[llvm] r244474 - [IndVarSimplify] Make cost estimation in RewriteLoopExitValues smarter
Igor Laevsky via llvm-commits
llvm-commits at lists.llvm.org
Mon Aug 10 11:23:59 PDT 2015
Author: igor.laevsky
Date: Mon Aug 10 13:23:58 2015
New Revision: 244474
URL: http://llvm.org/viewvc/llvm-project?rev=244474&view=rev
Log:
[IndVarSimplify] Make cost estimation in RewriteLoopExitValues smarter
Differential Revision: http://reviews.llvm.org/D11687
Modified:
llvm/trunk/include/llvm/Analysis/ScalarEvolutionExpander.h
llvm/trunk/lib/Analysis/ScalarEvolutionExpander.cpp
llvm/trunk/lib/Transforms/Scalar/IndVarSimplify.cpp
Modified: llvm/trunk/include/llvm/Analysis/ScalarEvolutionExpander.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Analysis/ScalarEvolutionExpander.h?rev=244474&r1=244473&r2=244474&view=diff
==============================================================================
--- llvm/trunk/include/llvm/Analysis/ScalarEvolutionExpander.h (original)
+++ llvm/trunk/include/llvm/Analysis/ScalarEvolutionExpander.h Mon Aug 10 13:23:58 2015
@@ -117,9 +117,13 @@ namespace llvm {
/// \brief Return true for expressions that may incur non-trivial cost to
/// evaluate at runtime.
- bool isHighCostExpansion(const SCEV *Expr, Loop *L) {
+ /// 'At' is an optional parameter which specifies point in code where user
+ /// is going to expand this expression. Sometimes this knowledge can lead to
+ /// a more accurate cost estimation.
+ bool isHighCostExpansion(const SCEV *Expr, Loop *L,
+ const Instruction *At = nullptr) {
SmallPtrSet<const SCEV *, 8> Processed;
- return isHighCostExpansionHelper(Expr, L, Processed);
+ return isHighCostExpansionHelper(Expr, L, At, Processed);
}
/// \brief This method returns the canonical induction variable of the
@@ -193,11 +197,20 @@ namespace llvm {
void setChainedPhi(PHINode *PN) { ChainedPhis.insert(PN); }
+ /// \brief Try to find LLVM IR value for 'S' available at the point 'At'.
+ // 'L' is a hint which tells in which loop to look for the suitable value.
+ // On success return value which is equivalent to the expanded 'S' at point
+ // 'At'. Return nullptr if value was not found.
+ // Note that this function does not perform exhaustive search. I.e if it
+ // didn't find any value it does not mean that there is no such value.
+ Value *findExistingExpansion(const SCEV *S, const Instruction *At, Loop *L);
+
private:
LLVMContext &getContext() const { return SE.getContext(); }
/// \brief Recursive helper function for isHighCostExpansion.
bool isHighCostExpansionHelper(const SCEV *S, Loop *L,
+ const Instruction *At,
SmallPtrSetImpl<const SCEV *> &Processed);
/// \brief Insert the specified binary operator, doing a small amount
Modified: llvm/trunk/lib/Analysis/ScalarEvolutionExpander.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ScalarEvolutionExpander.cpp?rev=244474&r1=244473&r2=244474&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/ScalarEvolutionExpander.cpp (original)
+++ llvm/trunk/lib/Analysis/ScalarEvolutionExpander.cpp Mon Aug 10 13:23:58 2015
@@ -1812,8 +1812,46 @@ unsigned SCEVExpander::replaceCongruentI
return NumElim;
}
+Value *SCEVExpander::findExistingExpansion(const SCEV *S,
+ const Instruction *At, Loop *L) {
+ using namespace llvm::PatternMatch;
+
+ SmallVector<BasicBlock *, 4> Latches;
+ L->getLoopLatches(Latches);
+
+ // Look for suitable value in simple conditions at the loop latches.
+ for (BasicBlock *BB : Latches) {
+ ICmpInst::Predicate Pred;
+ Instruction *LHS, *RHS;
+ BasicBlock *TrueBB, *FalseBB;
+
+ if (!match(BB->getTerminator(),
+ m_Br(m_ICmp(Pred, m_Instruction(LHS), m_Instruction(RHS)),
+ TrueBB, FalseBB)))
+ continue;
+
+ if (SE.getSCEV(LHS) == S && SE.DT->dominates(LHS, At))
+ return LHS;
+
+ if (SE.getSCEV(RHS) == S && SE.DT->dominates(RHS, At))
+ return RHS;
+ }
+
+ // There is potential to make this significantly smarter, but this simple
+ // heuristic already gets some interesting cases.
+
+ // Can not find suitable value.
+ return nullptr;
+}
+
bool SCEVExpander::isHighCostExpansionHelper(
- const SCEV *S, Loop *L, SmallPtrSetImpl<const SCEV *> &Processed) {
+ const SCEV *S, Loop *L, const Instruction *At,
+ SmallPtrSetImpl<const SCEV *> &Processed) {
+
+ // If we can find an existing value for this scev avaliable at the point "At"
+ // then consider the expression cheap.
