forked from OSchip/llvm-project
516 lines
20 KiB
C++
516 lines
20 KiB
C++
//===-- LCSSA.cpp - Convert loops into loop-closed SSA form ---------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// This pass transforms loops by placing phi nodes at the end of the loops for
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// all values that are live across the loop boundary. For example, it turns
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// the left into the right code:
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//
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// for (...) for (...)
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// if (c) if (c)
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// X1 = ... X1 = ...
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// else else
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// X2 = ... X2 = ...
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// X3 = phi(X1, X2) X3 = phi(X1, X2)
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// ... = X3 + 4 X4 = phi(X3)
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// ... = X4 + 4
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//
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// This is still valid LLVM; the extra phi nodes are purely redundant, and will
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// be trivially eliminated by InstCombine. The major benefit of this
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// transformation is that it makes many other loop optimizations, such as
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// LoopUnswitching, simpler.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Utils/LCSSA.h"
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#include "llvm/ADT/STLExtras.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/Analysis/BasicAliasAnalysis.h"
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#include "llvm/Analysis/BranchProbabilityInfo.h"
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#include "llvm/Analysis/GlobalsModRef.h"
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#include "llvm/Analysis/LoopPass.h"
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#include "llvm/Analysis/MemorySSA.h"
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#include "llvm/Analysis/ScalarEvolution.h"
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#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
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#include "llvm/IR/Constants.h"
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#include "llvm/IR/Dominators.h"
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#include "llvm/IR/Function.h"
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#include "llvm/IR/IRBuilder.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/IntrinsicInst.h"
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#include "llvm/IR/PredIteratorCache.h"
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#include "llvm/InitializePasses.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Transforms/Utils.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include "llvm/Transforms/Utils/LoopUtils.h"
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#include "llvm/Transforms/Utils/SSAUpdater.h"
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using namespace llvm;
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#define DEBUG_TYPE "lcssa"
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STATISTIC(NumLCSSA, "Number of live out of a loop variables");
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#ifdef EXPENSIVE_CHECKS
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static bool VerifyLoopLCSSA = true;
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#else
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static bool VerifyLoopLCSSA = false;
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#endif
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static cl::opt<bool, true>
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VerifyLoopLCSSAFlag("verify-loop-lcssa", cl::location(VerifyLoopLCSSA),
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cl::Hidden,
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cl::desc("Verify loop lcssa form (time consuming)"));
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/// 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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return is_contained(ExitBlocks, BB);
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}
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/// For every instruction from the worklist, check to see if it has any uses
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/// that are outside the current loop. If so, insert LCSSA PHI nodes and
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/// rewrite the uses.
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bool llvm::formLCSSAForInstructions(SmallVectorImpl<Instruction *> &Worklist,
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const DominatorTree &DT, const LoopInfo &LI,
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ScalarEvolution *SE, IRBuilderBase &Builder,
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SmallVectorImpl<PHINode *> *PHIsToRemove) {
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SmallVector<Use *, 16> UsesToRewrite;
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SmallSetVector<PHINode *, 16> LocalPHIsToRemove;
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PredIteratorCache PredCache;
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bool Changed = false;
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IRBuilderBase::InsertPointGuard InsertPtGuard(Builder);
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// Cache the Loop ExitBlocks across this loop. We expect to get a lot of
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// instructions within the same loops, computing the exit blocks is
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// expensive, and we're not mutating the loop structure.
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SmallDenseMap<Loop*, SmallVector<BasicBlock *,1>> LoopExitBlocks;
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while (!Worklist.empty()) {
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UsesToRewrite.clear();
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Instruction *I = Worklist.pop_back_val();
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assert(!I->getType()->isTokenTy() && "Tokens shouldn't be in the worklist");
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BasicBlock *InstBB = I->getParent();
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Loop *L = LI.getLoopFor(InstBB);
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assert(L && "Instruction belongs to a BB that's not part of a loop");
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if (!LoopExitBlocks.count(L))
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L->getExitBlocks(LoopExitBlocks[L]);
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assert(LoopExitBlocks.count(L));
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const SmallVectorImpl<BasicBlock *> &ExitBlocks = LoopExitBlocks[L];
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if (ExitBlocks.empty())
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continue;
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for (Use &U : I->uses()) {
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Instruction *User = cast<Instruction>(U.getUser());
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BasicBlock *UserBB = User->getParent();
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// For practical purposes, we consider that the use in a PHI
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// occurs in the respective predecessor block. For more info,
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// see the `phi` doc in LangRef and the LCSSA doc.
