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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@63198 91177308-0d34-0410-b5e6-96231b3b80d8
601 lines
23 KiB
C++
601 lines
23 KiB
C++
//===- LoopSimplify.cpp - Loop Canonicalization Pass ----------------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This pass performs several transformations to transform natural loops into a
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// simpler form, which makes subsequent analyses and transformations simpler and
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// more effective.
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//
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// Loop pre-header insertion guarantees that there is a single, non-critical
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// entry edge from outside of the loop to the loop header. This simplifies a
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// number of analyses and transformations, such as LICM.
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//
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// Loop exit-block insertion guarantees that all exit blocks from the loop
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// (blocks which are outside of the loop that have predecessors inside of the
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// loop) only have predecessors from inside of the loop (and are thus dominated
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// by the loop header). This simplifies transformations such as store-sinking
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// that are built into LICM.
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//
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// This pass also guarantees that loops will have exactly one backedge.
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//
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// Note that the simplifycfg pass will clean up blocks which are split out but
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// end up being unnecessary, so usage of this pass should not pessimize
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// generated code.
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//
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// This pass obviously modifies the CFG, but updates loop information and
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// dominator information.
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "loopsimplify"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Constants.h"
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#include "llvm/Instructions.h"
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#include "llvm/Function.h"
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#include "llvm/Type.h"
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/Transforms/Utils/BasicBlockUtils.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/ADT/SetOperations.h"
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#include "llvm/ADT/SetVector.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/ADT/DepthFirstIterator.h"
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using namespace llvm;
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STATISTIC(NumInserted, "Number of pre-header or exit blocks inserted");
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STATISTIC(NumNested , "Number of nested loops split out");
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namespace {
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struct VISIBILITY_HIDDEN LoopSimplify : public FunctionPass {
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static char ID; // Pass identification, replacement for typeid
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LoopSimplify() : FunctionPass(&ID) {}
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// AA - If we have an alias analysis object to update, this is it, otherwise
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// this is null.
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AliasAnalysis *AA;
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LoopInfo *LI;
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DominatorTree *DT;
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virtual bool runOnFunction(Function &F);
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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// We need loop information to identify the loops...
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AU.addRequired<LoopInfo>();
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AU.addRequired<DominatorTree>();
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AU.addPreserved<LoopInfo>();
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AU.addPreserved<DominatorTree>();
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AU.addPreserved<DominanceFrontier>();
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AU.addPreserved<AliasAnalysis>();
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AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added.
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}
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/// verifyAnalysis() - Verify loop nest.
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void verifyAnalysis() const {
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#ifndef NDEBUG
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LoopInfo *NLI = &getAnalysis<LoopInfo>();
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for (LoopInfo::iterator I = NLI->begin(), E = NLI->end(); I != E; ++I)
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(*I)->verifyLoop();
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#endif
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}
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private:
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bool ProcessLoop(Loop *L);
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BasicBlock *RewriteLoopExitBlock(Loop *L, BasicBlock *Exit);
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void InsertPreheaderForLoop(Loop *L);
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Loop *SeparateNestedLoop(Loop *L);
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void InsertUniqueBackedgeBlock(Loop *L);
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void PlaceSplitBlockCarefully(BasicBlock *NewBB,
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SmallVectorImpl<BasicBlock*> &SplitPreds,
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Loop *L);
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};
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}
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char LoopSimplify::ID = 0;
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static RegisterPass<LoopSimplify>
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X("loopsimplify", "Canonicalize natural loops", true);
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// Publically exposed interface to pass...
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const PassInfo *const llvm::LoopSimplifyID = &X;
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FunctionPass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
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/// runOnFunction - Run down all loops in the CFG (recursively, but we could do
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/// it in any convenient order) inserting preheaders...
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///
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bool LoopSimplify::runOnFunction(Function &F) {
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bool Changed = false;
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LI = &getAnalysis<LoopInfo>();
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AA = getAnalysisIfAvailable<AliasAnalysis>();
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DT = &getAnalysis<DominatorTree>();
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// Check to see that no blocks (other than the header) in loops have
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// predecessors that are not in loops. This is not valid for natural loops,
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// but can occur if the blocks are unreachable. Since they are unreachable we
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// can just shamelessly destroy their terminators to make them not branch into
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// the loop!
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for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
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// This case can only occur for unreachable blocks. Blocks that are
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// unreachable can't be in loops, so filter those blocks out.
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if (LI->getLoopFor(BB)) continue;
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bool BlockUnreachable = false;
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TerminatorInst *TI = BB->getTerminator();
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// Check to see if any successors of this block are non-loop-header loops
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// that are not the header.
