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

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Helps SCEV analyze some special phi nodes, allowing the computation of
loop trip count in cases like the following:

https://godbolt.org/z/xGs1d81TW
This commit is contained in:
Akshay Deodhar 2024-12-06 15:26:45 +05:30 committed by GitHub
parent 49abcd207f
commit 82c93b6f19
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3 changed files with 135 additions and 0 deletions
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4
llvm/include/llvm/Analysis/ScalarEvolution.h
View File

@ -1780,6 +1780,10 @@ private:
/// 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.
const SCEV *createNodeForPHIWithIdenticalOperands(PHINode *PN);
/// Provide the special handling we need to analyze PHI SCEVs.
const SCEV *createNodeForPHI(PHINode *PN);

39
llvm/lib/Analysis/ScalarEvolution.cpp
View File

@ -6019,6 +6019,42 @@ const SCEV *ScalarEvolution::createNodeFromSelectLikePHI(PHINode *PN) {
return nullptr;
}
/// Returns SCEV for the first operand of a phi if all phi operands have
/// 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;
// Check if instructions are identical.
for (Value *Incoming : PN->incoming_values()) {
auto *IncomingInst = dyn_cast<BinaryOperator>(Incoming);
if (!IncomingInst)
return nullptr;
if (CommonInst) {
if (!CommonInst->isIdenticalToWhenDefined(IncomingInst))
return nullptr; // Not identical, give up
} else {
// Remember binary operator
CommonInst = IncomingInst;
}
}
if (!CommonInst)
return nullptr;
// Check if SCEV exprs for instructions are identical.
const SCEV *CommonSCEV = getSCEV(CommonInst);
bool SCEVExprsIdentical =
all_of(drop_begin(PN->incoming_values()),
[this, CommonSCEV](Value *V) { return CommonSCEV == getSCEV(V); });
return SCEVExprsIdentical ? CommonSCEV : nullptr;
}
const SCEV *ScalarEvolution::createNodeForPHI(PHINode *PN) {
if (const SCEV *S = createAddRecFromPHI(PN))
return S;
@ -6030,6 +6066,9 @@ 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;

92
llvm/test/Analysis/ScalarEvolution/trip-count-phi-increment.ll Normal file
View File

@ -0,0 +1,92 @@
; 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 @test1(ptr %x, ptr %y) {
; CHECK-LABEL: 'test1'
; CHECK-NEXT: Classifying expressions for: @test1
; 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: @test1
; 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
}
define void @test2(ptr %x, ptr %y) {
; CHECK-LABEL: 'test2'
; CHECK-NEXT: Classifying expressions for: @test2
; 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 nuw 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: @test2
; 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 nuw 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
}