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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209763 91177308-0d34-0410-b5e6-96231b3b80d8
255 lines
10 KiB
C++
255 lines
10 KiB
C++
//===- ScalarEvolutionNormalization.cpp - See below -------------*- 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 utilities for working with "normalized" expressions.
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// See the comments at the top of ScalarEvolutionNormalization.h for details.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/IR/Dominators.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/Analysis/ScalarEvolutionExpressions.h"
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#include "llvm/Analysis/ScalarEvolutionNormalization.h"
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using namespace llvm;
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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)) 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) 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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namespace {
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/// Hold the state used during post-inc expression transformation, including a
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/// map of transformed expressions.
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class PostIncTransform {
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TransformKind Kind;
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PostIncLoopSet &Loops;
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ScalarEvolution &SE;
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DominatorTree &DT;
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DenseMap<const SCEV*, const SCEV*> Transformed;
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public:
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PostIncTransform(TransformKind kind, PostIncLoopSet &loops,
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ScalarEvolution &se, DominatorTree &dt):
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Kind(kind), Loops(loops), SE(se), DT(dt) {}
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const SCEV *TransformSubExpr(const SCEV *S, Instruction *User,
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Value *OperandValToReplace);
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protected:
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const SCEV *TransformImpl(const SCEV *S, Instruction *User,
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Value *OperandValToReplace);
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};
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} // namespace
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/// Implement post-inc transformation for all valid expression types.
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const SCEV *PostIncTransform::
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TransformImpl(const SCEV *S, Instruction *User, Value *OperandValToReplace) {
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if (const SCEVCastExpr *X = dyn_cast<SCEVCastExpr>(S)) {
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const SCEV *O = X->getOperand();
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const SCEV *N = TransformSubExpr(O, User, OperandValToReplace);
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if (O != N)
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switch (S->getSCEVType()) {
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case scZeroExtend: return SE.getZeroExtendExpr(N, S->getType());
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case scSignExtend: return SE.getSignExtendExpr(N, S->getType());
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case scTruncate: return SE.getTruncateExpr(N, S->getType());
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default: llvm_unreachable("Unexpected SCEVCastExpr kind!");
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}
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return S;
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}
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if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
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// An addrec. This is the interesting part.
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SmallVector<const SCEV *, 8> Operands;
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const Loop *L = AR->getLoop();
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// The addrec conceptually uses its operands at loop entry.
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Instruction *LUser = L->getHeader()->begin();
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// Transform each operand.
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for (SCEVNAryExpr::op_iterator I = AR->op_begin(), E = AR->op_end();
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I != E; ++I) {
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Operands.push_back(TransformSubExpr(*I, LUser, nullptr));
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}
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// Conservatively use AnyWrap until/unless we need FlagNW.
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const SCEV *Result = SE.getAddRecExpr(Operands, L, SCEV::FlagAnyWrap);
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switch (Kind) {
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case NormalizeAutodetect:
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// Normalize this SCEV by subtracting the expression for the final step.
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// We only allow affine AddRecs to be normalized, otherwise we would not
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// be able to correctly denormalize.
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// e.g. {1,+,3,+,2} == {-2,+,1,+,2} + {3,+,2}
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// Normalized form: {-2,+,1,+,2}
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// Denormalized form: {1,+,3,+,2}
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//
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// However, denormalization would use the a different step expression than
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// normalization (see getPostIncExpr), generating the wrong final
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// expression: {-2,+,1,+,2} + {1,+,2} => {-1,+,3,+,2}
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if (AR->isAffine() &&
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IVUseShouldUsePostIncValue(User, OperandValToReplace, L, &DT)) {
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const SCEV *TransformedStep =
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TransformSubExpr(AR->getStepRecurrence(SE),
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User, OperandValToReplace);
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Result = SE.getMinusSCEV(Result, TransformedStep);
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Loops.insert(L);
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}
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#if 0
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// This assert is conceptually correct, but ScalarEvolution currently
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// sometimes fails to canonicalize two equal SCEVs to exactly the same
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// form. It's possibly a pessimization when this happens, but it isn't a
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// correctness problem, so disable this assert for now.
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assert(S == TransformSubExpr(Result, User, OperandValToReplace) &&
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"SCEV normalization is not invertible!");
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#endif
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break;
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case Normalize:
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// We want to normalize step expression, because otherwise we might not be
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// able to denormalize to the original expression.
