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[SCEV] Simplify SCEVExpr for PHI to SCEV for operand if operands are identical #115945

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Dec 6, 2024
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[SCEV] Simplify SCEVExpr for PHI to SCEV for operand if operands are identical #115945

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[SCEV] Simplify SCEVExpr for PHI to SCEV for operand if operands are …
akshayrdeodhar Oct 30, 2024
24e4262
address review comments
akshayrdeodhar Nov 15, 2024
bd8642d
address review comments, compare SCEVExprs instead of instructions
akshayrdeodhar Nov 25, 2024
7c43e64
explicitly check equality of SCEVs
akshayrdeodhar Nov 26, 2024
231503f
comments
akshayrdeodhar Nov 26, 2024
ea17ced
add test, address comments
akshayrdeodhar Nov 27, 2024
68c8213
clang-format
akshayrdeodhar Nov 27, 2024
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4 changes: 4 additions & 0 deletions llvm/include/llvm/Analysis/ScalarEvolution.h
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Expand Up @@ -1761,6 +1761,10 @@ class ScalarEvolution {
/// V.
const SCEV *getOperandsToCreate(Value *V, SmallVectorImpl<Value *> &Ops);

/// Returns SCEV for the first operand of a phi if all phi operands have
/// identical opcodes and operands
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const SCEV *createNodeForPHIWithIdenticalOperands(PHINode *PN);

/// Provide the special handling we need to analyze PHI SCEVs.
const SCEV *createNodeForPHI(PHINode *PN);

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34 changes: 34 additions & 0 deletions llvm/lib/Analysis/ScalarEvolution.cpp
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Expand Up @@ -6019,6 +6019,37 @@ const SCEV *ScalarEvolution::createNodeFromSelectLikePHI(PHINode *PN) {
return nullptr;
}

// Returns SCEV for the first operand of a phi if all phi operands have
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// identical opcodes and operands
// eg.
// a: %add = %a + %b
// br %c
// b: %add1 = %a + %b
// br %c
// c: %phi = phi [%add, a], [%add1, b]
// scev(%phi) => scev(%add)
const SCEV *
ScalarEvolution::createNodeForPHIWithIdenticalOperands(PHINode *PN) {
BinaryOperator *CommonInst = nullptr;
for (Value *Incoming : PN->incoming_values()) {
BinaryOperator *IncomingInst = dyn_cast<BinaryOperator>(Incoming);
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if (CommonInst) {
if (!(IncomingInst && CommonInst->isIdenticalTo(IncomingInst))) {
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Should we check that all incoming instructions belong to the same loop?

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@akshayrdeodhar akshayrdeodhar Nov 15, 2024 •
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If the operands of the phi are identical, then the simplification is correct regardless of whether instructions belong to the same loop- I think there's no advantage to special-casing this.

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Ping?

// Not identical, give up
CommonInst = nullptr;
break;
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}
} else if (IncomingInst) {
// Remember binary operator
CommonInst = IncomingInst;
} else {
// Not a binary operator, give up
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return nullptr;
}
}
return CommonInst ? getSCEV(CommonInst) : nullptr;
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This looks dangerous to me. Even if the instructions are the same, I think that SCEV would be allowed to use context-sensitive reasoning when constructing the SCEV node. It would be safer to call getSCEV for each incoming value in a second loop and make sure they're all the same.

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Using a single loop with calls to getSCEV.

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I expect that you do need both loops -- not for correctness, but as a profitability heuristic. Computing SCEVs is expensive, and always recursing through phis would likely add significant cost.

I tried to confirm this but the stage2 build crashes (https://llvm-compile-time-tracker.com/show_error.php?commit=a4e3a0e648d7a3664ca6269e846abf2e6ddcdb08). I didn't investigate, but it might be that the recursion causes a stack overflow.

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Restored the original loop, and added an all_of check for SCEV exprs of the incoming values being identical.

}

const SCEV *ScalarEvolution::createNodeForPHI(PHINode *PN) {
if (const SCEV *S = createAddRecFromPHI(PN))
return S;
Expand All @@ -6030,6 +6061,9 @@ const SCEV *ScalarEvolution::createNodeForPHI(PHINode *PN) {
/*UseInstrInfo=*/true, /*CanUseUndef=*/false}))
return getSCEV(V);

if (const SCEV *S = createNodeForPHIWithIdenticalOperands(PN))
return S;

if (const SCEV *S = createNodeFromSelectLikePHI(PN))
return S;

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46 changes: 46 additions & 0 deletions llvm/test/Analysis/ScalarEvolution/trip-count16.ll
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Original file line number Diff line number Diff line change
@@ -0,0 +1,46 @@
; NOTE: Assertions have been autogenerated by utils/update_analyze_test_checks.py UTC_ARGS: --version 5
; RUN: opt < %s -disable-output "-passes=print<scalar-evolution>" 2>&1 | FileCheck %s
define void @test(ptr %x, ptr %y) {
; CHECK-LABEL: 'test'
; CHECK-NEXT: Classifying expressions for: @test
; CHECK-NEXT: %v1.0 = phi i32 [ 0, %entry ], [ %k.0, %if.end ]
; CHECK-NEXT: --> {0,+,1}<nuw><nsw><%for.cond> U: [0,7) S: [0,7) Exits: 6 LoopDispositions: { %for.cond: Computable }
; CHECK-NEXT: %add = add nsw i32 %v1.0, 1
; CHECK-NEXT: --> {1,+,1}<nuw><nsw><%for.cond> U: [1,8) S: [1,8) Exits: 7 LoopDispositions: { %for.cond: Computable }
; CHECK-NEXT: %add6 = add nsw i32 %v1.0, 1
; CHECK-NEXT: --> {1,+,1}<nuw><nsw><%for.cond> U: [1,8) S: [1,8) Exits: 7 LoopDispositions: { %for.cond: Computable }
; CHECK-NEXT: %k.0 = phi i32 [ %add, %if.then ], [ %add6, %if.else ]
; CHECK-NEXT: --> {1,+,1}<nuw><nsw><%for.cond> U: [1,8) S: [1,8) Exits: 7 LoopDispositions: { %for.cond: Computable }
; CHECK-NEXT: Determining loop execution counts for: @test
; CHECK-NEXT: Loop %for.cond: backedge-taken count is i32 6
; CHECK-NEXT: Loop %for.cond: constant max backedge-taken count is i32 6
; CHECK-NEXT: Loop %for.cond: symbolic max backedge-taken count is i32 6
; CHECK-NEXT: Loop %for.cond: Trip multiple is 7
;
entry:
br label %for.cond

for.cond: ; preds = %6, %0
%v1.0 = phi i32 [ 0, %entry ], [ %k.0, %if.end ]
%cmp = icmp slt i32 %v1.0, 6
br i1 %cmp, label %for.body, label %exit

for.body: ; preds = %1
%cmp3 = icmp slt i32 %v1.0, 2
br i1 %cmp3, label %if.then, label %if.else

if.then: ; preds = %2
%add = add nsw i32 %v1.0, 1
br label %if.end

if.else: ; preds = %2
%add6 = add nsw i32 %v1.0, 1
br label %if.end

if.end: ; preds = %4, %3
%k.0 = phi i32 [ %add, %if.then ], [ %add6, %if.else ]
br label %for.cond

exit: ; preds = %5
ret void
}
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