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[VPlan] Create resume phis in scalar preheader early. (NFC) #166099

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[VPlan] Create resume phis in scalar preheader early. (NFC) #166099

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5 changes: 4 additions & 1 deletion llvm/lib/Transforms/Vectorize/VPlan.h
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Original file line number Diff line number Diff line change
Expand Up @@ -1134,7 +1134,7 @@ class LLVM_ABI_FOR_TEST VPInstruction : public VPRecipeWithIRFlags,
OpcodeTy Opcode;

/// An optional name that can be used for the generated IR instruction.
const std::string Name;
std::string Name;

/// Returns true if we can generate a scalar for the first lane only if
/// needed.
Expand Down Expand Up @@ -1225,6 +1225,9 @@ class LLVM_ABI_FOR_TEST VPInstruction : public VPRecipeWithIRFlags,
/// Returns the symbolic name assigned to the VPInstruction.
StringRef getName() const { return Name; }

/// Set the symbolic name assigned to the VPinstruction.
void setName(StringRef NewName) { Name = NewName.str(); }

protected:
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
/// Print the VPInstruction to \p O.
Expand Down
9 changes: 9 additions & 0 deletions llvm/lib/Transforms/Vectorize/VPlanConstruction.cpp
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Expand Up @@ -554,6 +554,15 @@ static void addInitialSkeleton(VPlan &Plan, Type *InductionTy, DebugLoc IVDL,
Plan.getEntry()->swapSuccessors();

createExtractsForLiveOuts(Plan, MiddleVPBB);

VPBuilder ScalarPHBuilder(ScalarPH);
for (const auto &[PhiR, ScalarPhiR] : zip_equal(
drop_begin(HeaderVPBB->phis()), Plan.getScalarHeader()->phis())) {
auto *VectorPhiR = cast<VPPhi>(&PhiR);
auto *ResumePhiR = ScalarPHBuilder.createScalarPhi(
{VectorPhiR, VectorPhiR->getOperand(0)}, VectorPhiR->getDebugLoc());
cast<VPIRPhi>(&ScalarPhiR)->addOperand(ResumePhiR);
}
}

std::unique_ptr<VPlan>
Expand Down
36 changes: 15 additions & 21 deletions llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
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Expand Up @@ -4527,9 +4527,10 @@ void VPlanTransforms::addBranchWeightToMiddleTerminator(
/// Create and return a ResumePhi for \p WideIV, unless it is truncated. If the
/// induction recipe is not canonical, creates a VPDerivedIVRecipe to compute
/// the end value of the induction.
static VPInstruction *addResumePhiRecipeForInduction(
VPWidenInductionRecipe *WideIV, VPBuilder &VectorPHBuilder,
VPBuilder &ScalarPHBuilder, VPTypeAnalysis &TypeInfo, VPValue *VectorTC) {
static VPValue *addResumePhiRecipeForInduction(VPWidenInductionRecipe *WideIV,
VPBuilder &VectorPHBuilder,
VPTypeAnalysis &TypeInfo,
VPValue *VectorTC) {
auto *WideIntOrFp = dyn_cast<VPWidenIntOrFpInductionRecipe>(WideIV);
// Truncated wide inductions resume from the last lane of their vector value
// in the last vector iteration which is handled elsewhere.
Expand All @@ -4555,9 +4556,7 @@ static VPInstruction *addResumePhiRecipeForInduction(
WideIV->getDebugLoc());
}

auto *ResumePhiRecipe = ScalarPHBuilder.createScalarPhi(
{EndValue, Start}, WideIV->getDebugLoc(), "bc.resume.val");
return ResumePhiRecipe;
return EndValue;
}

void VPlanTransforms::addScalarResumePhis(
Expand All @@ -4570,21 +4569,18 @@ void VPlanTransforms::addScalarResumePhis(
VPBuilder VectorPHBuilder(
cast<VPBasicBlock>(VectorRegion->getSinglePredecessor()));
VPBuilder MiddleBuilder(MiddleVPBB, MiddleVPBB->getFirstNonPhi());
VPBuilder ScalarPHBuilder(ScalarPH);
for (VPRecipeBase &ScalarPhiR : Plan.getScalarHeader()->phis()) {
auto *ScalarPhiIRI = cast<VPIRPhi>(&ScalarPhiR);
for (VPRecipeBase &PhiR : Plan.getScalarPreheader()->phis()) {
auto *ResumePhiR = cast<VPPhi>(&PhiR);

// TODO: Extract final value from induction recipe initially, optimize to
// pre-computed end value together in optimizeInductionExitUsers.
auto *VectorPhiR =
cast<VPHeaderPHIRecipe>(Builder.getRecipe(&ScalarPhiIRI->getIRPhi()));
Comment on lines -4579 to -4580
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@fhahn fhahn Nov 11, 2025

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Removes one of the few remaining instances that retrieve recipes via IR referneces.