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if (auto *PN = dyn_cast<PHINode>(User))
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UserBB = PN->getIncomingBlock(U);
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if (InstBB != UserBB && !L->contains(UserBB))
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UsesToRewrite.push_back(&U);
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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())
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continue;
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++NumLCSSA; // We are applying the transformation
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// Invoke instructions are special in that their result value is not
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// available along their unwind edge. The code below tests to see whether
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// DomBB dominates the value, so adjust DomBB to the normal destination
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// block, which is effectively where the value is first usable.
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BasicBlock *DomBB = InstBB;
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if (auto *Inv = dyn_cast<InvokeInst>(I))
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DomBB = Inv->getNormalDest();
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const DomTreeNode *DomNode = DT.getNode(DomBB);
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SmallVector<PHINode *, 16> AddedPHIs;
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SmallVector<PHINode *, 8> PostProcessPHIs;
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SmallVector<PHINode *, 4> InsertedPHIs;
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SSAUpdater SSAUpdate(&InsertedPHIs);
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SSAUpdate.Initialize(I->getType(), I->getName());
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// Force re-computation of I, as some users now need to use the new PHI
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// node.
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if (SE)
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SE->forgetValue(I);
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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 (BasicBlock *ExitBB : ExitBlocks) {
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if (!DT.dominates(DomNode, DT.getNode(ExitBB)))
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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))
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continue;
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Builder.SetInsertPoint(&ExitBB->front());
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PHINode *PN = Builder.CreatePHI(I->getType(), PredCache.size(ExitBB),
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I->getName() + ".lcssa");
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// Get the debug location from the original instruction.
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PN->setDebugLoc(I->getDebugLoc());
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// Add inputs from inside the loop for this PHI. This is valid
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// because `I` dominates `ExitBB` (checked above). This implies
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// that every incoming block/edge is dominated by `I` as well,
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// i.e. we can add uses of `I` to those incoming edges/append to the incoming
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// blocks without violating the SSA dominance property.
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for (BasicBlock *Pred : PredCache.get(ExitBB)) {
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PN->addIncoming(I, Pred);
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// If the exit block has a predecessor not within the loop, arrange for
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// the incoming value use corresponding to that predecessor to be
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// rewritten in terms of a different LCSSA PHI.
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if (!L->contains(Pred))
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UsesToRewrite.push_back(
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&PN->getOperandUse(PN->getOperandNumForIncomingValue(
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PN->getNumIncomingValues() - 1)));
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}
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AddedPHIs.push_back(PN);
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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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// LoopSimplify might fail to simplify some loops (e.g. when indirect
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// branches are involved). In such situations, it might happen that an
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// exit for Loop L1 is the header of a disjoint Loop L2. Thus, when we
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// create PHIs in such an exit block, we are also inserting PHIs into L2's
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// header. This could break LCSSA form for L2 because these inserted PHIs
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// can also have uses outside of L2. Remember all PHIs in such situation
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// as to revisit than later on. FIXME: Remove this if indirectbr support
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// into LoopSimplify gets improved.
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if (auto *OtherLoop = LI.getLoopFor(ExitBB))
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if (!L->contains(OtherLoop))
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PostProcessPHIs.push_back(PN);
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}
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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 (Use *UseToRewrite : UsesToRewrite) {
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Instruction *User = cast<Instruction>(UseToRewrite->getUser());
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BasicBlock *UserBB = User->getParent();
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// For practical purposes, we consider that the use in a PHI
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// occurs in the respective predecessor block. For more info,
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// see the `phi` doc in LangRef and the LCSSA doc.
