool LoopRotate::rotateLoop(Loop *L) { // If the loop has only one block then there is not much to rotate. if (L->getBlocks().size() == 1) return false; - + BasicBlock *OrigHeader = L->getHeader(); - + BranchInst *BI = dyn_cast(OrigHeader->getTerminator()); if (BI == 0 || BI->isUnconditional()) return false; - + // If the loop header is not one of the loop exiting blocks then // either this loop is already rotated or it is not // suitable for loop rotation transformations. if (!L->isLoopExiting(OrigHeader)) return false; - // Updating PHInodes in loops with multiple exits adds complexity. + // Updating PHInodes in loops with multiple exits adds complexity. // Keep it simple, and restrict loop rotation to loops with one exit only. // In future, lift this restriction and support for multiple exits if // required. @@ -183,7 +287,11 @@ bool LoopRotate::rotateLoop(Loop *L) { // Now, this loop is suitable for rotation. BasicBlock *OrigPreheader = L->getLoopPreheader(); BasicBlock *OrigLatch = L->getLoopLatch(); - assert(OrigPreheader && OrigLatch && "Loop not in canonical form?"); + + // If the loop could not be converted to canonical form, it must have an + // indirectbr in it, just give up. + if (OrigPreheader == 0 || OrigLatch == 0) + return false; // Anything ScalarEvolution may know about this loop or the PHI nodes // in its header will soon be invalidated. @@ -198,9 +306,9 @@ bool LoopRotate::rotateLoop(Loop *L) { if (L->contains(Exit)) std::swap(Exit, NewHeader); assert(NewHeader && "Unable to determine new loop header"); - assert(L->contains(NewHeader) && !L->contains(Exit) && + assert(L->contains(NewHeader) && !L->contains(Exit) && "Unable to determine loop header and exit blocks"); - + // This code assumes that the new header has exactly one predecessor. // Remove any single-entry PHI nodes in it. assert(NewHeader->getSinglePredecessor() && @@ -215,14 +323,14 @@ bool LoopRotate::rotateLoop(Loop *L) { // For PHI nodes, the value available in OldPreHeader is just the // incoming value from OldPreHeader. for (; PHINode *PN = dyn_cast(I); ++I) - ValueMap[PN] = PN->getIncomingValue(PN->getBasicBlockIndex(OrigPreheader)); + ValueMap[PN] = PN->getIncomingValueForBlock(OrigPreheader); // For the rest of the instructions, either hoist to the OrigPreheader if // possible or create a clone in the OldPreHeader if not. TerminatorInst *LoopEntryBranch = OrigPreheader->getTerminator(); while (I != E) { Instruction *Inst = I++; - + // If the instruction's operands are invariant and it doesn't read or write // memory, then it is safe to hoist. Doing this doesn't change the order of // execution in the preheader, but does prevent the instruction from @@ -231,18 +339,19 @@ bool LoopRotate::rotateLoop(Loop *L) { // memory (without proving that the loop doesn't write). if (L->hasLoopInvariantOperands(Inst) && !Inst->mayReadFromMemory() && !Inst->mayWriteToMemory() && - !isa(Inst)) { + !isa(Inst) && !isa(Inst) && + !isa(Inst)) { Inst->moveBefore(LoopEntryBranch); continue; } - + // Otherwise, create a duplicate of the instruction. Instruction *C = Inst->clone(); - + // Eagerly remap the operands of the instruction. RemapInstruction(C, ValueMap, RF_NoModuleLevelChanges|RF_IgnoreMissingEntries); - + // With the operands remapped, see if the instruction constant folds or is // otherwise simplifyable. This commonly occurs because the entry from PHI // nodes allows icmps and other instructions to fold. @@ -282,39 +391,67 @@ bool LoopRotate::rotateLoop(Loop *L) { L->moveToHeader(NewHeader); assert(L->getHeader() == NewHeader && "Latch block is our new header"); - // Update DominatorTree to reflect the CFG change we just made. Then split - // edges as necessary to preserve LoopSimplify form. - if (DominatorTree *DT = getAnalysisIfAvailable()) { - // Since OrigPreheader now has the conditional branch to Exit block, it is - // the dominator of Exit. - DT->changeImmediateDominator(Exit, OrigPreheader); - DT->changeImmediateDominator(NewHeader, OrigPreheader); - - // Update OrigHeader to be