rewrite LCSSA to use SSAUpdate, to only return true if it modifies
the IR, and to implement the FIXME'd optimization. llvm-svn: 83748
This commit is contained in:
Chris Lattner
parent
101dde30ed
commit
71d353dd48
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@ -33,22 +33,19 @@
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#include "llvm/Pass.h"
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#include "llvm/Function.h"
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#include "llvm/Instructions.h"
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#include "llvm/LLVMContext.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/ADT/STLExtras.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/Analysis/LoopPass.h"
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#include "llvm/Analysis/ScalarEvolution.h"
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#include "llvm/Support/CFG.h"
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#include "llvm/Support/Compiler.h"
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#include "llvm/Transforms/Utils/SSAUpdater.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/ADT/STLExtras.h"
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#include "llvm/Support/PredIteratorCache.h"
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#include <map>
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using namespace llvm;
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STATISTIC(NumLCSSA, "Number of live out of a loop variables");
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namespace {
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struct VISIBILITY_HIDDEN LCSSA : public LoopPass {
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struct LCSSA : public LoopPass {
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static char ID; // Pass identification, replacement for typeid
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LCSSA() : LoopPass(&ID) {}
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@ -83,8 +80,8 @@ namespace {
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AU.addPreserved<DominanceFrontier>();
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}
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private:
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void ProcessInstruction(Instruction* Instr,
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const SmallVector<BasicBlock*, 8>& exitBlocks);
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bool ProcessInstruction(Instruction *Inst,
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const SmallVectorImpl<BasicBlock*> &ExitBlocks);
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/// verifyAnalysis() - Verify loop nest.
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virtual void verifyAnalysis() const {
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@ -92,12 +89,6 @@ namespace {
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assert(L->isLCSSAForm() && "LCSSA form not preserved!");
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}
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void getLoopValuesUsedOutsideLoop(Loop *L,
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SmallVectorImpl<Instruction*> &AffectedValues);
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Value *GetValueForBlock(DomTreeNode *BB, Instruction *OrigInst,
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DenseMap<DomTreeNode*, Value*> &Phis);
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/// inLoop - returns true if the given block is within the current loop
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bool inLoop(BasicBlock *B) const {
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return std::binary_search(LoopBlocks.begin(), LoopBlocks.end(), B);
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@ -111,178 +102,163 @@ static RegisterPass<LCSSA> X("lcssa", "Loop-Closed SSA Form Pass");
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Pass *llvm::createLCSSAPass() { return new LCSSA(); }
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const PassInfo *const llvm::LCSSAID = &X;
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/// BlockDominatesAnExit - Return true if the specified block dominates at least
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/// one of the blocks in the specified list.
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static bool BlockDominatesAnExit(BasicBlock *BB,
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const SmallVectorImpl<BasicBlock*> &ExitBlocks,
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DominatorTree *DT) {
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DomTreeNode *DomNode = DT->getNode(BB);
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for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
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if (DT->dominates(DomNode, DT->getNode(ExitBlocks[i])))
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return true;
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return false;
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}
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/// runOnFunction - Process all loops in the function, inner-most out.
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bool LCSSA::runOnLoop(Loop *l, LPPassManager &LPM) {
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L = l;
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PredCache.clear();
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bool LCSSA::runOnLoop(Loop *TheLoop, LPPassManager &LPM) {
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L = TheLoop;
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LI = &LPM.getAnalysis<LoopInfo>();
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DT = &getAnalysis<DominatorTree>();
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// Get the set of exiting blocks.
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SmallVector<BasicBlock*, 8> ExitBlocks;
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L->getExitBlocks(ExitBlocks);
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if (ExitBlocks.empty())
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return false;
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// Speed up queries by creating a sorted vector of blocks.
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LoopBlocks.clear();
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LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
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array_pod_sort(LoopBlocks.begin(), LoopBlocks.end());
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SmallVector<Instruction*, 16> AffectedValues;
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getLoopValuesUsedOutsideLoop(L, AffectedValues);
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// Look at all the instructions in the loop, checking to see if they have uses
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// outside the loop. If so, rewrite those uses.
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bool MadeChange = false;
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// If no values are affected, we can save a lot of work, since we know that
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// nothing will be changed.
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if (AffectedValues.empty())
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return false;
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SmallVector<BasicBlock*, 8> ExitBlocks;
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L->getExitBlocks(ExitBlocks);
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// Iterate over all affected values for this loop and insert Phi nodes
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// for them in the appropriate exit blocks.
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for (SmallVectorImpl<Instruction*>::iterator I = AffectedValues.begin(),
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E = AffectedValues.end(); I != E; ++I)
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ProcessInstruction(*I, ExitBlocks);
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for (Loop::block_iterator BBI = L->block_begin(), E = L->block_end();
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BBI != E; ++BBI) {
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BasicBlock *BB = *BBI;
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// For large loops, avoid use-scanning by using dominance information: In
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// particular, if a block does not dominate any of the loop exits, then none
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// of the values defined in the block could be used outside the loop.
