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Nov 25, 2025
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[HLSL] Update vector swizzle elements individually #169090

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3fe6c76
[HLSL] Update vector elements individually
hekota Nov 21, 2025
b167333
code review feedback - update comment
hekota Nov 25, 2025
38e580e
Revert unintentional change
hekota Nov 25, 2025
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46 changes: 38 additions & 8 deletions clang/lib/CodeGen/CGExpr.cpp
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Expand Up @@ -2801,26 +2801,56 @@ void CodeGenFunction::EmitStoreThroughExtVectorComponentLValue(RValue Src,
LValue Dst) {
llvm::Value *SrcVal = Src.getScalarVal();
Address DstAddr = Dst.getExtVectorAddress();
const llvm::Constant *Elts = Dst.getExtVectorElts();
if (DstAddr.getElementType()->getScalarSizeInBits() >
SrcVal->getType()->getScalarSizeInBits())
SrcVal = Builder.CreateZExt(
SrcVal, convertTypeForLoadStore(Dst.getType(), SrcVal->getType()));

// HLSL allows storing to scalar values through ExtVector component LValues.
// To support this we need to handle the case where the destination address is
// a scalar.
if (!DstAddr.getElementType()->isVectorTy()) {
assert(!Dst.getType()->isVectorType() &&
"this should only occur for non-vector l-values");
Builder.CreateStore(SrcVal, DstAddr, Dst.isVolatileQualified());
if (getLangOpts().HLSL) {
llvm::Type *DestAddrTy = DstAddr.getElementType();
// HLSL allows storing to scalar values through ExtVector component LValues.
// To support this we need to handle the case where the destination address
// is a scalar.
if (!DestAddrTy->isVectorTy()) {
assert(!Dst.getType()->isVectorType() &&
"this should only occur for non-vector l-values");
Builder.CreateStore(SrcVal, DstAddr, Dst.isVolatileQualified());
return;
}

// HLSL allows direct access to vector elements, so storing to individual
// elements of a vector through ExtVector is handled as separate store
// instructions.
// If we are updating multiple elements, Dst and Src are vectors; for
// a single element update they are scalars.
const VectorType *VTy = Dst.getType()->getAs<VectorType>();
unsigned NumSrcElts = VTy ? VTy->getNumElements() : 1;
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@spall spall Nov 24, 2025

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doesn't the above if case handle when Dst.getType isn't a vector? Could we assert that VTy != null instead?

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@hekota hekota Nov 24, 2025

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No, there are actually three cases:

  int i;
  int4 vec;

  i.x = 1.0; // case 1
  vec.y = 1; // case 2
  vec.xz = { 3, 4}; // case 3

Case 1 is storing a scalar to scalar via vector swizzle and it is handled by the code above this comment.
Case 2 is storing a scalar to vector via single element swizzle and VTy is going to be null in this case.
Case 3 is storing multiple scalars to a vector via multi-element swizzle and VTy != nullptr.

CharUnits ElemAlign = CharUnits::fromQuantity(
CGM.getDataLayout().getPrefTypeAlign(DestAddrTy->getScalarType()));
llvm::Value *Zero = llvm::ConstantInt::get(Int32Ty, 0);

for (unsigned I = 0; I != NumSrcElts; ++I) {
llvm::Value *Val = VTy ? Builder.CreateExtractElement(
SrcVal, llvm::ConstantInt::get(Int32Ty, I))
: SrcVal;
unsigned FieldNo = getAccessedFieldNo(I, Elts);
Address DstElemAddr = Address::invalid();
if (FieldNo == 0)
DstElemAddr = DstAddr.withAlignment(ElemAlign);
else
DstElemAddr = Builder.CreateGEP(
DstAddr, {Zero, llvm::ConstantInt::get(Int32Ty, FieldNo)},
DestAddrTy, ElemAlign);
Builder.CreateStore(Val, DstElemAddr, Dst.isVolatileQualified());
}
return;
}

