forked from OSchip/llvm-project
428 lines
16 KiB
C++
428 lines
16 KiB
C++
//===- IVUsers.cpp - Induction Variable Users -------------------*- C++ -*-===//
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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 file implements bookkeeping for "interesting" users of expressions
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// computed from induction variables.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Analysis/IVUsers.h"
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#include "llvm/ADT/STLExtras.h"
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#include "llvm/Analysis/AssumptionCache.h"
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#include "llvm/Analysis/CodeMetrics.h"
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#include "llvm/Analysis/LoopAnalysisManager.h"
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#include "llvm/Analysis/LoopPass.h"
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#include "llvm/Analysis/ScalarEvolutionExpressions.h"
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#include "llvm/Analysis/ValueTracking.h"
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#include "llvm/Config/llvm-config.h"
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#include "llvm/IR/Constants.h"
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#include "llvm/IR/DataLayout.h"
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#include "llvm/IR/DerivedTypes.h"
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#include "llvm/IR/Dominators.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/Module.h"
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#include "llvm/IR/Type.h"
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#include "llvm/InitializePasses.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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#include <algorithm>
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using namespace llvm;
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#define DEBUG_TYPE "iv-users"
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AnalysisKey IVUsersAnalysis::Key;
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IVUsers IVUsersAnalysis::run(Loop &L, LoopAnalysisManager &AM,
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LoopStandardAnalysisResults &AR) {
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return IVUsers(&L, &AR.AC, &AR.LI, &AR.DT, &AR.SE);
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}
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char IVUsersWrapperPass::ID = 0;
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INITIALIZE_PASS_BEGIN(IVUsersWrapperPass, "iv-users",
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"Induction Variable Users", false, true)
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INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
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INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
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INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
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INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
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INITIALIZE_PASS_END(IVUsersWrapperPass, "iv-users", "Induction Variable Users",
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false, true)
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Pass *llvm::createIVUsersPass() { return new IVUsersWrapperPass(); }
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/// isInteresting - Test whether the given expression is "interesting" when
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/// used by the given expression, within the context of analyzing the
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/// given loop.
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static bool isInteresting(const SCEV *S, const Instruction *I, const Loop *L,
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ScalarEvolution *SE, LoopInfo *LI) {
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// An addrec is interesting if it's affine or if it has an interesting start.
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if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
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// Keep things simple. Don't touch loop-variant strides unless they're
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// only used outside the loop and we can simplify them.
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if (AR->getLoop() == L)
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return AR->isAffine() ||
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(!L->contains(I) &&
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SE->getSCEVAtScope(AR, LI->getLoopFor(I->getParent())) != AR);
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// Otherwise recurse to see if the start value is interesting, and that
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// the step value is not interesting, since we don't yet know how to
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// do effective SCEV expansions for addrecs with interesting steps.
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return isInteresting(AR->getStart(), I, L, SE, LI) &&
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!isInteresting(AR->getStepRecurrence(*SE), I, L, SE, LI);
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}
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// An add is interesting if exactly one of its operands is interesting.
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if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
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bool AnyInterestingYet = false;
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for (const auto *Op : Add->operands())
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if (isInteresting(Op, I, L, SE, LI)) {
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if (AnyInterestingYet)
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return false;
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AnyInterestingYet = true;
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}
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return AnyInterestingYet;
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}
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// Nothing else is interesting here.
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return false;
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}
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/// Return true if all loop headers that dominate this block are in simplified
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/// form.
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static bool isSimplifiedLoopNest(BasicBlock *BB, const DominatorTree *DT,
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const LoopInfo *LI,
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SmallPtrSetImpl<Loop*> &SimpleLoopNests) {
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Loop *NearestLoop = nullptr;
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for (DomTreeNode *Rung = DT->getNode(BB);
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Rung; Rung = Rung->getIDom()) {
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BasicBlock *DomBB = Rung->getBlock();
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Loop *DomLoop = LI->getLoopFor(DomBB);
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if (DomLoop && DomLoop->getHeader() == DomBB) {
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// If the domtree walk reaches a loop with no preheader, return false.
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if (!DomLoop->isLoopSimplifyForm())
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return false;
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// If we have already checked this loop nest, stop checking.