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if (!BlockDominatesAnExit(BB, ExitBlocks, DT))
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continue;
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for (BasicBlock::iterator I = BB->begin(), E = BB->end();
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I != E; ++I) {
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// Reject two common cases fast: instructions with no uses (like stores)
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// and instructions with one use that is in the same block as this.
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if (I->use_empty() ||
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(I->hasOneUse() && I->use_back()->getParent() == BB &&
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!isa<PHINode>(I->use_back())))
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continue;
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MadeChange |= ProcessInstruction(I, ExitBlocks);
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}
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}
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assert(L->isLCSSAForm());
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return true;
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PredCache.clear();
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return MadeChange;
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}
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/// processInstruction - Given a live-out instruction, insert LCSSA Phi nodes,
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/// eliminate all out-of-loop uses.
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void LCSSA::ProcessInstruction(Instruction *Instr,
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const SmallVector<BasicBlock*, 8> &ExitBlocks) {
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/// isExitBlock - Return true if the specified block is in the list.
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static bool isExitBlock(BasicBlock *BB,
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const SmallVectorImpl<BasicBlock*> &ExitBlocks) {
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for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
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if (ExitBlocks[i] == BB)
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return true;
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return false;
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}
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/// ProcessInstruction - Given an instruction in the loop, check to see if it
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/// has any uses that are outside the current loop. If so, insert LCSSA PHI
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/// nodes and rewrite the uses.
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bool LCSSA::ProcessInstruction(Instruction *Inst,
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const SmallVectorImpl<BasicBlock*> &ExitBlocks) {
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SmallVector<Use*, 16> UsesToRewrite;
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BasicBlock *InstBB = Inst->getParent();
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for (Value::use_iterator UI = Inst->use_begin(), E = Inst->use_end();
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UI != E; ++UI) {
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BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
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if (PHINode *PN = dyn_cast<PHINode>(*UI))
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UserBB = PN->getIncomingBlock(UI);
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if (InstBB != UserBB && !inLoop(UserBB))
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UsesToRewrite.push_back(&UI.getUse());
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}
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// If there are no uses outside the loop, exit with no change.
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if (UsesToRewrite.empty()) return false;
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++NumLCSSA; // We are applying the transformation
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// Keep track of the blocks that have the value available already.
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DenseMap<DomTreeNode*, Value*> Phis;
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BasicBlock *DomBB = Instr->getParent();
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// Invoke instructions are special in that their result value is not available
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// along their unwind edge. The code below tests to see whether DomBB dominates
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// the value, so adjust DomBB to the normal destination block, which is
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// effectively where the value is first usable.
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if (InvokeInst *Inv = dyn_cast<InvokeInst>(Instr))
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BasicBlock *DomBB = Inst->getParent();
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if (InvokeInst *Inv = dyn_cast<InvokeInst>(Inst))
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DomBB = Inv->getNormalDest();
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DomTreeNode *DomNode = DT->getNode(DomBB);
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// Insert the LCSSA phi's into the exit blocks (dominated by the value), and
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// add them to the Phi's map.
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for (SmallVector<BasicBlock*, 8>::const_iterator BBI = ExitBlocks.begin(),
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SSAUpdater SSAUpdate;
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SSAUpdate.Initialize(Inst);
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// Insert the LCSSA phi's into all of the exit blocks dominated by the
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// value., and add them to the Phi's map.
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for (SmallVectorImpl<BasicBlock*>::const_iterator BBI = ExitBlocks.begin(),
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BBE = ExitBlocks.end(); BBI != BBE; ++BBI) {
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BasicBlock *BB = *BBI;
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DomTreeNode *ExitBBNode = DT->getNode(BB);
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Value *&Phi = Phis[ExitBBNode];
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if (!Phi && DT->dominates(DomNode, ExitBBNode)) {
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PHINode *PN = PHINode::Create(Instr->getType(), Instr->getName()+".lcssa",
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BB->begin());
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PN->reserveOperandSpace(PredCache.GetNumPreds(BB));
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BasicBlock *ExitBB = *BBI;
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if (!DT->dominates(DomNode, DT->getNode(ExitBB))) continue;
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// If we already inserted something for this BB, don't reprocess it.
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if (SSAUpdate.HasValueForBlock(ExitBB)) continue;
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PHINode *PN = PHINode::Create(Inst->getType(), Inst->getName()+".lcssa",
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ExitBB->begin());
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PN->reserveOperandSpace(PredCache.GetNumPreds(ExitBB));
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// Remember that this phi makes the value alive in this block.
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Phi = PN;
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// Add inputs from inside the loop for this PHI.
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for (BasicBlock** PI = PredCache.GetPreds(BB); *PI; ++PI)
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PN->addIncoming(Instr, *PI);
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}
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// Add inputs from inside the loop for this PHI.
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for (BasicBlock **PI = PredCache.GetPreds(ExitBB); *PI; ++PI)
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PN->addIncoming(Inst, *PI);
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// Remember that this phi makes the value alive in this block.
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SSAUpdate.AddAvailableValue(ExitBB, PN);
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}
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// Record all uses of Instr outside the loop. We need to rewrite these. The
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// LCSSA phis won't be included because they use the value in the loop.
