[LV] Use VPReductionRecipe for partial reductions #144908
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Partial reductions can easily be represented by the VPReductionRecipe class by setting their scale factor to something greater than 1. This PR merges the two together and gives VPReductionRecipe a VFScaleFactor so that it can choose to generate the partial reduction intrinsic at execute time. This also leads to partial reductions naturally being included in VPMulAccumulateRecipe, which is nice for hiding the cost of the extends and mul, but it does have the side effect of generating an unnecessary extend for chained partial reduction cases. I don't think this can be avoided nicely, and it should be eliminated by DCE anyway.
SamTebbs33
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gbossu,
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ElvisWang123
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@llvm/pr-subscribers-llvm-transforms @llvm/pr-subscribers-vectorizers Author: Sam Tebbs (SamTebbs33) ChangesPartial reductions can easily be represented by the VPReductionRecipe class by setting their scale factor to something greater than 1. This PR merges the two together and gives VPReductionRecipe a VFScaleFactor so that it can choose to generate the partial reduction intrinsic at execute time. This also leads to partial reductions naturally being included in VPMulAccumulateRecipe, which is nice for hiding the cost of the extends and mul, but it does have the side effect of generating an unnecessary extend for chained partial reduction cases due to how those extends are re-generated. I don't think this can be avoided nicely, and it should be eliminated by DCE anyway. Patch is 247.38 KiB, truncated to 20.00 KiB below, full version: https://github.com/llvm/llvm-project/pull/144908.diff 15 Files Affected:
diff --git a/llvm/include/llvm/Analysis/TargetTransformInfo.h b/llvm/include/llvm/Analysis/TargetTransformInfo.h
index 8f4ce80ada5ed..588e58f053cb7 100644
--- a/llvm/include/llvm/Analysis/TargetTransformInfo.h
+++ b/llvm/include/llvm/Analysis/TargetTransformInfo.h
@@ -223,6 +223,8 @@ class TargetTransformInfo {
/// Get the kind of extension that an instruction represents.
LLVM_ABI static PartialReductionExtendKind
getPartialReductionExtendKind(Instruction *I);
+ LLVM_ABI static PartialReductionExtendKind
+ getPartialReductionExtendKind(Instruction::CastOps CastOpc);
/// Construct a TTI object using a type implementing the \c Concept
/// API below.
diff --git a/llvm/lib/Analysis/TargetTransformInfo.cpp b/llvm/lib/Analysis/TargetTransformInfo.cpp
index 2d053e55bdfa9..06f43a4303fa8 100644
--- a/llvm/lib/Analysis/TargetTransformInfo.cpp
+++ b/llvm/lib/Analysis/TargetTransformInfo.cpp
@@ -994,11 +994,22 @@ InstructionCost TargetTransformInfo::getShuffleCost(
}
TargetTransformInfo::PartialReductionExtendKind
-TargetTransformInfo::getPartialReductionExtendKind(Instruction *I) {
- if (isa<SExtInst>(I))
- return PR_SignExtend;
- if (isa<ZExtInst>(I))
+TargetTransformInfo::getPartialReductionExtendKind(
+ Instruction::CastOps CastOpc) {
+ switch (CastOpc) {
+ case Instruction::CastOps::ZExt:
return PR_ZeroExtend;
+ case Instruction::CastOps::SExt:
+ return PR_SignExtend;
+ default:
+ return PR_None;
+ }
+}
+
+TargetTransformInfo::PartialReductionExtendKind
+TargetTransformInfo::getPartialReductionExtendKind(Instruction *I) {
+ if (auto *Cast = dyn_cast<CastInst>(I))
+ return getPartialReductionExtendKind(Cast->getOpcode());
return PR_None;
}
diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index f1470fd1f7314..fff8a856b3248 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -7050,7 +7050,8 @@ static bool planContainsAdditionalSimplifications(VPlan &Plan,
}
// The VPlan-based cost model is more accurate for partial reduction and
// comparing against the legacy cost isn't desirable.
- if (isa<VPPartialReductionRecipe>(&R))
+ if (auto *VPR = dyn_cast<VPReductionRecipe>(&R);
+ VPR && VPR->isPartialReduction())
return true;
/// If a VPlan transform folded a recipe to one producing a single-scalar,
@@ -8278,11 +8279,15 @@ VPRecipeBase *VPRecipeBuilder::tryToCreateWidenRecipe(VPSingleDefRecipe *R,
Phi->getIncomingValueForBlock(OrigLoop->getLoopPreheader()));
// If the PHI is used by a partial reduction, set the scale factor.