+ if (At && findExistingExpansion(S, At, L) != nullptr)
+ return false;
// Zero/One operand expressions
switch (S->getSCEVType()) {
@@ -1821,14 +1859,14 @@ bool SCEVExpander::isHighCostExpansionHe
case scConstant:
return false;
case scTruncate:
- return isHighCostExpansionHelper(cast<SCEVTruncateExpr>(S)->getOperand(), L,
- Processed);
+ return isHighCostExpansionHelper(cast<SCEVTruncateExpr>(S)->getOperand(),
+ L, At, Processed);
case scZeroExtend:
return isHighCostExpansionHelper(cast<SCEVZeroExtendExpr>(S)->getOperand(),
- L, Processed);
+ L, At, Processed);
case scSignExtend:
return isHighCostExpansionHelper(cast<SCEVSignExtendExpr>(S)->getOperand(),
- L, Processed);
+ L, At, Processed);
}
if (!Processed.insert(S).second)
@@ -1884,7 +1922,7 @@ bool SCEVExpander::isHighCostExpansionHe
if (const SCEVNAryExpr *NAry = dyn_cast<SCEVNAryExpr>(S)) {
for (SCEVNAryExpr::op_iterator I = NAry->op_begin(), E = NAry->op_end();
I != E; ++I) {
- if (isHighCostExpansionHelper(*I, L, Processed))
+ if (isHighCostExpansionHelper(*I, L, At, Processed))
return true;
}
}
Modified: llvm/trunk/lib/Transforms/Scalar/IndVarSimplify.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/IndVarSimplify.cpp?rev=244474&r1=244473&r2=244474&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/IndVarSimplify.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/IndVarSimplify.cpp Mon Aug 10 13:23:58 2015
@@ -138,8 +138,7 @@ namespace {
void SinkUnusedInvariants(Loop *L);
Value *ExpandSCEVIfNeeded(SCEVExpander &Rewriter, const SCEV *S, Loop *L,
- Instruction *InsertPt, Type *Ty,
- bool &IsHighCostExpansion);
+ Instruction *InsertPt, Type *Ty);
};
}
@@ -503,47 +502,13 @@ struct RewritePhi {
Value *IndVarSimplify::ExpandSCEVIfNeeded(SCEVExpander &Rewriter, const SCEV *S,
Loop *L, Instruction *InsertPt,
- Type *ResultTy,
- bool &IsHighCostExpansion) {
- using namespace llvm::PatternMatch;
-
- if (!Rewriter.isHighCostExpansion(S, L)) {
- IsHighCostExpansion = false;
- return Rewriter.expandCodeFor(S, ResultTy, InsertPt);
- }
-
+ Type *ResultTy) {
// Before expanding S into an expensive LLVM expression, see if we can use an
- // already existing value as the expansion for S. There is potential to make
- // this significantly smarter, but this simple heuristic already gets some
- // interesting cases.
-
- SmallVector<BasicBlock *, 4> Latches;
- L->getLoopLatches(Latches);
-
- for (BasicBlock *BB : Latches) {
- ICmpInst::Predicate Pred;
- Instruction *LHS, *RHS;
- BasicBlock *TrueBB, *FalseBB;
-
- if (!match(BB->getTerminator(),
- m_Br(m_ICmp(Pred, m_Instruction(LHS), m_Instruction(RHS)),
- TrueBB, FalseBB)))
- continue;
-
- if (SE->getSCEV(LHS) == S && DT->dominates(LHS, InsertPt)) {
- IsHighCostExpansion = false;
- return LHS;
- }
-
- if (SE->getSCEV(RHS) == S && DT->dominates(RHS, InsertPt)) {
- IsHighCostExpansion = false;
- return RHS;
- }
- }
+ // already existing value as the expansion for S.
+ if (Value *RetValue = Rewriter.findExistingExpansion(S, InsertPt, L))
+ return RetValue;
// We didn't find anything, fall back to using SCEVExpander.
- assert(Rewriter.isHighCostExpansion(S, L) && "this should not have changed!");
- IsHighCostExpansion = true;
return Rewriter.expandCodeFor(S, ResultTy, InsertPt);
}
@@ -679,9 +644,9 @@ void IndVarSimplify::RewriteLoopExitValu
continue;
}
- bool HighCost = false;
- Value *ExitVal = ExpandSCEVIfNeeded(Rewriter, ExitValue, L, Inst,
- PN->getType(), HighCost);
+ bool HighCost = Rewriter.isHighCostExpansion(ExitValue, L, Inst);
+ Value *ExitVal =
+ ExpandSCEVIfNeeded(Rewriter, ExitValue, L, Inst, PN->getType());
DEBUG(dbgs() << "INDVARS: RLEV: AfterLoopVal = " << *ExitVal << '\n'
<< " LoopVal = " << *Inst << "\n");
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