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if (auto *PN = dyn_cast<PHINode>(User))
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UserBB = PN->getIncomingBlock(*UseToRewrite);
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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
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// in the same block. It assumes the PHI we inserted is at the end of the
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// block.
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if (isa<PHINode>(UserBB->begin()) && isExitBlock(UserBB, ExitBlocks)) {
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UseToRewrite->set(&UserBB->front());
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continue;
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}
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// If we added a single PHI, it must dominate all uses and we can directly
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// rename it.
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if (AddedPHIs.size() == 1) {
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UseToRewrite->set(AddedPHIs[0]);
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continue;
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}
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// Otherwise, do full PHI insertion.
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SSAUpdate.RewriteUse(*UseToRewrite);
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}
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SmallVector<DbgValueInst *, 4> DbgValues;
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llvm::findDbgValues(DbgValues, I);
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// Update pre-existing debug value uses that reside outside the loop.
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for (auto DVI : DbgValues) {
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BasicBlock *UserBB = DVI->getParent();
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if (InstBB == UserBB || L->contains(UserBB))
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continue;
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// We currently only handle debug values residing in blocks that were
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// traversed while rewriting the uses. If we inserted just a single PHI,
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// we will handle all relevant debug values.
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Value *V = AddedPHIs.size() == 1 ? AddedPHIs[0]
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: SSAUpdate.FindValueForBlock(UserBB);
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if (V)
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DVI->replaceVariableLocationOp(I, V);
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}
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// SSAUpdater might have inserted phi-nodes inside other loops. We'll need
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// to post-process them to keep LCSSA form.
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for (PHINode *InsertedPN : InsertedPHIs) {
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if (auto *OtherLoop = LI.getLoopFor(InsertedPN->getParent()))
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if (!L->contains(OtherLoop))
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PostProcessPHIs.push_back(InsertedPN);
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}
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// Post process PHI instructions that were inserted into another disjoint
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// loop and update their exits properly.
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for (auto *PostProcessPN : PostProcessPHIs)
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if (!PostProcessPN->use_empty())
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Worklist.push_back(PostProcessPN);
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// Keep track of PHI nodes that we want to remove because they did not have
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// any uses rewritten.
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for (PHINode *PN : AddedPHIs)
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if (PN->use_empty())
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LocalPHIsToRemove.insert(PN);
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Changed = true;
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}
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// Remove PHI nodes that did not have any uses rewritten or add them to
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// PHIsToRemove, so the caller can remove them after some additional cleanup.
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// We need to redo the use_empty() check here, because even if the PHI node
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// wasn't used when added to LocalPHIsToRemove, later added PHI nodes can be
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// using it. This cleanup is not guaranteed to handle trees/cycles of PHI
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// nodes that only are used by each other. Such situations has only been
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// noticed when the input IR contains unreachable code, and leaving some extra
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// redundant PHI nodes in such situations is considered a minor problem.
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if (PHIsToRemove) {
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PHIsToRemove->append(LocalPHIsToRemove.begin(), LocalPHIsToRemove.end());
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} else {
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for (PHINode *PN : LocalPHIsToRemove)
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if (PN->use_empty())
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PN->eraseFromParent();
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}
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return Changed;
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}
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// Compute the set of BasicBlocks in the loop `L` dominating at least one exit.
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static void computeBlocksDominatingExits(
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Loop &L, const DominatorTree &DT, SmallVector<BasicBlock *, 8> &ExitBlocks,
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SmallSetVector<BasicBlock *, 8> &BlocksDominatingExits) {
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// We start from the exit blocks, as every block trivially dominates itself
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// (not strictly).
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SmallVector<BasicBlock *, 8> BBWorklist(ExitBlocks);
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while (!BBWorklist.empty()) {
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BasicBlock *BB = BBWorklist.pop_back_val();
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// Check if this is a loop header. If this is the case, we're done.