- unsigned ScaleFactor =
- getScalingForReduction(RdxDesc.getLoopExitInstr()).value_or(1);
- PhiRecipe = new VPReductionPHIRecipe(
- Phi, RdxDesc, *StartV, CM.isInLoopReduction(Phi),
- CM.useOrderedReductions(RdxDesc), ScaleFactor);
+ bool UseInLoopReduction = CM.isInLoopReduction(Phi);
+ bool UseOrderedReductions = CM.useOrderedReductions(RdxDesc);
+ auto ScaleFactor = ElementCount::getFixed(
+ (UseOrderedReductions || UseInLoopReduction)
+ ? 0
+ : getScalingForReduction(RdxDesc.getLoopExitInstr()).value_or(1));
+ PhiRecipe = new VPReductionPHIRecipe(Phi, RdxDesc, *StartV,
+ CM.isInLoopReduction(Phi),
+ UseOrderedReductions, ScaleFactor);
} else {
// TODO: Currently fixed-order recurrences are modeled as chains of
// first-order recurrences. If there are no users of the intermediate
@@ -8315,7 +8320,8 @@ VPRecipeBase *VPRecipeBuilder::tryToCreateWidenRecipe(VPSingleDefRecipe *R,
return tryToWidenMemory(Instr, Operands, Range);
if (std::optional<unsigned> ScaleFactor = getScalingForReduction(Instr))
- return tryToCreatePartialReduction(Instr, Operands, ScaleFactor.value());
+ return tryToCreatePartialReduction(
+ Instr, Operands, ElementCount::getFixed(ScaleFactor.value()));
if (!shouldWiden(Instr, Range))
return nullptr;
@@ -8338,7 +8344,7 @@ VPRecipeBase *VPRecipeBuilder::tryToCreateWidenRecipe(VPSingleDefRecipe *R,
VPRecipeBase *
VPRecipeBuilder::tryToCreatePartialReduction(Instruction *Reduction,
ArrayRef<VPValue *> Operands,
- unsigned ScaleFactor) {
+ ElementCount ScaleFactor) {
assert(Operands.size() == 2 &&
"Unexpected number of operands for partial reduction");
@@ -8346,7 +8352,8 @@ VPRecipeBuilder::tryToCreatePartialReduction(Instruction *Reduction,
VPValue *Accumulator = Operands[1];
VPRecipeBase *BinOpRecipe = BinOp->getDefiningRecipe();
if (isa<VPReductionPHIRecipe>(BinOpRecipe) ||
- isa<VPPartialReductionRecipe>(BinOpRecipe))
+ (isa<VPReductionRecipe>(BinOpRecipe) &&
+ cast<VPReductionRecipe>(BinOpRecipe)->isPartialReduction()))
std::swap(BinOp, Accumulator);
unsigned ReductionOpcode = Reduction->getOpcode();
@@ -8367,12 +8374,10 @@ VPRecipeBuilder::tryToCreatePartialReduction(Instruction *Reduction,
"Expected an ADD or SUB operation for predicated partial "
"reductions (because the neutral element in the mask is zero)!");
Cond = getBlockInMask(Builder.getInsertBlock());
- VPValue *Zero =
- Plan.getOrAddLiveIn(ConstantInt::get(Reduction->getType(), 0));
- BinOp = Builder.createSelect(Cond, BinOp, Zero, Reduction->getDebugLoc());
}
- return new VPPartialReductionRecipe(ReductionOpcode, Accumulator, BinOp, Cond,
- ScaleFactor, Reduction);
+
+ return new VPReductionRecipe(RecurKind::Add, FastMathFlags(), Reduction,
+ Accumulator, BinOp, Cond, false, ScaleFactor);
}
void LoopVectorizationPlanner::buildVPlansWithVPRecipes(ElementCount MinVF,
@@ -9139,9 +9144,12 @@ void LoopVectorizationPlanner::adjustRecipesForReductions(
FastMathFlags FMFs = isa<FPMathOperator>(CurrentLinkI)
? RdxDesc.getFastMathFlags()
: FastMathFlags();
+ bool UseOrderedReductions = CM.useOrderedReductions(RdxDesc);
+ ElementCount VFScaleFactor =
+ ElementCount::getFixed(!UseOrderedReductions);
auto *RedRecipe = new VPReductionRecipe(
Kind, FMFs, CurrentLinkI, PreviousLink, VecOp, CondOp,
- CM.useOrderedReductions(RdxDesc), CurrentLinkI->getDebugLoc());
+ UseOrderedReductions, VFScaleFactor, CurrentLinkI->getDebugLoc());
// Append the recipe to the end of the VPBasicBlock because we need to
// ensure that it comes after all of it's inputs, including CondOp.