auto *VectorPhiR = cast<VPHeaderPHIRecipe>(ResumePhiR->getOperand(0));
if (auto *WideIVR = dyn_cast<VPWidenInductionRecipe>(VectorPhiR)) {
if (VPInstruction *ResumePhi = addResumePhiRecipeForInduction(
WideIVR, VectorPHBuilder, ScalarPHBuilder, TypeInfo,
&Plan.getVectorTripCount())) {
assert(isa<VPPhi>(ResumePhi) && "Expected a phi");
IVEndValues[WideIVR] = ResumePhi->getOperand(0);
ScalarPhiIRI->addOperand(ResumePhi);
if (VPValue *ResumeV = addResumePhiRecipeForInduction(
WideIVR, VectorPHBuilder, TypeInfo, &Plan.getVectorTripCount())) {
IVEndValues[WideIVR] = ResumeV;
ResumePhiR->setOperand(0, ResumeV);
ResumePhiR->setName("bc.resume.val");
continue;
}
// TODO: Also handle truncated inductions here. Computing end-values
Expand All @@ -4606,10 +4602,8 @@ void VPlanTransforms::addScalarResumePhis(
ResumeFromVectorLoop = MiddleBuilder.createNaryOp(
VPInstruction::ExtractLastElement, {ResumeFromVectorLoop}, {},
"vector.recur.extract");
StringRef Name = IsFOR ? "scalar.recur.init" : "bc.merge.rdx";
auto *ResumePhiR = ScalarPHBuilder.createScalarPhi(
{ResumeFromVectorLoop, VectorPhiR->getStartValue()}, {}, Name);
ScalarPhiIRI->addOperand(ResumePhiR);
ResumePhiR->setName(IsFOR ? "scalar.recur.init" : "bc.merge.rdx");
ResumePhiR->setOperand(0, ResumeFromVectorLoop);
}
}

Expand Down
6 changes: 4 additions & 2 deletions llvm/unittests/Transforms/Vectorize/VPlanHCFGTest.cpp
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Expand Up @@ -113,12 +113,13 @@ compound=true
N0 -> N2 [ label="F"]
N1 [label =
"scalar.ph:\l" +
" EMIT-SCALAR vp\<%6\> = phi [ ir\<%indvars.iv\>, middle.block ], [ ir\<0\>, ir-bb\<entry\> ]\l" +
"Successor(s): ir-bb\<for.body\>\l"
]
N1 -> N3 [ label=""]
N3 [label =
"ir-bb\<for.body\>:\l" +
" IR %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]\l" +
" IR %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ] (extra operand: vp\<%6\> from scalar.ph)\l" +
" IR %arr.idx = getelementptr inbounds i32, ptr %A, i64 %indvars.iv\l" +
" IR %l1 = load i32, ptr %arr.idx, align 4\l" +
" IR %res = add i32 %l1, 10\l" +
Expand Down Expand Up @@ -282,12 +283,13 @@ compound=true
N0 -> N2 [ label="F"]
N1 [label =
"scalar.ph:\l" +
" EMIT-SCALAR vp\<%6\> = phi [ ir\<%iv\>, middle.block ], [ ir\<0\>, ir-bb\<entry\> ]\l" +
"Successor(s): ir-bb\<loop.header\>\l"
]
N1 -> N3 [ label=""]
N3 [label =
"ir-bb\<loop.header\>:\l" +
" IR %iv = phi i64 [ 0, %entry ], [ %iv.next, %loop.latch ]\l" +
" IR %iv = phi i64 [ 0, %entry ], [ %iv.next, %loop.latch ] (extra operand: vp\<%6\> from scalar.ph)\l" +
" IR %arr.idx = getelementptr inbounds i32, ptr %A, i64 %iv\l" +
" IR %l1 = load i32, ptr %arr.idx, align 4\l" +
" IR %c = icmp eq i32 %l1, 0\l" +
Expand Down
4 changes: 2 additions & 2 deletions llvm/unittests/Transforms/Vectorize/VPlanVerifierTest.cpp
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Expand Up @@ -346,6 +346,8 @@ TEST_F(VPIRVerifierTest, testVerifyIRPhiInScalarHeaderVPIRBB) {
Function *F = M.getFunction("f");
BasicBlock *LoopHeader = F->getEntryBlock().getSingleSuccessor();
auto Plan = buildVPlan(LoopHeader);
VPValue *Zero = Plan->getConstantInt(32, 0);
Plan->getScalarHeader()->front().addOperand(Zero);
Comment on lines +349 to +350
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@fhahn fhahn Nov 4, 2025

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The initial plan now already has the operands of the scalar header phis set up correctly, so here we now add anothe operand to trigger a mismatch between #incoming values and #predecessors


#if GTEST_HAS_STREAM_REDIRECTION
::testing::internal::CaptureStderr();
Expand Down Expand Up @@ -387,8 +389,6 @@ TEST_F(VPIRVerifierTest, testVerifyIRPhiInExitVPIRBB) {
{HeaderBlock->front().getVPSingleValue()});
DefI->insertBefore(Plan->getMiddleBlock()->getTerminator());
Plan->getExitBlocks()[0]->front().addOperand(DefI);
VPValue *Zero = Plan->getConstantInt(32, 0);
Plan->getScalarHeader()->front().addOperand(Zero);
Comment on lines -390 to -391
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@fhahn fhahn Nov 4, 2025

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The initial plan now already has the operands of the scalar header phis set up correctly, so this change is not needed any more to fix them up.


#if GTEST_HAS_STREAM_REDIRECTION
::testing::internal::CaptureStderr();
Expand Down