// This access turns into a read/modify/write of the vector. Load the input
// value now.
llvm::Value *Vec = Builder.CreateLoad(DstAddr, Dst.isVolatileQualified());
llvm::Type *VecTy = Vec->getType();
const llvm::Constant *Elts = Dst.getExtVectorElts();

if (const VectorType *VTy = Dst.getType()->getAs<VectorType>()) {
unsigned NumSrcElts = VTy->getNumElements();
Expand Down
12 changes: 9 additions & 3 deletions clang/test/CodeGenHLSL/BasicFeatures/OutputArguments.hlsl
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Expand Up @@ -101,10 +101,16 @@ void funky(inout int3 X) {
// Call the function with the temporary.
// CHECK: call void {{.*}}funky{{.*}}(ptr noalias noundef nonnull align 16 dereferenceable(16) [[ArgTmp]])

// Shuffle it back.
// Write it back.
// CHECK: [[RetVal:%.*]] = load <3 x i32>, ptr [[ArgTmp]]
// CHECK: [[Vxyz:%.*]] = shufflevector <3 x i32> [[RetVal]], <3 x i32> poison, <3 x i32> <i32 2, i32 0, i32 1>
// CHECK: store <3 x i32> [[Vxyz]], ptr [[V]]
// CHECK: [[Src0:%.*]] = extractelement <3 x i32> [[RetVal]], i32 0
// CHECK: [[PtrY:%.*]] = getelementptr <3 x i32>, ptr %V, i32 0, i32 1
// CHECK: store i32 [[Src0]], ptr [[PtrY]], align 4
// CHECK: [[Src1:%.*]] = extractelement <3 x i32> [[RetVal]], i32 1
// CHECK: [[PtrZ:%.*]] = getelementptr <3 x i32>, ptr %V, i32 0, i32 2
// CHECK: store i32 [[Src1]], ptr [[PtrZ]], align 4
// CHECK: [[Src2:%.*]] = extractelement <3 x i32> [[RetVal]], i32 2
// CHECK: store i32 [[Src2]], ptr %V, align 4

// OPT: ret <3 x i32> <i32 3, i32 1, i32 2>
export int3 case4() {
Expand Down
25 changes: 15 additions & 10 deletions clang/test/CodeGenHLSL/builtins/ScalarSwizzles.hlsl
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Expand Up @@ -259,9 +259,8 @@ bool AssignBool(bool V) {
// CHECK-NEXT: [[B:%.*]] = load i32, ptr [[VAddr]], align 4
// CHECK-NEXT: [[LV1:%.*]] = trunc i32 [[B]] to i1
// CHECK-NEXT: [[D:%.*]] = zext i1 [[LV1]] to i32
// CHECK-NEXT: [[C:%.*]] = load <2 x i32>, ptr [[X]], align 8
// CHECK-NEXT: [[E:%.*]] = insertelement <2 x i32> [[C]], i32 [[D]], i32 1
// CHECK-NEXT: store <2 x i32> [[E]], ptr [[X]], align 8
// CHECK-NEXT: [[C:%.*]] = getelementptr <2 x i32>, ptr [[X]], i32 0, i32 1
// CHECK-NEXT: store i32 [[D]], ptr [[C]], align 4
// CHECK-NEXT: ret void
void AssignBool2(bool V) {
bool2 X = true.xx;
Expand All @@ -277,10 +276,13 @@ void AssignBool2(bool V) {
// CHECK-NEXT: [[Z:%.*]] = load <2 x i32>, ptr [[VAddr]], align 8
// CHECK-NEXT: [[LV:%.*]] = trunc <2 x i32> [[Z]] to <2 x i1>
// CHECK-NEXT: [[B:%.*]] = zext <2 x i1> [[LV]] to <2 x i32>
// CHECK-NEXT: [[A:%.*]] = load <2 x i32>, ptr [[X]], align 8
// CHECK-NEXT: [[C:%.*]] = shufflevector <2 x i32> [[B]], <2 x i32> poison, <2 x i32> <i32 0, i32 1>
// CHECK-NEXT: store <2 x i32> [[C]], ptr [[X]], align 8
// CHECK-NEXT: [[V1:%.*]] = extractelement <2 x i32> [[B]], i32 0
// CHECK-NEXT: store i32 [[V1]], ptr [[X]], align 4
// CHECK-NEXT: [[V2:%.*]] = extractelement <2 x i32> [[B]], i32 1
// CHECK-NEXT: [[X2:%.*]] = getelementptr <2 x i32>, ptr [[X]], i32 0, i32 1
// CHECK-NEXT: store i32 [[V2]], ptr [[X2]], align 4
// CHECK-NEXT: ret void