// Delete CurrentLink as it will be invalid if its operand is replaced
@@ -9175,8 +9183,9 @@ void LoopVectorizationPlanner::adjustRecipesForReductions(
// Don't output selects for partial reductions because they have an output
// with fewer lanes than the VF. So the operands of the select would have
// different numbers of lanes. Partial reductions mask the input instead.
+ auto *RR = dyn_cast<VPReductionRecipe>(OrigExitingVPV->getDefiningRecipe());
if (!PhiR->isInLoop() && CM.foldTailByMasking() &&
- !isa<VPPartialReductionRecipe>(OrigExitingVPV->getDefiningRecipe())) {
+ (!RR || !RR->isPartialReduction())) {
VPValue *Cond = RecipeBuilder.getBlockInMask(PhiR->getParent());
std::optional<FastMathFlags> FMFs =
PhiTy->isFloatingPointTy()
diff --git a/llvm/lib/Transforms/Vectorize/VPRecipeBuilder.h b/llvm/lib/Transforms/Vectorize/VPRecipeBuilder.h
index 8369c78a2d78f..ac345a0d020e1 100644
--- a/llvm/lib/Transforms/Vectorize/VPRecipeBuilder.h
+++ b/llvm/lib/Transforms/Vectorize/VPRecipeBuilder.h
@@ -172,7 +172,7 @@ class VPRecipeBuilder {
/// along with binary operation and reduction phi operands.
VPRecipeBase *tryToCreatePartialReduction(Instruction *Reduction,
ArrayRef<VPValue *> Operands,
- unsigned ScaleFactor);
+ ElementCount ScaleFactor);
/// Set the recipe created for given ingredient.
void setRecipe(Instruction *I, VPRecipeBase *R) {
diff --git a/llvm/lib/Transforms/Vectorize/VPlan.h b/llvm/lib/Transforms/Vectorize/VPlan.h
index 5a3c4a514a5dd..9e38771e746df 100644
--- a/llvm/lib/Transforms/Vectorize/VPlan.h
+++ b/llvm/lib/Transforms/Vectorize/VPlan.h
@@ -553,7 +553,6 @@ class VPSingleDefRecipe : public VPRecipeBase, public VPValue {
case VPRecipeBase::VPWidenIntOrFpInductionSC:
case VPRecipeBase::VPWidenPointerInductionSC:
case VPRecipeBase::VPReductionPHISC:
- case VPRecipeBase::VPPartialReductionSC:
return true;
case VPRecipeBase::VPBranchOnMaskSC:
case VPRecipeBase::VPInterleaveSC:
@@ -2189,18 +2188,21 @@ class VPReductionPHIRecipe : public VPHeaderPHIRecipe,
/// When expanding the reduction PHI, the plan's VF element count is divided
/// by this factor to form the reduction phi's VF.
- unsigned VFScaleFactor = 1;
+ ElementCount VFScaleFactor;
public:
/// Create a new VPReductionPHIRecipe for the reduction \p Phi described by \p
/// RdxDesc.
VPReductionPHIRecipe(PHINode *Phi, const RecurrenceDescriptor &RdxDesc,
VPValue &Start, bool IsInLoop = false,
- bool IsOrdered = false, unsigned VFScaleFactor = 1)
+ bool IsOrdered = false,
+ ElementCount VFScaleFactor = ElementCount::getFixed(1))
: VPHeaderPHIRecipe(VPDef::VPReductionPHISC, Phi, &Start),
RdxDesc(RdxDesc), IsInLoop(IsInLoop), IsOrdered(IsOrdered),
VFScaleFactor(VFScaleFactor) {
assert((!IsOrdered || IsInLoop) && "IsOrdered requires IsInLoop");
+ assert(((!IsInLoop && !IsOrdered) || VFScaleFactor.isZero()) &&
+ "Invalid VFScaleFactor");
}
~VPReductionPHIRecipe() override = default;
@@ -2219,7 +2221,7 @@ class VPReductionPHIRecipe : public VPHeaderPHIRecipe,
void execute(VPTransformState &State) override;
/// Get the factor that the VF of this recipe's output should be scaled by.
