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[DAG] Handle truncated splat in isBoolConstant #145473

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[DAG] Handle truncated splat in isBoolConstant #145473

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ed89e3b
[DAG] Handle truncated splat in isBoolConstant
davemgreen Jun 24, 2025
d84d9de
Update test and add comments
davemgreen Jun 24, 2025
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3 changes: 1 addition & 2 deletions llvm/include/llvm/CodeGen/SelectionDAG.h
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Expand Up @@ -2479,8 +2479,7 @@ class SelectionDAG {

/// Check if a value \op N is a constant using the target's BooleanContent for
/// its type.
LLVM_ABI std::optional<bool>
isBoolConstant(SDValue N, bool AllowTruncation = false) const;
LLVM_ABI std::optional<bool> isBoolConstant(SDValue N) const;

/// Set CallSiteInfo to be associated with Node.
void addCallSiteInfo(const SDNode *Node, CallSiteInfo &&CallInfo) {
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5 changes: 5 additions & 0 deletions llvm/include/llvm/CodeGen/TargetLowering.h
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Expand Up @@ -4375,6 +4375,11 @@ class LLVM_ABI TargetLowering : public TargetLoweringBase {
Op.getOpcode() == ISD::SPLAT_VECTOR_PARTS;
}

/// Return true if the given select/vselect should be considered canonical and
/// not be transformed. Currently only used for "vselect (not Cond), N1, N2 ->
/// vselect Cond, N2, N1".
virtual bool isTargetCanonicalSelect(SDNode *N) const { return false; }
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(style) add a description of what a canonical select means

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This seems like a very specific hack, can you do this another way

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We could try moving the AVX512 fold to X86DAGToDAGISel / isel patterns? @phoebewang WDYT?

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X86DAGToDAGISel sounds good to me. We have hundreds of AVX512 instructions, adding patterns for them is too verbose.

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@phoebewang I've created #145724 to investigate this - I'm not yet sure how much yak shaving will be necessary to address the remaining regressions

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It looks to me like there are only broadcasting missing except for a scalar case?


struct DAGCombinerInfo {
void *DC; // The DAG Combiner object.
CombineLevel Level;
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5 changes: 3 additions & 2 deletions llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
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Expand Up @@ -12967,8 +12967,9 @@ SDValue DAGCombiner::visitVSELECT(SDNode *N) {
return V;

// vselect (not Cond), N1, N2 -> vselect Cond, N2, N1
if (SDValue F = extractBooleanFlip(N0, DAG, TLI, false))
return DAG.getSelect(DL, VT, F, N2, N1);
if (!TLI.isTargetCanonicalSelect(N))
if (SDValue F = extractBooleanFlip(N0, DAG, TLI, false))
return DAG.getSelect(DL, VT, F, N2, N1);

// select (sext m), (add X, C), X --> (add X, (and C, (sext m))))
if (N1.getOpcode() == ISD::ADD && N1.getOperand(0) == N2 && N1->hasOneUse() &&
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11 changes: 6 additions & 5 deletions llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
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Expand Up @@ -10349,7 +10349,7 @@ SDValue SelectionDAG::simplifySelect(SDValue Cond, SDValue T, SDValue F) {

// select true, T, F --> T
// select false, T, F --> F
if (auto C = isBoolConstant(Cond, /*AllowTruncation=*/true))
if (auto C = isBoolConstant(Cond))
return *C ? T : F;

// select ?, T, T --> T
Expand Down Expand Up @@ -13562,13 +13562,14 @@ bool SelectionDAG::isConstantFPBuildVectorOrConstantFP(SDValue N) const {
return false;
}

std::optional<bool> SelectionDAG::isBoolConstant(SDValue N,
bool AllowTruncation) const {
ConstantSDNode *Const = isConstOrConstSplat(N, false, AllowTruncation);
std::optional<bool> SelectionDAG::isBoolConstant(SDValue N) const {
ConstantSDNode *Const =
isConstOrConstSplat(N, false, /*AllowTruncation=*/true);
if (!Const)
return std::nullopt;

const APInt &CVal = Const->getAPIntValue();
EVT VT = N->getValueType(0);
const APInt CVal = Const->getAPIntValue().trunc(VT.getScalarSizeInBits());
switch (TLI->getBooleanContents(N.getValueType())) {
case TargetLowering::ZeroOrOneBooleanContent:
if (CVal.isOne())
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10 changes: 10 additions & 0 deletions llvm/lib/Target/X86/X86ISelLowering.cpp
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Expand Up @@ -4975,6 +4975,16 @@ X86TargetLowering::getTargetConstantFromLoad(LoadSDNode *LD) const {
return getTargetConstantFromNode(LD);
}

bool X86TargetLowering::isTargetCanonicalSelect(SDNode *N) const {
// Do not fold (vselect not(C), X, 0s) to (vselect C, Os, X)
SDValue Cond = N->getOperand(0);
SDValue RHS = N->getOperand(2);
EVT CondVT = Cond.getValueType();
return N->getOpcode() == ISD::VSELECT && Subtarget.hasAVX512() &&
CondVT.getVectorElementType() == MVT::i1 &&
ISD::isBuildVectorAllZeros(RHS.getNode());
}

// Extract raw constant bits from constant pools.
static bool getTargetConstantBitsFromNode(SDValue Op, unsigned EltSizeInBits,
APInt &UndefElts,
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2 changes: 2 additions & 0 deletions llvm/lib/Target/X86/X86ISelLowering.h
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Expand Up @@ -1356,6 +1356,8 @@ namespace llvm {
TargetLowering::isTargetCanonicalConstantNode(Op);
}

bool isTargetCanonicalSelect(SDNode *N) const override;

const Constant *getTargetConstantFromLoad(LoadSDNode *LD) const override;

SDValue unwrapAddress(SDValue N) const override;
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