void AssignBool3(bool2 V) {
bool2 X = {true,true};
X.xy = V;
Expand Down Expand Up @@ -313,10 +315,13 @@ bool2 AccessBools() {
// CHECK-NEXT: [[L1:%.*]] = shufflevector <1 x i32> [[L0]], <1 x i32> poison, <3 x i32> zeroinitializer
// CHECK-NEXT: [[TruncV:%.*]] = trunc <3 x i32> [[L1]] to <3 x i1>
// CHECK-NEXT: [[L2:%.*]] = zext <3 x i1> [[TruncV]] to <3 x i32>
// CHECK-NEXT: [[L3:%.*]] = load <4 x i32>, ptr [[B]], align 16
// CHECK-NEXT: [[L4:%.*]] = shufflevector <3 x i32> [[L2]], <3 x i32> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 poison>
// CHECK-NEXT: [[L5:%.*]] = shufflevector <4 x i32> [[L3]], <4 x i32> [[L4]], <4 x i32> <i32 4, i32 5, i32 6, i32 3>
// CHECK-NEXT: store <4 x i32> [[L5]], ptr [[B]], align 16
// CHECK-NEXT: [[V1:%.*]] = extractelement <3 x i32> [[L2]], i32 0
// CHECK-NEXT: store i32 [[V1]], ptr %B, align 4
// CHECK-NEXT: [[V2:%.*]] = extractelement <3 x i32> [[L2]], i32 1
// CHECK-NEXT: [[B2:%.*]] = getelementptr <4 x i32>, ptr %B, i32 0, i32 1
// CHECK-NEXT: store i32 [[V2]], ptr [[B2]], align 4
// CHECK-NEXT: [[V3:%.*]] = extractelement <3 x i32> [[L2]], i32 2
// CHECK-NEXT: [[B3:%.*]] = getelementptr <4 x i32>, ptr %B, i32 0, i32 2
void BoolSizeMismatch() {
bool4 B = {true,true,true,true};
B.xyz = false.xxx;
Expand Down
96 changes: 96 additions & 0 deletions clang/test/CodeGenHLSL/builtins/VectorSwizzles.hlsl
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@@ -0,0 +1,96 @@
// RUN: %clang_cc1 -finclude-default-header -fnative-half-type \
// RUN: -triple dxil-pc-shadermodel6.3-library %s -disable-llvm-passes \
// RUN: -emit-llvm -o - | FileCheck %s

// CHECK-LABEL: Single

// Setup local vars.
// CHECK: [[VecAddr:%.*]] = alloca <3 x i64>, align 32
// CHECK-NEXT: [[AAddr:%.*]] = alloca i64, align 8
// CHECK-NEXT: store <3 x i64> %vec, ptr [[VecAddr]], align 32
// CHECK-NEXT: store i64 %a, ptr [[AAddr]], align 8

// Update single element of the vector.
// CHECK-NEXT: [[A:%.*]] = load i64, ptr [[AAddr]], align 8
// CHECK-NEXT: [[Vy:%.*]] = getelementptr <3 x i64>, ptr [[VecAddr]], i32 0, i32 1
// CHECK-NEXT: store i64 [[A]], ptr [[Vy]], align 8

// Return.
// CHECK-NEXT: [[RetVal:%.*]] = load <3 x i64>, ptr [[VecAddr]], align 32
// CHECK-NEXT: ret <3 x i64> [[RetVal]]
uint64_t3 Single(uint64_t3 vec, uint64_t a){
vec.y = a;
return vec;
}

// CHECK-LABEL: Double

// Setup local vars.
// CHECK: [[VecAddr:%.*]] = alloca <3 x float>, align 16
// CHECK-NEXT: [[AAddr:%.*]] = alloca float, align 4
// CHECK-NEXT: [[BAddr:%.*]] = alloca float, align 4
// CHECK-NEXT: store <3 x float> %vec, ptr [[VecAddr]], align 16
// CHECK-NEXT: store float %a, ptr [[AAddr]], align 4
// CHECK-NEXT: store float %b, ptr [[BAddr]], align 4