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for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) {
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// If this successor is not in a loop, BB is clearly ok.
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Loop *L = LI->getLoopFor(TI->getSuccessor(i));
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if (!L) continue;
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// If the succ is the loop header, and if L is a top-level loop, then this
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// is an entrance into a loop through the header, which is also ok.
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if (L->getHeader() == TI->getSuccessor(i) && L->getParentLoop() == 0)
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continue;
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// Otherwise, this is an entrance into a loop from some place invalid.
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// Either the loop structure is invalid and this is not a natural loop (in
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// which case the compiler is buggy somewhere else) or BB is unreachable.
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BlockUnreachable = true;
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break;
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}
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// If this block is ok, check the next one.
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if (!BlockUnreachable) continue;
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// Otherwise, this block is dead. To clean up the CFG and to allow later
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// loop transformations to ignore this case, we delete the edges into the
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// loop by replacing the terminator.
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// Remove PHI entries from the successors.
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for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
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TI->getSuccessor(i)->removePredecessor(BB);
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// Add a new unreachable instruction before the old terminator.
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new UnreachableInst(TI);
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// Delete the dead terminator.
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if (AA) AA->deleteValue(TI);
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if (!TI->use_empty())
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TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
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TI->eraseFromParent();
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Changed |= true;
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}
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for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
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Changed |= ProcessLoop(*I);
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return Changed;
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}
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/// ProcessLoop - Walk the loop structure in depth first order, ensuring that
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/// all loops have preheaders.
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///
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bool LoopSimplify::ProcessLoop(Loop *L) {
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bool Changed = false;
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ReprocessLoop:
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// Canonicalize inner loops before outer loops. Inner loop canonicalization
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// can provide work for the outer loop to canonicalize.
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for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
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Changed |= ProcessLoop(*I);
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assert(L->getBlocks()[0] == L->getHeader() &&
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"Header isn't first block in loop?");
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// Does the loop already have a preheader? If so, don't insert one.
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if (L->getLoopPreheader() == 0) {
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InsertPreheaderForLoop(L);
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NumInserted++;
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Changed = true;
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}
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// Next, check to make sure that all exit nodes of the loop only have
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// predecessors that are inside of the loop. This check guarantees that the
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// loop preheader/header will dominate the exit blocks. If the exit block has
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// predecessors from outside of the loop, split the edge now.
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SmallVector<BasicBlock*, 8> ExitBlocks;
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L->getExitBlocks(ExitBlocks);
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SetVector<BasicBlock*> ExitBlockSet(ExitBlocks.begin(), ExitBlocks.end());
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for (SetVector<BasicBlock*>::iterator I = ExitBlockSet.begin(),
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E = ExitBlockSet.end(); I != E; ++I) {
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BasicBlock *ExitBlock = *I;
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for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock);
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PI != PE; ++PI)
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// Must be exactly this loop: no subloops, parent loops, or non-loop preds
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// allowed.
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if (!L->contains(*PI)) {
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RewriteLoopExitBlock(L, ExitBlock);
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NumInserted++;
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Changed = true;
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break;
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}
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}
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// If the header has more than two predecessors at this point (from the
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// preheader and from multiple backedges), we must adjust the loop.
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unsigned NumBackedges = L->getNumBackEdges();
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if (NumBackedges != 1) {
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// If this is really a nested loop, rip it out into a child loop. Don't do
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// this for loops with a giant number of backedges, just factor them into a
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// common backedge instead.
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if (NumBackedges < 8) {
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if (Loop *NL = SeparateNestedLoop(L)) {
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++NumNested;
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// This is a big restructuring change, reprocess the whole loop.
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ProcessLoop(NL);
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Changed = true;
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// GCC doesn't tail recursion eliminate this.
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goto ReprocessLoop;
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}
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}
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// If we either couldn't, or didn't want to, identify nesting of the loops,
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// insert a new block that all backedges target, then make it jump to the
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// loop header.
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InsertUniqueBackedgeBlock(L);
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NumInserted++;
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Changed = true;
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}
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// Scan over the PHI nodes in the loop header. Since they now have only two
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// incoming values (the loop is canonicalized), we may have simplified the PHI
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// down to 'X = phi [X, Y]', which should be replaced with 'Y'.
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PHINode *PN;
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for (BasicBlock::iterator I = L->getHeader()->begin();
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(PN = dyn_cast<PHINode>(I++)); )
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if (Value *V = PN->hasConstantValue()) {
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if (AA) AA->deleteValue(PN);
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PN->replaceAllUsesWith(V);
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PN->eraseFromParent();
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}
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return Changed;
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}
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/// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
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/// preheader, this method is called to insert one. This method has two phases:
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/// preheader insertion and analysis updating.