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//
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// Here is an example what will happen if we don't normalize step:
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// ORIGINAL ISE:
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// {(100 /u {1,+,1}<%bb16>),+,(100 /u {1,+,1}<%bb16>)}<%bb25>
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// NORMALIZED ISE:
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// {((-1 * (100 /u {1,+,1}<%bb16>)) + (100 /u {0,+,1}<%bb16>)),+,
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// (100 /u {0,+,1}<%bb16>)}<%bb25>
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// DENORMALIZED BACK ISE:
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// {((2 * (100 /u {1,+,1}<%bb16>)) + (-1 * (100 /u {2,+,1}<%bb16>))),+,
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// (100 /u {1,+,1}<%bb16>)}<%bb25>
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// Note that the initial value changes after normalization +
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// denormalization, which isn't correct.
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if (Loops.count(L)) {
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const SCEV *TransformedStep =
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TransformSubExpr(AR->getStepRecurrence(SE),
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User, OperandValToReplace);
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Result = SE.getMinusSCEV(Result, TransformedStep);
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}
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#if 0
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// See the comment on the assert above.
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assert(S == TransformSubExpr(Result, User, OperandValToReplace) &&
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"SCEV normalization is not invertible!");
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#endif
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break;
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case Denormalize:
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// Here we want to normalize step expressions for the same reasons, as
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// stated above.
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if (Loops.count(L)) {
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const SCEV *TransformedStep =
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TransformSubExpr(AR->getStepRecurrence(SE),
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User, OperandValToReplace);
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Result = SE.getAddExpr(Result, TransformedStep);
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}
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break;
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}
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return Result;
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}
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if (const SCEVNAryExpr *X = dyn_cast<SCEVNAryExpr>(S)) {
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SmallVector<const SCEV *, 8> Operands;
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bool Changed = false;
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// Transform each operand.
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for (SCEVNAryExpr::op_iterator I = X->op_begin(), E = X->op_end();
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I != E; ++I) {
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const SCEV *O = *I;
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const SCEV *N = TransformSubExpr(O, User, OperandValToReplace);
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Changed |= N != O;
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Operands.push_back(N);
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}
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// If any operand actually changed, return a transformed result.
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if (Changed)
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switch (S->getSCEVType()) {
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case scAddExpr: return SE.getAddExpr(Operands);
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case scMulExpr: return SE.getMulExpr(Operands);
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case scSMaxExpr: return SE.getSMaxExpr(Operands);
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case scUMaxExpr: return SE.getUMaxExpr(Operands);
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default: llvm_unreachable("Unexpected SCEVNAryExpr kind!");
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}
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return S;
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}
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if (const SCEVUDivExpr *X = dyn_cast<SCEVUDivExpr>(S)) {
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const SCEV *LO = X->getLHS();
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const SCEV *RO = X->getRHS();
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const SCEV *LN = TransformSubExpr(LO, User, OperandValToReplace);
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const SCEV *RN = TransformSubExpr(RO, User, OperandValToReplace);
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if (LO != LN || RO != RN)
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return SE.getUDivExpr(LN, RN);
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return S;
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}
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llvm_unreachable("Unexpected SCEV kind!");
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}
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/// Manage recursive transformation across an expression DAG. Revisiting
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/// expressions would lead to exponential recursion.
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const SCEV *PostIncTransform::
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TransformSubExpr(const SCEV *S, Instruction *User, Value *OperandValToReplace) {
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if (isa<SCEVConstant>(S) || isa<SCEVUnknown>(S))
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return S;
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const SCEV *Result = Transformed.lookup(S);
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if (Result)
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return Result;
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Result = TransformImpl(S, User, OperandValToReplace);
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Transformed[S] = Result;
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return Result;
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}
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/// Top level driver for transforming an expression DAG into its requested
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/// post-inc form (either "Normalized" or "Denormalized").
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const SCEV *llvm::TransformForPostIncUse(TransformKind Kind,
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const SCEV *S,
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Instruction *User,
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Value *OperandValToReplace,
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PostIncLoopSet &Loops,
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ScalarEvolution &SE,
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DominatorTree &DT) {
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PostIncTransform Transform(Kind, Loops, SE, DT);
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return Transform.TransformSubExpr(S, User, OperandValToReplace);
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}
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