- unsigned getVFScaleFactor() const { return VFScaleFactor; }
+ ElementCount getVFScaleFactor() const { return VFScaleFactor; }
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
/// Print the recipe.
@@ -2237,6 +2239,10 @@ class VPReductionPHIRecipe : public VPHeaderPHIRecipe,
/// Returns true, if the phi is part of an in-loop reduction.
bool isInLoop() const { return IsInLoop; }
+ bool isPartialReduction() const {
+ return ElementCount::isKnownGT(VFScaleFactor, ElementCount::getFixed(1));
+ }
+
/// Returns true if the recipe only uses the first lane of operand \p Op.
bool onlyFirstLaneUsed(const VPValue *Op) const override {
assert(is_contained(operands(), Op) &&
@@ -2408,23 +2414,32 @@ class VPInterleaveRecipe : public VPRecipeBase {
Instruction *getInsertPos() const { return IG->getInsertPos(); }
};
-/// A recipe to represent inloop reduction operations, performing a reduction on
-/// a vector operand into a scalar value, and adding the result to a chain.
-/// The Operands are {ChainOp, VecOp, [Condition]}.
+/// A recipe to represent inloop, ordered or partial reduction operations. It
+/// performs a reduction on a vector operand into a scalar (vector in the case
+/// of a partial reduction) value, and adds the result to a chain. The Operands
+/// are {ChainOp, VecOp, [Condition]}.
class VPReductionRecipe : public VPRecipeWithIRFlags {
/// The recurrence kind for the reduction in question.
RecurKind RdxKind;
bool IsOrdered;
/// Whether the reduction is conditional.
bool IsConditional = false;
+ /// The scaling factor, relative to the VF, that this recipe's output is
+ /// divided by.
+ /// For outer-loop reductions this is equal to 1.
+ /// For in-loop reductions this is equal to 0, to specify that this is equal
+ /// to the VF (which may not be known yet). For partial-reductions this is
+ /// equal to another scalar value.
+ ElementCount VFScaleFactor;
protected:
VPReductionRecipe(const unsigned char SC, RecurKind RdxKind,
FastMathFlags FMFs, Instruction *I,
ArrayRef<VPValue *> Operands, VPValue *CondOp,
- bool IsOrdered, DebugLoc DL)
+ bool IsOrdered, ElementCount VFScaleFactor, DebugLoc DL)
: VPRecipeWithIRFlags(SC, Operands, FMFs, DL), RdxKind(RdxKind),
- IsOrdered(IsOrdered) {
+ IsOrdered(IsOrdered), VFScaleFactor(VFScaleFactor) {
+ assert((!IsOrdered || VFScaleFactor.isZero()) && "Invalid scale factor");
if (CondOp) {
IsConditional = true;
addOperand(CondOp);
@@ -2436,9 +2451,11 @@ class VPReductionRecipe : public VPRecipeWithIRFlags {
/// Note that the debug location is from the extend.
VPReductionRecipe(const unsigned char SC, const RecurKind RdxKind,
ArrayRef<VPValue *> Operands, VPValue *CondOp,
- bool IsOrdered, DebugLoc DL)
+ bool IsOrdered, ElementCount VFScaleFactor, DebugLoc DL)
: VPRecipeWithIRFlags(SC, Operands, DL), RdxKind(RdxKind),
- IsOrdered(IsOrdered), IsConditional(CondOp) {
+ IsOrdered(IsOrdered), IsConditional(CondOp),
+ VFScaleFactor(VFScaleFactor) {
+ assert((!IsOrdered || VFScaleFactor.isZero()) && "Invalid scale factor");
if (CondOp)
addOperand(CondOp);
}
@@ -2447,9 +2464,12 @@ class VPReductionRecipe : public VPRecipeWithIRFlags {
/// Note that the NUW/NSW flags and the debug location are from the Mul.