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///
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void LoopSimplify::InsertPreheaderForLoop(Loop *L) {
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BasicBlock *Header = L->getHeader();
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// Compute the set of predecessors of the loop that are not in the loop.
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SmallVector<BasicBlock*, 8> OutsideBlocks;
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for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
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PI != PE; ++PI)
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if (!L->contains(*PI)) // Coming in from outside the loop?
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OutsideBlocks.push_back(*PI); // Keep track of it...
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// Split out the loop pre-header.
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BasicBlock *NewBB =
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SplitBlockPredecessors(Header, &OutsideBlocks[0], OutsideBlocks.size(),
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".preheader", this);
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//===--------------------------------------------------------------------===//
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// Update analysis results now that we have performed the transformation
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//
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// We know that we have loop information to update... update it now.
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if (Loop *Parent = L->getParentLoop())
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Parent->addBasicBlockToLoop(NewBB, LI->getBase());
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// Make sure that NewBB is put someplace intelligent, which doesn't mess up
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// code layout too horribly.
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PlaceSplitBlockCarefully(NewBB, OutsideBlocks, L);
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}
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/// RewriteLoopExitBlock - Ensure that the loop preheader dominates all exit
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/// blocks. This method is used to split exit blocks that have predecessors
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/// outside of the loop.
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BasicBlock *LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
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SmallVector<BasicBlock*, 8> LoopBlocks;
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for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I)
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if (L->contains(*I))
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LoopBlocks.push_back(*I);
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assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
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BasicBlock *NewBB = SplitBlockPredecessors(Exit, &LoopBlocks[0],
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LoopBlocks.size(), ".loopexit",
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this);
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// Update Loop Information - we know that the new block will be in whichever
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// loop the Exit block is in. Note that it may not be in that immediate loop,
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// if the successor is some other loop header. In that case, we continue
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// walking up the loop tree to find a loop that contains both the successor
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// block and the predecessor block.
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Loop *SuccLoop = LI->getLoopFor(Exit);
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while (SuccLoop && !SuccLoop->contains(L->getHeader()))
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SuccLoop = SuccLoop->getParentLoop();
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if (SuccLoop)
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SuccLoop->addBasicBlockToLoop(NewBB, LI->getBase());
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return NewBB;
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}
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/// AddBlockAndPredsToSet - Add the specified block, and all of its
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/// predecessors, to the specified set, if it's not already in there. Stop
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/// predecessor traversal when we reach StopBlock.
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static void AddBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
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std::set<BasicBlock*> &Blocks) {
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std::vector<BasicBlock *> WorkList;
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WorkList.push_back(InputBB);
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do {
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BasicBlock *BB = WorkList.back(); WorkList.pop_back();
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if (Blocks.insert(BB).second && BB != StopBlock)
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// If BB is not already processed and it is not a stop block then
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// insert its predecessor in the work list
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for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
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BasicBlock *WBB = *I;
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WorkList.push_back(WBB);
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}
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} while(!WorkList.empty());
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}
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/// FindPHIToPartitionLoops - The first part of loop-nestification is to find a
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/// PHI node that tells us how to partition the loops.
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static PHINode *FindPHIToPartitionLoops(Loop *L, DominatorTree *DT,
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AliasAnalysis *AA) {
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for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
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PHINode *PN = cast<PHINode>(I);
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++I;
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if (Value *V = PN->hasConstantValue())
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if (!isa<Instruction>(V) || DT->dominates(cast<Instruction>(V), PN)) {
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// This is a degenerate PHI already, don't modify it!
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PN->replaceAllUsesWith(V);
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if (AA) AA->deleteValue(PN);
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PN->eraseFromParent();
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continue;
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}
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// Scan this PHI node looking for a use of the PHI node by itself.
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for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
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if (PN->getIncomingValue(i) == PN &&
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L->contains(PN->getIncomingBlock(i)))
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// We found something tasty to remove.
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return PN;
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}
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return 0;
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}
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// PlaceSplitBlockCarefully - If the block isn't already, move the new block to
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// right after some 'outside block' block. This prevents the preheader from
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// being placed inside the loop body, e.g. when the loop hasn't been rotated.
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void LoopSimplify::PlaceSplitBlockCarefully(BasicBlock *NewBB,
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SmallVectorImpl<BasicBlock*> &SplitPreds,
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Loop *L) {
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// Check to see if NewBB is already well placed.
