//===- ScalarEvolutionNormalization.cpp - See below -----------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements utilities for working with "normalized" expressions.
// See the comments at the top of ScalarEvolutionNormalization.h for details.
//
//===----------------------------------------------------------------------===//
#include "llvm/IR/Dominators.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/ScalarEvolutionNormalization.h"
using namespace llvm;
/// IVUseShouldUsePostIncValue - We have discovered a "User" of an IV expression
/// and now we need to decide whether the user should use the preinc or post-inc
/// value. If this user should use the post-inc version of the IV, return true.
///
/// Choosing wrong here can break dominance properties (if we choose to use the
/// post-inc value when we cannot) or it can end up adding extra live-ranges to
/// the loop, resulting in reg-reg copies (if we use the pre-inc value when we
/// should use the post-inc value).
static bool IVUseShouldUsePostIncValue(Instruction *User, Value *Operand,
const Loop *L, DominatorTree *DT) {
// If the user is in the loop, use the preinc value.
if (L->contains(User)) return false;
BasicBlock *LatchBlock = L->getLoopLatch();
if (!LatchBlock)
return false;
// Ok, the user is outside of the loop. If it is dominated by the latch
// block, use the post-inc value.
if (DT->dominates(LatchBlock, User->getParent()))
return true;
// There is one case we have to be careful of: PHI nodes. These little guys
// can live in blocks that are not dominated by the latch block, but (since
// their uses occur in the predecessor block, not the block the PHI lives in)
// should still use the post-inc value. Check for this case now.
PHINode *PN = dyn_cast<PHINode>(User);
if (!PN || !Operand) return false; // not a phi, not dominated by latch block.
// Look at all of the uses of Operand by the PHI node. If any use corresponds
// to a block that is not dominated by the latch block, give up and use the
// preincremented value.
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
if (PN->getIncomingValue(i) == Operand &&
!DT->dominates(LatchBlock, PN->getIncomingBlock(i)))
return false;
// Okay, all uses of Operand by PN are in predecessor blocks that really are
// dominated by the latch block. Use the post-incremented value.
return true;
}
namespace {
/// Hold the state used during post-inc expression transformation, including a
/// map of transformed expressions.
class PostIncTransform {
TransformKind Kind;
PostIncLoopSet &Loops;
ScalarEvolution &SE;
DominatorTree &DT;
DenseMap<const SCEV*, const SCEV*> Transformed;
public:
PostIncTransform(TransformKind kind, PostIncLoopSet &loops,
ScalarEvolution &se, DominatorTree &dt):
Kind(kind), Loops(loops), SE(se), DT(dt) {}
const SCEV *TransformSubExpr(const SCEV *S, Instruction *User,
Value *OperandValToReplace);
protected:
const SCEV *TransformImpl(const SCEV *S, Instruction *User,
Value *OperandValToReplace);
};
} // namespace
/// Implement post-inc transformation for all valid expression types.
const SCEV *PostIncTransform::
TransformImpl(const SCEV *S, Instruction *User, Value *OperandValToReplace) {
if (const SCEVCastExpr *X = dyn_cast<SCEVCastExpr>(S)) {
const SCEV *O = X->getOperand();
const SCEV *N = TransformSubExpr(O, User, OperandValToReplace);
if (O != N)
switch (S->getSCEVType()) {
case scZeroExtend: return SE.getZeroExtendExpr(N, S->getType());
case scSignExtend: return SE.getSignExtendExpr(N, S->getType());
case scTruncate: return SE.getTruncateExpr(N, S->getType());
default: llvm_unreachable("Unexpected SCEVCastExpr kind!");
}
return S;
}
if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
// An addrec. This is the interesting part.
SmallVector<const SCEV *, 8> Operands;
const Loop *L = AR->getLoop();
// The addrec conceptually uses its operands at loop entry.
Instruction *LUser = &L->getHeader()->front();
// Transform each operand.
for (SCEVNAryExpr::op_iterator I = AR->op_begin(), E = AR->op_end();
I != E; ++I) {
Operands.push_back(TransformSubExpr(*I, LUser, nullptr));
}
// Conservatively use AnyWrap until/unless we need FlagNW.
const SCEV *Result = SE.getAddRecExpr(Operands, L, SCEV::FlagAnyWrap);
switch (Kind) {
case NormalizeAutodetect:
// Normalize this SCEV by subtracting the expression for the final step.
// We only allow affine AddRecs to be normalized, otherwise we would not
// be able to correctly denormalize.