dominated by the new header block. - DT->changeImmediateDominator(OrigHeader, OrigLatch); + + // At this point, we've finished our major CFG changes. As part of cloning + // the loop into the preheader we've simplified instructions and the + // duplicated conditional branch may now be branching on a constant. If it is + // branching on a constant and if that constant means that we enter the loop, + // then we fold away the cond branch to an uncond branch. This simplifies the + // loop in cases important for nested loops, and it also means we don't have + // to split as many edges. + BranchInst *PHBI = cast(OrigPreheader->getTerminator()); + assert(PHBI->isConditional() && "Should be clone of BI condbr!"); + if (!isa(PHBI->getCondition()) || + PHBI->getSuccessor(cast(PHBI->getCondition())->isZero()) + != NewHeader) { + // The conditional branch can't be folded, handle the general case. + // Update DominatorTree to reflect the CFG change we just made. Then split + // edges as necessary to preserve LoopSimplify form. + if (DominatorTree *DT = getAnalysisIfAvailable()) { + // Since OrigPreheader now has the conditional branch to Exit block, it is + // the dominator of Exit. + DT->changeImmediateDominator(Exit, OrigPreheader); + DT->changeImmediateDominator(NewHeader, OrigPreheader); + + // Update OrigHeader to be dominated by the new header block. + DT->changeImmediateDominator(OrigHeader, OrigLatch); + } + + // Right now OrigPreHeader has two successors, NewHeader and ExitBlock, and + // thus is not a preheader anymore. + // Split the edge to form a real preheader. + BasicBlock *NewPH = SplitCriticalEdge(OrigPreheader, NewHeader, this); + NewPH->setName(NewHeader->getName() + ".lr.ph"); + + // Preserve canonical loop form, which means that 'Exit' should have only + // one predecessor. + BasicBlock *ExitSplit = SplitCriticalEdge(L->getLoopLatch(), Exit, this); + ExitSplit->moveBefore(Exit); + } else { + // We can fold the conditional branch in the preheader, this makes things + // simpler. The first step is to remove the extra edge to the Exit block. + Exit->removePredecessor(OrigPreheader, true /*preserve LCSSA*/); + BranchInst *NewBI = BranchInst::Create(NewHeader, PHBI); + NewBI->setDebugLoc(PHBI->getDebugLoc()); + PHBI->eraseFromParent(); + + // With our CFG finalized, update DomTree if it is available. + if (DominatorTree *DT = getAnalysisIfAvailable()) { + // Update OrigHeader to be dominated by the new header block. + DT->changeImmediateDominator(NewHeader, OrigPreheader); + DT->changeImmediateDominator(OrigHeader, OrigLatch); + } } - - // Right now OrigPreHeader has two successors, NewHeader and ExitBlock, and - // thus is not a preheader anymore. Split the edge to form a real preheader. - BasicBlock *NewPH = SplitCriticalEdge(OrigPreheader, NewHeader, this); - NewPH->setName(NewHeader->getName() + ".lr.ph"); - - // Preserve canonical loop form, which means that 'Exit' should have only one - // predecessor. - BasicBlock *ExitSplit = SplitCriticalEdge(L->getLoopLatch(), Exit, this); - ExitSplit->moveBefore(Exit); - - assert(L->getLoopPreheader() == NewPH && - "Invalid loop preheader after loop rotation"); + + assert(L->getLoopPreheader() && "Invalid loop preheader after loop rotation"); assert(L->getLoopLatch() && "Invalid loop latch after loop rotation"); - // Now that the CFG and DomTree are in a consistent state again, merge the - // OrigHeader block into OrigLatch. We know that they are joined by an - // unconditional branch. This is just a cleanup so the emitted code isn't - // too gross. - bool DidIt = MergeBlockIntoPredecessor(OrigHeader, this); - assert(DidIt && "Block merge failed??"); (void)DidIt; - + // Now that the CFG and DomTree are in a consistent state again, try to merge + // the OrigHeader block into OrigLatch. This will succeed if they are + // connected by an unconditional branch. This is just a cleanup so the + // emitted code isn't too gross in this common case. + MergeBlockIntoPredecessor(OrigHeader, this); + ++NumRotated; return true; }