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Function::iterator BBI = NewBB; --BBI;
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for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
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if (&*BBI == SplitPreds[i])
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return;
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}
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// If it isn't already after an outside block, move it after one. This is
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// always good as it makes the uncond branch from the outside block into a
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// fall-through.
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// Figure out *which* outside block to put this after. Prefer an outside
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// block that neighbors a BB actually in the loop.
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BasicBlock *FoundBB = 0;
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for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
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Function::iterator BBI = SplitPreds[i];
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if (++BBI != NewBB->getParent()->end() &&
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L->contains(BBI)) {
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FoundBB = SplitPreds[i];
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break;
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}
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}
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// If our heuristic for a *good* bb to place this after doesn't find
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// anything, just pick something. It's likely better than leaving it within
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// the loop.
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if (!FoundBB)
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FoundBB = SplitPreds[0];
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NewBB->moveAfter(FoundBB);
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}
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/// SeparateNestedLoop - If this loop has multiple backedges, try to pull one of
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/// them out into a nested loop. This is important for code that looks like
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/// this:
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///
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/// Loop:
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/// ...
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/// br cond, Loop, Next
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/// ...
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/// br cond2, Loop, Out
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///
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/// To identify this common case, we look at the PHI nodes in the header of the
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/// loop. PHI nodes with unchanging values on one backedge correspond to values
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/// that change in the "outer" loop, but not in the "inner" loop.
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///
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/// If we are able to separate out a loop, return the new outer loop that was
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/// created.
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///
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Loop *LoopSimplify::SeparateNestedLoop(Loop *L) {
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PHINode *PN = FindPHIToPartitionLoops(L, DT, AA);
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if (PN == 0) return 0; // No known way to partition.
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// Pull out all predecessors that have varying values in the loop. This
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// handles the case when a PHI node has multiple instances of itself as
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// arguments.
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SmallVector<BasicBlock*, 8> OuterLoopPreds;
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for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
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if (PN->getIncomingValue(i) != PN ||
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!L->contains(PN->getIncomingBlock(i)))
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OuterLoopPreds.push_back(PN->getIncomingBlock(i));
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BasicBlock *Header = L->getHeader();
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BasicBlock *NewBB = SplitBlockPredecessors(Header, &OuterLoopPreds[0],
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OuterLoopPreds.size(),
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".outer", this);
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// Make sure that NewBB is put someplace intelligent, which doesn't mess up
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// code layout too horribly.
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PlaceSplitBlockCarefully(NewBB, OuterLoopPreds, L);
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// Create the new outer loop.
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Loop *NewOuter = new Loop();
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// Change the parent loop to use the outer loop as its child now.
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if (Loop *Parent = L->getParentLoop())
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Parent->replaceChildLoopWith(L, NewOuter);
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else
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LI->changeTopLevelLoop(L, NewOuter);
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// This block is going to be our new header block: add it to this loop and all
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// parent loops.
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NewOuter->addBasicBlockToLoop(NewBB, LI->getBase());
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// L is now a subloop of our outer loop.
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NewOuter->addChildLoop(L);
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for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
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I != E; ++I)
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NewOuter->addBlockEntry(*I);
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// Determine which blocks should stay in L and which should be moved out to
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// the Outer loop now.
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std::set<BasicBlock*> BlocksInL;
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for (pred_iterator PI = pred_begin(Header), E = pred_end(Header); PI!=E; ++PI)
|
|
if (DT->dominates(Header, *PI))
|
|
AddBlockAndPredsToSet(*PI, Header, BlocksInL);
|
|
|
|
|
|
// Scan all of the loop children of L, moving them to OuterLoop if they are
|
|
// not part of the inner loop.
|
|
const std::vector<Loop*> &SubLoops = L->getSubLoops();
|
|
for (size_t I = 0; I != SubLoops.size(); )
|
|
if (BlocksInL.count(SubLoops[I]->getHeader()))
|
|
++I; // Loop remains in L
|
|
else
|
|
NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I));
|
|
|
|
// Now that we know which blocks are in L and which need to be moved to
|
|
// OuterLoop, move any blocks that need it.
|
|
for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
|
|
BasicBlock *BB = L->getBlocks()[i];
|
|
if (!BlocksInL.count(BB)) {
|
|
// Move this block to the parent, updating the exit blocks sets
|
|
L->removeBlockFromLoop(BB);
|
|
if ((*LI)[BB] == L)
|
|
LI->changeLoopFor(BB, NewOuter);
|
|
--i;
|
|
}
|
|
}
|
|
|
|
return NewOuter;
|
|
}
|
|
|
|
|
|
|
|
/// InsertUniqueBackedgeBlock - This method is called when the specified loop
|
|
/// has more than one backedge in it. If this occurs, revector all of these
|
|
/// backedges to target a new basic block and have that block branch to the loop
|
|
/// header. This ensures that loops have exactly one backedge.
