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365 lines
14 KiB
C++
365 lines
14 KiB
C++
//===- IVUsers.cpp - Induction Variable Users -------------------*- C++ -*-===//
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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 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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#define DEBUG_TYPE "iv-users"
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#include "llvm/Analysis/IVUsers.h"
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#include "llvm/Constants.h"
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#include "llvm/Instructions.h"
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#include "llvm/Type.h"
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#include "llvm/DerivedTypes.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/Analysis/LoopPass.h"
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#include "llvm/Analysis/ScalarEvolutionExpressions.h"
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#include "llvm/Assembly/AsmAnnotationWriter.h"
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#include "llvm/ADT/STLExtras.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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char IVUsers::ID = 0;
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static RegisterPass<IVUsers>
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X("iv-users", "Induction Variable Users", false, true);
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Pass *llvm::createIVUsersPass() {
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return new IVUsers();
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}
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/// CollectSubexprs - Split S into subexpressions which can be pulled out into
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/// separate registers.
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static void CollectSubexprs(const SCEV *S,
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SmallVectorImpl<const SCEV *> &Ops,
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ScalarEvolution &SE) {
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if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
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// Break out add operands.
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for (SCEVAddExpr::op_iterator I = Add->op_begin(), E = Add->op_end();
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I != E; ++I)
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CollectSubexprs(*I, Ops, SE);
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return;
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} else if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
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// Split a non-zero base out of an addrec.
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if (!AR->getStart()->isZero()) {
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CollectSubexprs(AR->getStart(), Ops, SE);
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CollectSubexprs(SE.getAddRecExpr(SE.getIntegerSCEV(0, AR->getType()),
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AR->getStepRecurrence(SE),
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AR->getLoop()), Ops, SE);
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return;
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}
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}
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// Otherwise use the value itself.
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Ops.push_back(S);
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}
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/// getSCEVStartAndStride - Compute the start and stride of this expression,
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/// returning false if the expression is not a start/stride pair, or true if it
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/// is. The stride must be a loop invariant expression, but the start may be
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/// a mix of loop invariant and loop variant expressions. The start cannot,
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/// however, contain an AddRec from a different loop, unless that loop is an
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/// outer loop of the current loop.
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static bool getSCEVStartAndStride(const SCEV *&SH, Loop *L, Loop *UseLoop,
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const SCEV *&Start, const SCEV *&Stride,
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ScalarEvolution *SE, DominatorTree *DT) {
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const SCEV *TheAddRec = Start; // Initialize to zero.
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// If the outer level is an AddExpr, the operands are all start values except
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// for a nested AddRecExpr.
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if (const SCEVAddExpr *AE = dyn_cast<SCEVAddExpr>(SH)) {
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for (unsigned i = 0, e = AE->getNumOperands(); i != e; ++i)
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if (const SCEVAddRecExpr *AddRec =
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dyn_cast<SCEVAddRecExpr>(AE->getOperand(i)))
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TheAddRec = SE->getAddExpr(AddRec, TheAddRec);
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else
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Start = SE->getAddExpr(Start, AE->getOperand(i));
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} else if (isa<SCEVAddRecExpr>(SH)) {
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TheAddRec = SH;
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} else {
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return false; // not analyzable.
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}
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// Break down TheAddRec into its component parts.
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SmallVector<const SCEV *, 4> Subexprs;
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CollectSubexprs(TheAddRec, Subexprs, *SE);
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// Look for an addrec on the current loop among the parts.
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const SCEV *AddRecStride = 0;
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for (SmallVectorImpl<const SCEV *>::iterator I = Subexprs.begin(),
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E = Subexprs.end(); I != E; ++I) {
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const SCEV *S = *I;
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if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S))
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if (AR->getLoop() == L) {
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*I = AR->getStart();
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AddRecStride = AR->getStepRecurrence(*SE);
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break;
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}
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}
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if (!AddRecStride)
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return false;
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// Add up everything else into a start value (which may not be
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// loop-invariant).
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const SCEV *AddRecStart = SE->getAddExpr(Subexprs);
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// Use getSCEVAtScope to attempt to simplify other loops out of
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// the picture.
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AddRecStart = SE->getSCEVAtScope(AddRecStart, UseLoop);
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Start = SE->getAddExpr(Start, AddRecStart);
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// If stride is an instruction, make sure it properly dominates the header.
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// Otherwise we could end up with a use before def situation.
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if (!isa<SCEVConstant>(AddRecStride)) {
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BasicBlock *Header = L->getHeader();
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if (!AddRecStride->properlyDominates(Header, DT))
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return false;
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DEBUG(dbgs() << "[";
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WriteAsOperand(dbgs(), L->getHeader(), /*PrintType=*/false);
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dbgs() << "] Variable stride: " << *AddRecStride << "\n");
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}
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Stride = AddRecStride;
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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, Instruction *IV,
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Loop *L, LoopInfo *LI, DominatorTree *DT,
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Pass *P) {
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// If the user is in the loop, use the preinc value.
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if (L->contains(User)) 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) return false; // not a phi, not dominated by latch block.
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// Look at all of the uses of IV by the PHI node. If any use corresponds to
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// 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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unsigned NumUses = 0;
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for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
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if (PN->getIncomingValue(i) == IV) {
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++NumUses;
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if (!DT->dominates(LatchBlock, PN->getIncomingBlock(i)))
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return false;
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}
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// Okay, all uses of IV 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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/// AddUsersIfInteresting - 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::AddUsersIfInteresting(Instruction *I) {
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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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// LSR is not APInt clean, do not touch integers bigger than 64-bits.