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if (SimpleLoopNests.count(DomLoop))
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break;
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// If we have not already checked this loop nest, remember the loop
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// header nearest to BB. The nearest loop may not contain BB.
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if (!NearestLoop)
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NearestLoop = DomLoop;
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}
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}
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if (NearestLoop)
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SimpleLoopNests.insert(NearestLoop);
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return true;
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}
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/// IVUseShouldUsePostIncValue - We have discovered a "User" of an IV expression
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/// and now we need to decide whether the user should use the preinc or post-inc
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/// value. If this user should use the post-inc version of the IV, return true.
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///
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/// Choosing wrong here can break dominance properties (if we choose to use the
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/// post-inc value when we cannot) or it can end up adding extra live-ranges to
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/// the loop, resulting in reg-reg copies (if we use the pre-inc value when we
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/// should use the post-inc value).
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static bool IVUseShouldUsePostIncValue(Instruction *User, Value *Operand,
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const Loop *L, DominatorTree *DT) {
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// If the user is in the loop, use the preinc value.
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if (L->contains(User))
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return false;
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BasicBlock *LatchBlock = L->getLoopLatch();
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if (!LatchBlock)
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return false;
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// Ok, the user is outside of the loop. If it is dominated by the latch
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// block, use the post-inc value.
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if (DT->dominates(LatchBlock, User->getParent()))
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return true;
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// There is one case we have to be careful of: PHI nodes. These little guys
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// can live in blocks that are not dominated by the latch block, but (since
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// their uses occur in the predecessor block, not the block the PHI lives in)
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// should still use the post-inc value. Check for this case now.
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PHINode *PN = dyn_cast<PHINode>(User);
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if (!PN || !Operand)
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return false; // not a phi, not dominated by latch block.
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// Look at all of the uses of Operand by the PHI node. If any use corresponds
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// to a block that is not dominated by the latch block, give up and use the
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// preincremented value.
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for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
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if (PN->getIncomingValue(i) == Operand &&
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!DT->dominates(LatchBlock, PN->getIncomingBlock(i)))
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return false;
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// Okay, all uses of Operand by PN are in predecessor blocks that really are
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// dominated by the latch block. Use the post-incremented value.
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return true;
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}
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/// AddUsersImpl - Inspect the specified instruction. If it is a
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/// reducible SCEV, recursively add its users to the IVUsesByStride set and
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/// return true. Otherwise, return false.
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bool IVUsers::AddUsersImpl(Instruction *I,
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SmallPtrSetImpl<Loop*> &SimpleLoopNests) {
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const DataLayout &DL = I->getModule()->getDataLayout();
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// Add this IV user to the Processed set before returning false to ensure that
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// all IV users are members of the set. See IVUsers::isIVUserOrOperand.
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if (!Processed.insert(I).second)
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return true; // Instruction already handled.
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if (!SE->isSCEVable(I->getType()))
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return false; // Void and FP expressions cannot be reduced.
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// IVUsers is used by LSR which assumes that all SCEV expressions are safe to
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// pass to SCEVExpander. Expressions are not safe to expand if they represent
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// operations that are not safe to speculate, namely integer division.
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if (!isa<PHINode>(I) && !isSafeToSpeculativelyExecute(I))
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return false;
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// LSR is not APInt clean, do not touch integers bigger than 64-bits.
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// Also avoid creating IVs of non-native types. For example, we don't want a
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// 64-bit IV in 32-bit code just because the loop has one 64-bit cast.
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uint64_t Width = SE->getTypeSizeInBits(I->getType());
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if (Width > 64 || !DL.isLegalInteger(Width))
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return false;
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// Don't attempt to promote ephemeral values to indvars. They will be removed
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// later anyway.
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if (EphValues.count(I))
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return false;
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// Get the symbolic expression for this instruction.
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const SCEV *ISE = SE->getSCEV(I);
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// If we've come to an uninteresting expression, stop the traversal and
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// call this a user.
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if (!isInteresting(ISE, I, L, SE, LI))
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return false;
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SmallPtrSet<Instruction *, 4> UniqueUsers;
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for (Use &U : I->uses()) {
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Instruction *User = cast<Instruction>(U.getUser());
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if (!UniqueUsers.insert(User).second)
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continue;
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// Do not infinitely recurse on PHI nodes.