VPReductionRecipe(const unsigned char SC, const RecurKind RdxKind,
ArrayRef<VPValue *> Operands, VPValue *CondOp,
- bool IsOrdered, WrapFlagsTy WrapFlags, DebugLoc DL)
+ bool IsOrdered, ElementCount VFScaleFactor,
+ WrapFlagsTy WrapFlags, DebugLoc DL)
: VPRecipeWithIRFlags(SC, Operands, WrapFlags, DL), RdxKind(RdxKind),
- IsOrdered(IsOrdered), IsConditional(CondOp) {
+ IsOrdered(IsOrdered), IsConditional(CondOp),
+ VFScaleFactor(VFScaleFactor) {
+ assert((!IsOrdered || VFScaleFactor.isZero()) && "Invalid scale factor");
if (CondOp)
addOperand(CondOp);
}
@@ -2457,24 +2477,26 @@ class VPReductionRecipe : public VPRecipeWithIRFlags {
public:
VPReductionRecipe(RecurKind RdxKind, FastMathFlags FMFs, Instruction *I,
VPValue *ChainOp, VPValue *VecOp, VPValue *CondOp,
- bool IsOrdered, DebugLoc DL = {})
+ bool IsOrdered, ElementCount VFScaleFactor,
+ DebugLoc DL = {})
: VPReductionRecipe(VPDef::VPReductionSC, RdxKind, FMFs, I,
ArrayRef<VPValue *>({ChainOp, VecOp}), CondOp,
- IsOrdered, DL) {}
+ IsOrdered, VFScaleFactor, DL) {}
VPReductionRecipe(const RecurKind RdxKind, FastMathFlags FMFs,
VPValue *ChainOp, VPValue *VecOp, VPValue *CondOp,
- bool IsOrdered, DebugLoc DL = {})
+ bool IsOrdered, ElementCount VFScaleFactor,
+ DebugLoc DL = {})
: VPReductionRecipe(VPDef::VPReductionSC, RdxKind, FMFs, nullptr,
ArrayRef<VPValue *>({ChainOp, VecOp}), CondOp,
- IsOrdered, DL) {}
+ IsOrdered, VFScaleFactor, DL) {}
~VPReductionRecipe() override = default;
VPReductionRecipe *clone() override {
- return new VPReductionRecipe(RdxKind, getFastMathFlags(),
- getUnderlyingInstr(), getChainOp(), getVecOp(),
- getCondOp(), IsOrdered, getDebugLoc());
+ return new VPReductionRecipe(
+ RdxKind, getFastMathFlags(), getUnderlyingInstr(), getChainOp(),
+ getVecOp(), getCondOp(), IsOrdered, VFScaleFactor, getDebugLoc());
}
static inline bool classof(const VPRecipeBase *R) {
@@ -2508,6 +2530,10 @@ class VPReductionRecipe : public VPRecipeWithIRFlags {
bool isOrdered() const { return IsOrdered; };
/// Return true if the in-loop reduction is conditional.
bool isConditional() const { return IsConditional; };
+ /// Return true if the reduction is a partial reduction.
+ bool isPartialReduction() const {
+ return ElementCount::isKnownGT(VFScaleFactor, ElementCount::getFixed(1));
+ }
/// The VPValue of the scalar Chain being accumulated.
VPValue *getChainOp() const { return getOperand(0); }
/// The VPValue of the vector value to be reduced.
@@ -2516,65 +2542,8 @@ class VPReductionRecipe : public VPRecipeWithIRFlags {
VPValue *getCondOp() const {
return isConditional() ? getOperand(getNumOperands() - 1) : nullptr;
}
-};
-
-/// A recipe for forming partial reductions. In the loop, an accumulator and
-/// vector operand are added together and passed to the next iteration as the
-/// next accumulator. After the loop body, the accumulator is reduced to a
-/// scalar value.
-class VPPartialReductionRecipe : public VPReductionRecipe {
- unsigned Opcode;
-
- /// The divisor by which the VF of this recipe's output should be divided
- /// during execution.