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if (L.getHeader() == BB)
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continue;
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// Otherwise, add its immediate predecessor in the dominator tree to the
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// worklist, unless we visited it already.
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BasicBlock *IDomBB = DT.getNode(BB)->getIDom()->getBlock();
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// Exit blocks can have an immediate dominator not beloinging to the
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// loop. For an exit block to be immediately dominated by another block
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// outside the loop, it implies not all paths from that dominator, to the
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// exit block, go through the loop.
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// Example:
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//
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// |---- A
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// | |
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// | B<--
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// | | |
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// |---> C --
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// |
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// D
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//
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// C is the exit block of the loop and it's immediately dominated by A,
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// which doesn't belong to the loop.
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if (!L.contains(IDomBB))
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continue;
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if (BlocksDominatingExits.insert(IDomBB))
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BBWorklist.push_back(IDomBB);
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}
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}
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bool llvm::formLCSSA(Loop &L, const DominatorTree &DT, const LoopInfo *LI,
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ScalarEvolution *SE) {
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bool Changed = false;
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#ifdef EXPENSIVE_CHECKS
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// Verify all sub-loops are in LCSSA form already.
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for (Loop *SubLoop: L)
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assert(SubLoop->isRecursivelyLCSSAForm(DT, *LI) && "Subloop not in LCSSA!");
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#endif
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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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SmallSetVector<BasicBlock *, 8> BlocksDominatingExits;
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// We want to avoid use-scanning leveraging dominance informations.
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// If a block doesn't dominate any of the loop exits, the none of the values
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// defined in the loop can be used outside.
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// We compute the set of blocks fullfilling the conditions in advance
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// walking the dominator tree upwards until we hit a loop header.
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computeBlocksDominatingExits(L, DT, ExitBlocks, BlocksDominatingExits);
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SmallVector<Instruction *, 8> Worklist;
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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, put them into the worklist to rewrite those uses.
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for (BasicBlock *BB : BlocksDominatingExits) {
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// Skip blocks that are part of any sub-loops, they must be in LCSSA
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// already.
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if (LI->getLoopFor(BB) != &L)
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continue;
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for (Instruction &I : *BB) {
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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.user_back()->getParent() == BB &&
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!isa<PHINode>(I.user_back())))
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continue;
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// Tokens cannot be used in PHI nodes, so we skip over them.
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// We can run into tokens which are live out of a loop with catchswitch
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// instructions in Windows EH if the catchswitch has one catchpad which
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// is inside the loop and another which is not.
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if (I.getType()->isTokenTy())
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continue;
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Worklist.push_back(&I);
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}
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}
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IRBuilder<> Builder(L.getHeader()->getContext());
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Changed = formLCSSAForInstructions(Worklist, DT, *LI, SE, Builder);
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// If we modified the code, remove any caches about the loop from SCEV to
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// avoid dangling entries.
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// FIXME: This is a big hammer, can we clear the cache more selectively?
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if (SE && Changed)
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SE->forgetLoop(&L);
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assert(L.isLCSSAForm(DT));
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return Changed;
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}
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/// Process a loop nest depth first.
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bool llvm::formLCSSARecursively(Loop &L, const DominatorTree &DT,
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const LoopInfo *LI, ScalarEvolution *SE) {
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bool Changed = false;
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// Recurse depth-first through inner loops.
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for (Loop *SubLoop : L.getSubLoops())
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Changed |= formLCSSARecursively(*SubLoop, DT, LI, SE);
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Changed |= formLCSSA(L, DT, LI, SE);
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return Changed;
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}
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/// Process all loops in the function, inner-most out.
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static bool formLCSSAOnAllLoops(const LoopInfo *LI, const DominatorTree &DT,
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ScalarEvolution *SE) {
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bool Changed = false;
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for (auto &L : *LI)
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Changed |= formLCSSARecursively(*L, DT, LI, SE);
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return Changed;
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}
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namespace {
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struct LCSSAWrapperPass : public FunctionPass {
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static char ID; // Pass identification, replacement for typeid
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LCSSAWrapperPass() : FunctionPass(ID) {
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initializeLCSSAWrapperPassPass(*PassRegistry::getPassRegistry());
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}
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// Cached analysis information for the current function.