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if (isa<PHINode>(User) && Processed.count(User))
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continue;
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// Only consider IVUsers that are dominated by simplified loop
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// headers. Otherwise, SCEVExpander will crash.
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BasicBlock *UseBB = User->getParent();
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// A phi's use is live out of its predecessor block.
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if (PHINode *PHI = dyn_cast<PHINode>(User)) {
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unsigned OperandNo = U.getOperandNo();
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unsigned ValNo = PHINode::getIncomingValueNumForOperand(OperandNo);
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UseBB = PHI->getIncomingBlock(ValNo);
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}
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if (!isSimplifiedLoopNest(UseBB, DT, LI, SimpleLoopNests))
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return false;
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// Descend recursively, but not into PHI nodes outside the current loop.
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// It's important to see the entire expression outside the loop to get
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// choices that depend on addressing mode use right, although we won't
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// consider references outside the loop in all cases.
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// If User is already in Processed, we don't want to recurse into it again,
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// but do want to record a second reference in the same instruction.
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bool AddUserToIVUsers = false;
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if (LI->getLoopFor(User->getParent()) != L) {
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if (isa<PHINode>(User) || Processed.count(User) ||
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!AddUsersImpl(User, SimpleLoopNests)) {
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LLVM_DEBUG(dbgs() << "FOUND USER in other loop: " << *User << '\n'
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<< " OF SCEV: " << *ISE << '\n');
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AddUserToIVUsers = true;
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}
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} else if (Processed.count(User) || !AddUsersImpl(User, SimpleLoopNests)) {
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LLVM_DEBUG(dbgs() << "FOUND USER: " << *User << '\n'
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<< " OF SCEV: " << *ISE << '\n');
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AddUserToIVUsers = true;
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}
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if (AddUserToIVUsers) {
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// Okay, we found a user that we cannot reduce.
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IVStrideUse &NewUse = AddUser(User, I);
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// Autodetect the post-inc loop set, populating NewUse.PostIncLoops.
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// The regular return value here is discarded; instead of recording
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// it, we just recompute it when we need it.
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const SCEV *OriginalISE = ISE;
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auto NormalizePred = [&](const SCEVAddRecExpr *AR) {
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auto *L = AR->getLoop();
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bool Result = IVUseShouldUsePostIncValue(User, I, L, DT);
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if (Result)
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NewUse.PostIncLoops.insert(L);
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return Result;
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};
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ISE = normalizeForPostIncUseIf(ISE, NormalizePred, *SE);
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// PostIncNormalization effectively simplifies the expression under
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// pre-increment assumptions. Those assumptions (no wrapping) might not
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// hold for the post-inc value. Catch such cases by making sure the
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// transformation is invertible.
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if (OriginalISE != ISE) {
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const SCEV *DenormalizedISE =
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denormalizeForPostIncUse(ISE, NewUse.PostIncLoops, *SE);
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// If we normalized the expression, but denormalization doesn't give the
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// original one, discard this user.
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if (OriginalISE != DenormalizedISE) {
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LLVM_DEBUG(dbgs()
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<< " DISCARDING (NORMALIZATION ISN'T INVERTIBLE): "
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<< *ISE << '\n');
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IVUses.pop_back();
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return false;
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}
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}
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LLVM_DEBUG(if (SE->getSCEV(I) != ISE) dbgs()
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<< " NORMALIZED TO: " << *ISE << '\n');
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}
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}
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return true;
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}
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bool IVUsers::AddUsersIfInteresting(Instruction *I) {
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// SCEVExpander can only handle users that are dominated by simplified loop
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// entries. Keep track of all loops that are only dominated by other simple
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// loops so we don't traverse the domtree for each user.
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SmallPtrSet<Loop*,16> SimpleLoopNests;
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return AddUsersImpl(I, SimpleLoopNests);
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}
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IVStrideUse &IVUsers::AddUser(Instruction *User, Value *Operand) {
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IVUses.push_back(new IVStrideUse(this, User, Operand));
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return IVUses.back();
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}
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IVUsers::IVUsers(Loop *L, AssumptionCache *AC, LoopInfo *LI, DominatorTree *DT,
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ScalarEvolution *SE)
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: L(L), AC(AC), LI(LI), DT(DT), SE(SE), IVUses() {
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// Collect ephemeral values so that AddUsersIfInteresting skips them.