|
|
///
|
|
void LoopSimplify::InsertUniqueBackedgeBlock(Loop *L) {
|
|
assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
|
|
|
|
// Get information about the loop
|
|
BasicBlock *Preheader = L->getLoopPreheader();
|
|
BasicBlock *Header = L->getHeader();
|
|
Function *F = Header->getParent();
|
|
|
|
// Figure out which basic blocks contain back-edges to the loop header.
|
|
std::vector<BasicBlock*> BackedgeBlocks;
|
|
for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I)
|
|
if (*I != Preheader) BackedgeBlocks.push_back(*I);
|
|
|
|
// Create and insert the new backedge block...
|
|
BasicBlock *BEBlock = BasicBlock::Create(Header->getName()+".backedge", F);
|
|
BranchInst *BETerminator = BranchInst::Create(Header, BEBlock);
|
|
|
|
// Move the new backedge block to right after the last backedge block.
|
|
Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos;
|
|
F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
|
|
|
|
// Now that the block has been inserted into the function, create PHI nodes in
|
|
// the backedge block which correspond to any PHI nodes in the header block.
|
|
for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
|
|
PHINode *PN = cast<PHINode>(I);
|
|
PHINode *NewPN = PHINode::Create(PN->getType(), PN->getName()+".be",
|
|
BETerminator);
|
|
NewPN->reserveOperandSpace(BackedgeBlocks.size());
|
|
if (AA) AA->copyValue(PN, NewPN);
|
|
|
|
// Loop over the PHI node, moving all entries except the one for the
|
|
// preheader over to the new PHI node.
|
|
unsigned PreheaderIdx = ~0U;
|
|
bool HasUniqueIncomingValue = true;
|
|
Value *UniqueValue = 0;
|
|
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
|
|
BasicBlock *IBB = PN->getIncomingBlock(i);
|
|
Value *IV = PN->getIncomingValue(i);
|
|
if (IBB == Preheader) {
|
|
PreheaderIdx = i;
|
|
} else {
|
|
NewPN->addIncoming(IV, IBB);
|
|
if (HasUniqueIncomingValue) {
|
|
if (UniqueValue == 0)
|
|
UniqueValue = IV;
|
|
else if (UniqueValue != IV)
|
|
HasUniqueIncomingValue = false;
|
|
}
|
|
}
|
|
}
|
|
|
|
// Delete all of the incoming values from the old PN except the preheader's
|
|
assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
|
|
if (PreheaderIdx != 0) {
|
|
PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
|
|
PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
|
|
}
|
|
// Nuke all entries except the zero'th.
|
|
for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i)
|
|
PN->removeIncomingValue(e-i, false);
|
|
|
|
// Finally, add the newly constructed PHI node as the entry for the BEBlock.
|
|
PN->addIncoming(NewPN, BEBlock);
|
|
|
|
// As an optimization, if all incoming values in the new PhiNode (which is a
|
|
// subset of the incoming values of the old PHI node) have the same value,
|
|
// eliminate the PHI Node.
|
|
if (HasUniqueIncomingValue) {
|
|
NewPN->replaceAllUsesWith(UniqueValue);
|
|
if (AA) AA->deleteValue(NewPN);
|
|
BEBlock->getInstList().erase(NewPN);
|
|
}
|
|
}
|
|
|
|
// Now that all of the PHI nodes have been inserted and adjusted, modify the
|
|
// backedge blocks to just to the BEBlock instead of the header.
|
|
for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
|
|
TerminatorInst *TI = BackedgeBlocks[i]->getTerminator();
|
|
for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
|
|
if (TI->getSuccessor(Op) == Header)
|
|
TI->setSuccessor(Op, BEBlock);
|
|
}
|
|
|
|
//===--- Update all analyses which we must preserve now -----------------===//
|
|
|
|
// Update Loop Information - we know that this block is now in the current
|
|
// loop and all parent loops.
|
|
L->addBasicBlockToLoop(BEBlock, LI->getBase());
|
|
|
|
// Update dominator information
|
|
DT->splitBlock(BEBlock);
|
|
if (DominanceFrontier *DF = getAnalysisIfAvailable<DominanceFrontier>())
|
|
DF->splitBlock(BEBlock);
|
|
}
|