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DominatorTree *DT;
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LoopInfo *LI;
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ScalarEvolution *SE;
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bool runOnFunction(Function &F) override;
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void verifyAnalysis() const override {
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// This check is very expensive. On the loop intensive compiles it may cause
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// up to 10x slowdown. Currently it's disabled by default. LPPassManager
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// always does limited form of the LCSSA verification. Similar reasoning
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// was used for the LoopInfo verifier.
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if (VerifyLoopLCSSA) {
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assert(all_of(*LI,
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[&](Loop *L) {
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return L->isRecursivelyLCSSAForm(*DT, *LI);
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}) &&
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"LCSSA form is broken!");
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}
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};
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/// This transformation requires natural loop information & requires that
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/// loop preheaders be inserted into the CFG. It maintains both of these,
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/// as well as the CFG. It also requires dominator information.
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.setPreservesCFG();
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AU.addRequired<DominatorTreeWrapperPass>();
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AU.addRequired<LoopInfoWrapperPass>();
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AU.addPreservedID(LoopSimplifyID);
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AU.addPreserved<AAResultsWrapperPass>();
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AU.addPreserved<BasicAAWrapperPass>();
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AU.addPreserved<GlobalsAAWrapperPass>();
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AU.addPreserved<ScalarEvolutionWrapperPass>();
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AU.addPreserved<SCEVAAWrapperPass>();
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AU.addPreserved<BranchProbabilityInfoWrapperPass>();
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AU.addPreserved<MemorySSAWrapperPass>();
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// This is needed to perform LCSSA verification inside LPPassManager
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AU.addRequired<LCSSAVerificationPass>();
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AU.addPreserved<LCSSAVerificationPass>();
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|
}
|
|
};
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|
}
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|
|
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char LCSSAWrapperPass::ID = 0;
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INITIALIZE_PASS_BEGIN(LCSSAWrapperPass, "lcssa", "Loop-Closed SSA Form Pass",
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false, false)
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|
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
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|
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
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|
INITIALIZE_PASS_DEPENDENCY(LCSSAVerificationPass)
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|
INITIALIZE_PASS_END(LCSSAWrapperPass, "lcssa", "Loop-Closed SSA Form Pass",
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|
false, false)
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|
|
|
Pass *llvm::createLCSSAPass() { return new LCSSAWrapperPass(); }
|
|
char &llvm::LCSSAID = LCSSAWrapperPass::ID;
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|
|
|
/// Transform \p F into loop-closed SSA form.
|
|
bool LCSSAWrapperPass::runOnFunction(Function &F) {
|
|
LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
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|
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
|
|
auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();
|
|
SE = SEWP ? &SEWP->getSE() : nullptr;
|
|
|
|
return formLCSSAOnAllLoops(LI, *DT, SE);
|
|
}
|
|
|
|
PreservedAnalyses LCSSAPass::run(Function &F, FunctionAnalysisManager &AM) {
|
|
auto &LI = AM.getResult<LoopAnalysis>(F);
|
|
auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
|
|
auto *SE = AM.getCachedResult<ScalarEvolutionAnalysis>(F);
|
|
if (!formLCSSAOnAllLoops(&LI, DT, SE))
|
|
return PreservedAnalyses::all();
|
|
|
|
PreservedAnalyses PA;
|
|
PA.preserveSet<CFGAnalyses>();
|
|
PA.preserve<BasicAA>();
|
|
PA.preserve<GlobalsAA>();
|
|
PA.preserve<SCEVAA>();
|
|
PA.preserve<ScalarEvolutionAnalysis>();
|
|
// BPI maps terminators to probabilities, since we don't modify the CFG, no
|
|
// updates are needed to preserve it.
|
|
PA.preserve<BranchProbabilityAnalysis>();
|
|
PA.preserve<MemorySSAAnalysis>();
|
|
return PA;
|
|
}
|