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if (SE->getTypeSizeInBits(I->getType()) > 64)
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return false;
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if (!Processed.insert(I))
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return true; // Instruction already handled.
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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 (isa<SCEVCouldNotCompute>(ISE)) return false;
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// Get the start and stride for this expression.
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Loop *UseLoop = LI->getLoopFor(I->getParent());
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const SCEV *Start = SE->getIntegerSCEV(0, ISE->getType());
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const SCEV *Stride = Start;
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if (!getSCEVStartAndStride(ISE, L, UseLoop, Start, Stride, SE, DT))
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return false; // Non-reducible symbolic expression, bail out.
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// Keep things simple. Don't touch loop-variant strides.
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if (!Stride->isLoopInvariant(L) && L->contains(I))
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return false;
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SmallPtrSet<Instruction *, 4> UniqueUsers;
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for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
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UI != E; ++UI) {
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Instruction *User = cast<Instruction>(*UI);
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if (!UniqueUsers.insert(User))
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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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// 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 ouside 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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!AddUsersIfInteresting(User)) {
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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) ||
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!AddUsersIfInteresting(User)) {
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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. Analyze the instruction
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// and decide what to do with it. If we are a use inside of the loop, use
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// the value before incrementation, otherwise use it after incrementation.
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if (IVUseShouldUsePostIncValue(User, I, L, LI, DT, this)) {
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// The value used will be incremented by the stride more than we are
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// expecting, so subtract this off.
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const SCEV *NewStart = SE->getMinusSCEV(Start, Stride);
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IVUses.push_back(new IVStrideUse(this, Stride, NewStart, User, I));
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IVUses.back().setIsUseOfPostIncrementedValue(true);
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DEBUG(dbgs() << " USING POSTINC SCEV, START=" << *NewStart<< "\n");
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} else {
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IVUses.push_back(new IVStrideUse(this, Stride, Start, User, I));
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}
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}
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}
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return true;
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}
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IVStrideUse &IVUsers::AddUser(const SCEV *Stride, const SCEV *Offset,
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Instruction *User, Value *Operand) {
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IVUses.push_back(new IVStrideUse(this, Stride, Offset, User, Operand));
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return IVUses.back();
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}
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IVUsers::IVUsers()
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: LoopPass(&ID) {
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}
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void IVUsers::getAnalysisUsage(AnalysisUsage &AU) const {
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AU.addRequired<LoopInfo>();
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AU.addRequired<DominatorTree>();
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AU.addRequired<ScalarEvolution>();
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AU.setPreservesAll();
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}
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bool IVUsers::runOnLoop(Loop *l, LPPassManager &LPM) {
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L = l;
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LI = &getAnalysis<LoopInfo>();
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DT = &getAnalysis<DominatorTree>();
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SE = &getAnalysis<ScalarEvolution>();
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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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AddUsersIfInteresting(I);
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return false;
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}
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/// getReplacementExpr - Return a SCEV expression which computes the
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/// value of the OperandValToReplace of the given IVStrideUse.
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const SCEV *IVUsers::getReplacementExpr(const IVStrideUse &U) const {
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// Start with zero.
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const SCEV *RetVal = SE->getIntegerSCEV(0, U.getStride()->getType());
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// Create the basic add recurrence.
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RetVal = SE->getAddRecExpr(RetVal, U.getStride(), L);
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// Add the offset in a separate step, because it may be loop-variant.
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RetVal = SE->getAddExpr(RetVal, U.getOffset());
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// For uses of post-incremented values, add an extra stride to compute
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// the actual replacement value.
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if (U.isUseOfPostIncrementedValue())
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RetVal = SE->getAddExpr(RetVal, U.getStride());
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return RetVal;
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}
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/// getCanonicalExpr - Return a SCEV expression which computes the
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/// value of the SCEV of the given IVStrideUse, ignoring the
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/// isUseOfPostIncrementedValue flag.
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const SCEV *IVUsers::getCanonicalExpr(const IVStrideUse &U) const {
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// Start with zero.
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const SCEV *RetVal = SE->getIntegerSCEV(0, U.getStride()->getType());
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// Create the basic add recurrence.
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RetVal = SE->getAddRecExpr(RetVal, U.getStride(), L);
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// Add the offset in a separate step, because it may be loop-variant.
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RetVal = SE->getAddExpr(RetVal, U.getOffset());
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return RetVal;
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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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WriteAsOperand(OS, L->getHeader(), false);
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if (SE->hasLoopInvariantBackedgeTakenCount(L)) {
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OS << " with backedge-taken count "
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<< *SE->getBackedgeTakenCount(L);
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}
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OS << ":\n";
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// Use a defualt AssemblyAnnotationWriter to suppress the default info
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// comments, which aren't relevant here.
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AssemblyAnnotationWriter Annotator;
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for (ilist<IVStrideUse>::const_iterator UI = IVUses.begin(),
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E = IVUses.end(); UI != E; ++UI) {
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OS << " ";
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WriteAsOperand(OS, UI->getOperandValToReplace(), false);
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OS << " = "
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<< *getReplacementExpr(*UI);
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if (UI->isUseOfPostIncrementedValue())
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OS << " (post-inc)";
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OS << " in ";
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UI->getUser()->print(OS, &Annotator);
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OS << '\n';
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}
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}
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void IVUsers::dump() const {
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print(dbgs());
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}
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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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void IVStrideUse::deleted() {
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// Remove this user from the list.
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Parent->IVUses.erase(this);
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// this now dangles!
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}
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