- unsigned VFScaleFactor;
-
-public:
- VPPartialReductionRecipe(Instruction *ReductionInst, VPValue *Op0,
- VPValue *Op1, VPValue *Cond, unsigned VFScaleFactor)
- : VPPartialReductionRecipe(ReductionInst->getOpcode(), Op0, Op1, Cond,
- VFScaleFactor, ReductionInst) {}
- VPPartialReductionRecipe(unsigned Opcode, VPValue *Op0, VPValue *Op1,
- VPValue *Cond, unsigned ScaleFactor,
- Instruction *ReductionInst = nullptr)
- : VPReductionRecipe(VPDef::VPPartialReductionSC, RecurKind::Add,
- FastMathFlags(), ReductionInst,
- ArrayRef<VPValue *>({Op0, Op1}), Cond, false, {}),
- Opcode(Opcode), VFScaleFactor(ScaleFactor) {
- [[maybe_unused]] auto *AccumulatorRecipe =
- getChainOp()->getDefiningRecipe();
- assert((isa<VPReductionPHIRecipe>(AccumulatorRecipe) ||
- isa<VPPartialReductionRecipe>(AccumulatorRecipe)) &&
- "Unexpected operand order for partial reduction recipe");
- }
- ~VPPartialReductionRecipe() override = default;
-
- VPPartialReductionRecipe *clone() override {
- return new VPPartialReductionRecipe(Opcode, getOperand(0), getOperand(1),
- getCondOp(), VFScaleFactor,
- getUnderlyingInstr());
- }
-
- VP_CLASSOF_IMPL(VPDef::VPPartialReductionSC)
-
- /// Generate the reduction in the loop.
- void execute(VPTransformState &State) override;
-
- /// Return the cost of this VPPartialReductionRecipe.
- InstructionCost computeCost(ElementCount VF,
- VPCostContext &Ctx) const override;
-
- /// Get the binary op's opcode.
- unsigned getOpcode() const { return Opcode; }
-
/// Get the factor that the VF of this recipe's output should be scaled by.
- unsigned getVFScaleFactor() const { return VFScaleFactor; }
-
-#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
- /// Print the recipe.
- void print(raw_ostream &O, const Twine &Indent,
- VPSlotTracker &SlotTracker) const override;
-#endif
+ ElementCount getVFScaleFactor() const { return VFScaleFactor; }
};
/// A recipe to represent inloop reduction operations with vector-predication
@@ -2590,7 +2559,7 @@ class VPReductionEVLRecipe : public VPReductionRecipe {
R.getFastMathFlags(),
cast_or_null<Instruction>(R.getUnderlyingValue()),
ArrayRef<VPValue *>({R.getChainOp(), R.getVecOp(), &EVL}), CondOp,
- R.isOrdered(), DL) {}
+ R.isOrdered(), ElementCount::getFixed(0), DL) {}
~VPReductionEVLRecipe() override = default;
@@ -2634,10 +2603,11 @@ class VPExtendedReductionRecipe : public VPReductionRecipe {
/// For cloning VPExtendedReductionRecipe.
VPExtendedReductionRecipe(VPExtendedReductionRecipe *ExtRed)
- ...
[truncated]
|
This was referenced Jun 19, 2025
lukel97
reviewed
Jun 19, 2025
| class VPReductionRecipe : public VPRecipeWithIRFlags { | ||
| /// The recurrence kind for the reduction in question. | ||
| RecurKind RdxKind; | ||
| bool IsOrdered; | ||
| /// Whether the reduction is conditional. | ||
| bool IsConditional = false; | ||
| /// The scaling factor, relative to the VF, that this recipe's output is | ||
| /// divided by. | ||
| /// For outer-loop reductions this is equal to 1. |
There was a problem hiding this comment.
Aren't VPReductionRecipes for in-loop reductions only? Should this say for unordered reductions?
I'm also a bit confused since I thought both unordered and ordered reductions also produced a scalar result, so the VFScaleFactor would be always be 1 for them?
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Partial reductions can easily be represented by the VPReductionRecipe class by setting their scale factor to something greater than 1. This PR merges the two together and gives VPReductionRecipe a VFScaleFactor so that it can choose to generate the partial reduction intrinsic at execute time.
This also leads to partial reductions naturally being included in VPMulAccumulateRecipe, which is nice for hiding the cost of the extends and mul, but it does have the side effect of generating an unnecessary extend for chained partial reduction cases due to how those extends are re-generated. I don't think this can be avoided nicely, and it should be eliminated by DCE anyway.