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EphValues.clear();
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CodeMetrics::collectEphemeralValues(L, AC, EphValues);
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// Find all uses of induction variables in this loop, and categorize
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// them by stride. Start by finding all of the PHI nodes in the header for
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// this loop. If they are induction variables, inspect their uses.
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for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I)
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(void)AddUsersIfInteresting(&*I);
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}
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void IVUsers::print(raw_ostream &OS, const Module *M) const {
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OS << "IV Users for loop ";
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L->getHeader()->printAsOperand(OS, false);
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if (SE->hasLoopInvariantBackedgeTakenCount(L)) {
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OS << " with backedge-taken count " << *SE->getBackedgeTakenCount(L);
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}
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OS << ":\n";
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for (const IVStrideUse &IVUse : IVUses) {
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OS << " ";
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IVUse.getOperandValToReplace()->printAsOperand(OS, false);
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OS << " = " << *getReplacementExpr(IVUse);
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for (auto PostIncLoop : IVUse.PostIncLoops) {
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OS << " (post-inc with loop ";
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PostIncLoop->getHeader()->printAsOperand(OS, false);
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OS << ")";
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}
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OS << " in ";
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if (IVUse.getUser())
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IVUse.getUser()->print(OS);
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else
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OS << "Printing <null> User";
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OS << '\n';
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}
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}
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#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
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LLVM_DUMP_METHOD void IVUsers::dump() const { print(dbgs()); }
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#endif
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void IVUsers::releaseMemory() {
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Processed.clear();
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IVUses.clear();
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}
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IVUsersWrapperPass::IVUsersWrapperPass() : LoopPass(ID) {
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initializeIVUsersWrapperPassPass(*PassRegistry::getPassRegistry());
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}
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void IVUsersWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
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AU.addRequired<AssumptionCacheTracker>();
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AU.addRequired<LoopInfoWrapperPass>();
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AU.addRequired<DominatorTreeWrapperPass>();
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AU.addRequired<ScalarEvolutionWrapperPass>();
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AU.setPreservesAll();
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}
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bool IVUsersWrapperPass::runOnLoop(Loop *L, LPPassManager &LPM) {
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auto *AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(
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*L->getHeader()->getParent());
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auto *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
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auto *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
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auto *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
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IU.reset(new IVUsers(L, AC, LI, DT, SE));
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return false;
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}
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void IVUsersWrapperPass::print(raw_ostream &OS, const Module *M) const {
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IU->print(OS, M);
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}
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void IVUsersWrapperPass::releaseMemory() { IU->releaseMemory(); }
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/// getReplacementExpr - Return a SCEV expression which computes the
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/// value of the OperandValToReplace.
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const SCEV *IVUsers::getReplacementExpr(const IVStrideUse &IU) const {
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return SE->getSCEV(IU.getOperandValToReplace());
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}
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/// getExpr - Return the expression for the use.
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const SCEV *IVUsers::getExpr(const IVStrideUse &IU) const {
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return normalizeForPostIncUse(getReplacementExpr(IU), IU.getPostIncLoops(),
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*SE);
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}
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static const SCEVAddRecExpr *findAddRecForLoop(const SCEV *S, const Loop *L) {
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if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
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if (AR->getLoop() == L)
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return AR;
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return findAddRecForLoop(AR->getStart(), L);
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}
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if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
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for (const auto *Op : Add->operands())
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if (const SCEVAddRecExpr *AR = findAddRecForLoop(Op, L))
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return AR;
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return nullptr;
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}
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return nullptr;
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}
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const SCEV *IVUsers::getStride(const IVStrideUse &IU, const Loop *L) const {
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if (const SCEVAddRecExpr *AR = findAddRecForLoop(getExpr(IU), L))
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return AR->getStepRecurrence(*SE);
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return nullptr;
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}
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void IVStrideUse::transformToPostInc(const Loop *L) {
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PostIncLoops.insert(L);
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}
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void IVStrideUse::deleted() {
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// Remove this user from the list.
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Parent->Processed.erase(this->getUser());
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Parent->IVUses.erase(this);
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// this now dangles!
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}
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