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for (Value::use_iterator UI = Instr->use_begin(), E = Instr->use_end();
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UI != E;) {
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BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
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if (PHINode *P = dyn_cast<PHINode>(*UI))
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UserBB = P->getIncomingBlock(UI);
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// If the user is in the loop, don't rewrite it!
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if (UserBB == Instr->getParent() || inLoop(UserBB)) {
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++UI;
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// Rewrite all uses outside the loop in terms of the new PHIs we just
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// inserted.
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for (unsigned i = 0, e = UsesToRewrite.size(); i != e; ++i) {
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// If this use is in an exit block, rewrite to use the newly inserted PHI.
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// This is required for correctness because SSAUpdate doesn't handle uses in
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// the same block. It assumes the PHI we inserted is at the end of the
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// block.
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Instruction *User = cast<Instruction>(UsesToRewrite[i]->getUser());
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BasicBlock *UserBB = User->getParent();
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if (PHINode *PN = dyn_cast<PHINode>(User))
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UserBB = PN->getIncomingBlock(*UsesToRewrite[i]);
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if (isa<PHINode>(UserBB->begin()) &&
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isExitBlock(UserBB, ExitBlocks)) {
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UsesToRewrite[i]->set(UserBB->begin());
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continue;
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}
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// Otherwise, patch up uses of the value with the appropriate LCSSA Phi,
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// inserting PHI nodes into join points where needed.
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Value *Val = GetValueForBlock(DT->getNode(UserBB), Instr, Phis);
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// Preincrement the iterator to avoid invalidating it when we change the
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// value.
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Use &U = UI.getUse();
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++UI;
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U.set(Val);
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}
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}
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/// getLoopValuesUsedOutsideLoop - Return any values defined in the loop that
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/// are used by instructions outside of it.
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void LCSSA::getLoopValuesUsedOutsideLoop(Loop *L,
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SmallVectorImpl<Instruction*> &AffectedValues) {
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// FIXME: For large loops, we may be able to avoid a lot of use-scanning
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// by using dominance information. In particular, if a block does not
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// dominate any of the loop exits, then none of the values defined in the
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// block could be used outside the loop.
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for (Loop::block_iterator BB = L->block_begin(), BE = L->block_end();
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BB != BE; ++BB) {
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for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ++I)
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for (Value::use_iterator UI = I->use_begin(), UE = I->use_end();
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UI != UE; ++UI) {
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BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
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if (PHINode *PN = dyn_cast<PHINode>(*UI))
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UserBB = PN->getIncomingBlock(UI);
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if (*BB != UserBB && !inLoop(UserBB)) {
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AffectedValues.push_back(I);
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break;
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}
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}
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}
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}
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/// GetValueForBlock - Get the value to use within the specified basic block.
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/// available values are in Phis.
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Value *LCSSA::GetValueForBlock(DomTreeNode *BB, Instruction *OrigInst,
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DenseMap<DomTreeNode*, Value*> &Phis) {
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// If there is no dominator info for this BB, it is unreachable.
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if (BB == 0)
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return UndefValue::get(OrigInst->getType());
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// If we have already computed this value, return the previously computed val.
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if (Phis.count(BB)) return Phis[BB];
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DomTreeNode *IDom = BB->getIDom();
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// Otherwise, there are two cases: we either have to insert a PHI node or we
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// don't. We need to insert a PHI node if this block is not dominated by one
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// of the exit nodes from the loop (the loop could have multiple exits, and
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// though the value defined *inside* the loop dominated all its uses, each
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// exit by itself may not dominate all the uses).
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//
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// The simplest way to check for this condition is by checking to see if the
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// idom is in the loop. If so, we *know* that none of the exit blocks
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// dominate this block. Note that we *know* that the block defining the
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// original instruction is in the idom chain, because if it weren't, then the
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// original value didn't dominate this use.
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if (!inLoop(IDom->getBlock())) {
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// Idom is not in the loop, we must still be "below" the exit block and must
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// be fully dominated by the value live in the idom.
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Value* val = GetValueForBlock(IDom, OrigInst, Phis);
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Phis.insert(std::make_pair(BB, val));
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return val;
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// Otherwise, do full PHI insertion.
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SSAUpdate.RewriteUse(*UsesToRewrite[i]);
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}
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BasicBlock *BBN = BB->getBlock();
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// Otherwise, the idom is the loop, so we need to insert a PHI node. Do so
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// now, then get values to fill in the incoming values for the PHI.
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PHINode *PN = PHINode::Create(OrigInst->getType(),
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OrigInst->getName() + ".lcssa", BBN->begin());
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PN->reserveOperandSpace(PredCache.GetNumPreds(BBN));
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Phis.insert(std::make_pair(BB, PN));
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// Fill in the incoming values for the block.
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for (BasicBlock **PI = PredCache.GetPreds(BBN); *PI; ++PI)
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PN->addIncoming(GetValueForBlock(DT->getNode(*PI), OrigInst, Phis), *PI);
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return PN;
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return true;
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}
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