// e.g. {1,+,3,+,2} == {-2,+,1,+,2} + {3,+,2}
// Normalized form: {-2,+,1,+,2}
// Denormalized form: {1,+,3,+,2}
//
// However, denormalization would use a different step expression than
// normalization (see getPostIncExpr), generating the wrong final
// expression: {-2,+,1,+,2} + {1,+,2} => {-1,+,3,+,2}
if (AR->isAffine() &&
IVUseShouldUsePostIncValue(User, OperandValToReplace, L, &DT)) {
const SCEV *TransformedStep =
TransformSubExpr(AR->getStepRecurrence(SE),
User, OperandValToReplace);
Result = SE.getMinusSCEV(Result, TransformedStep);
Loops.insert(L);
}
#if 0
// This assert is conceptually correct, but ScalarEvolution currently
// sometimes fails to canonicalize two equal SCEVs to exactly the same
// form. It's possibly a pessimization when this happens, but it isn't a
// correctness problem, so disable this assert for now.
assert(S == TransformSubExpr(Result, User, OperandValToReplace) &&
"SCEV normalization is not invertible!");
#endif
break;
case Normalize:
// We want to normalize step expression, because otherwise we might not be
// able to denormalize to the original expression.
//
// Here is an example what will happen if we don't normalize step:
// ORIGINAL ISE:
// {(100 /u {1,+,1}<%bb16>),+,(100 /u {1,+,1}<%bb16>)}<%bb25>
// NORMALIZED ISE:
// {((-1 * (100 /u {1,+,1}<%bb16>)) + (100 /u {0,+,1}<%bb16>)),+,
// (100 /u {0,+,1}<%bb16>)}<%bb25>
// DENORMALIZED BACK ISE:
// {((2 * (100 /u {1,+,1}<%bb16>)) + (-1 * (100 /u {2,+,1}<%bb16>))),+,
// (100 /u {1,+,1}<%bb16>)}<%bb25>
// Note that the initial value changes after normalization +
// denormalization, which isn't correct.
if (Loops.count(L)) {
const SCEV *TransformedStep =
TransformSubExpr(AR->getStepRecurrence(SE),
User, OperandValToReplace);
Result = SE.getMinusSCEV(Result, TransformedStep);
}
#if 0
// See the comment on the assert above.
assert(S == TransformSubExpr(Result, User, OperandValToReplace) &&
"SCEV normalization is not invertible!");
#endif
break;
case Denormalize:
// Here we want to normalize step expressions for the same reasons, as
// stated above.
if (Loops.count(L)) {
const SCEV *TransformedStep =
TransformSubExpr(AR->getStepRecurrence(SE),
User, OperandValToReplace);
Result = SE.getAddExpr(Result, TransformedStep);
}
break;
}
return Result;
}
if (const SCEVNAryExpr *X = dyn_cast<SCEVNAryExpr>(S)) {
SmallVector<const SCEV *, 8> Operands;
bool Changed = false;
// Transform each operand.
for (SCEVNAryExpr::op_iterator I = X->op_begin(), E = X->op_end();
I != E; ++I) {
const SCEV *O = *I;
const SCEV *N = TransformSubExpr(O, User, OperandValToReplace);
Changed |= N != O;
Operands.push_back(N);
}
// If any operand actually changed, return a transformed result.
if (Changed)
switch (S->getSCEVType()) {
case scAddExpr: return SE.getAddExpr(Operands);
case scMulExpr: return SE.getMulExpr(Operands);
case scSMaxExpr: return SE.getSMaxExpr(Operands);
case scUMaxExpr: return SE.getUMaxExpr(Operands);
default: llvm_unreachable("Unexpected SCEVNAryExpr kind!");
}
return S;
}
if (const SCEVUDivExpr *X = dyn_cast<SCEVUDivExpr>(S)) {
const SCEV *LO = X->getLHS();
const SCEV *RO = X->getRHS();
const SCEV *LN = TransformSubExpr(LO, User, OperandValToReplace);
const SCEV *RN = TransformSubExpr(RO, User, OperandValToReplace);
if (LO != LN || RO != RN)
return SE.getUDivExpr(LN, RN);
return S;
}
llvm_unreachable("Unexpected SCEV kind!");
}
/// Manage recursive transformation across an expression DAG. Revisiting
/// expressions would lead to exponential recursion.
const SCEV *PostIncTransform::
TransformSubExpr(const SCEV *S, Instruction *User, Value *OperandValToReplace) {
if (isa<SCEVConstant>(S) || isa<SCEVUnknown>(S))
return S;
const SCEV *Result = Transformed.lookup(S);
if (Result)
return Result;
Result = TransformImpl(S, User, OperandValToReplace);
Transformed[S] = Result;
return Result;
}
/// Top level driver for transforming an expression DAG into its requested
/// post-inc form (either "Normalized" or "Denormalized").
const SCEV *llvm::TransformForPostIncUse(TransformKind Kind,
const SCEV *S,
Instruction *User,
Value *OperandValToReplace,
PostIncLoopSet &Loops,
ScalarEvolution &SE,
DominatorTree &DT) {
PostIncTransform Transform(Kind, Loops, SE, DT);
return Transform.TransformSubExpr(S, User, OperandValToReplace);
}