// Create temporary vector {a, b}.
// CHECK-NEXT: [[A:%.*]] = load float, ptr [[AAddr]], align 4
// CHECK-NEXT: [[TmpVec0:%.*]] = insertelement <2 x float> poison, float [[A]], i32 0
// CHECK-NEXT: [[B:%.*]] = load float, ptr [[BAddr]], align 4
// CHECK-NEXT: [[TmpVec1:%.*]] = insertelement <2 x float> [[TmpVec0]], float [[B]], i32 1

// Update two elements of the vector from temporary vector.
// CHECK-NEXT: [[TmpX:%.*]] = extractelement <2 x float> [[TmpVec1]], i32 0
// CHECK-NEXT: [[VecZ:%.*]] = getelementptr <3 x float>, ptr [[VecAddr]], i32 0, i32 2
// CHECK-NEXT: store float [[TmpX]], ptr [[VecZ]], align 4
// CHECK-NEXT: [[TmpY:%.*]] = extractelement <2 x float> [[TmpVec1]], i32 1
// CHECK-NEXT: [[VecY:%.*]] = getelementptr <3 x float>, ptr [[VecAddr]], i32 0, i32 1
// CHECK-NEXT: store float [[TmpY]], ptr [[VecY]], align 4

// Return.
// CHECK-NEXT: [[RetVal:%.*]] = load <3 x float>, ptr [[VecAddr]], align 16
// CHECK-NEXT: ret <3 x float> [[RetVal]]
float3 Double(float3 vec, float a, float b) {
vec.zy = {a, b};
return vec;
}

// CHECK-LABEL: Shuffle

// Setup local vars.
// CHECK: [[VecAddr:%.*]] = alloca <4 x half>, align 8
// CHECK-NEXT: [[AAddr:%.*]] = alloca half, align 2
// CHECK-NEXT: [[BAddr:%.*]] = alloca half, align 2
// CHECK-NEXT: store <4 x half> %vec, ptr [[VecAddr]], align 8
// CHECK-NEXT: store half %a, ptr [[AAddr]], align 2
// CHECK-NEXT: store half %b, ptr [[BAddr]], align 2

// Create temporary vector {a, b, 13.74, a}.
// CHECK-NEXT: [[A:%.*]] = load half, ptr [[AAddr]], align 2
// CHECK-NEXT: [[TmpVec0:%.*]] = insertelement <4 x half> poison, half [[A]], i32 0
// CHECK-NEXT: [[B:%.*]] = load half, ptr [[BAddr]], align 2
// CHECK-NEXT: [[TmpVec1:%.*]] = insertelement <4 x half> [[TmpVec0]], half [[B]], i32 1
// CHECK-NEXT: [[TmpVec2:%.*]] = insertelement <4 x half> %vecinit1, half 0xH4ADF, i32 2
// CHECK-NEXT: [[A:%.*]] = load half, ptr [[AAddr]], align 2
// CHECK-NEXT: [[TmpVec3:%.*]] = insertelement <4 x half> [[TmpVec2]], half [[A]], i32 3

// Update four elements of the vector via mixed up swizzle from the temporary vector.
// CHECK-NEXT: [[TmpX:%.*]] = extractelement <4 x half> [[TmpVec3]], i32 0
// CHECK-NEXT: [[VecZ:%.*]] = getelementptr <4 x half>, ptr [[VecAddr]], i32 0, i32 2
// CHECK-NEXT: store half [[TmpX]], ptr [[VecZ]], align 2
// CHECK-NEXT: [[TmpY:%.*]] = extractelement <4 x half> [[TmpVec3]], i32 1
// CHECK-NEXT: [[VecW:%.*]] = getelementptr <4 x half>, ptr [[VecAddr]], i32 0, i32 3
// CHECK-NEXT: store half [[TmpY]], ptr [[VecW]], align 2
// CHECK-NEXT: [[TmpZ:%.*]] = extractelement <4 x half> [[TmpVec3]], i32 2
// CHECK-NEXT: store half [[TmpZ]], ptr [[VecAddr]], align 2
// CHECK-NEXT: [[TmpW:%.*]] = extractelement <4 x half> [[TmpVec3]], i32 3
// CHECK-NEXT: [[VecY:%.*]] = getelementptr <4 x half>, ptr [[VecAddr]], i32 0, i32 1
// CHECK-NEXT: store half [[TmpW]], ptr [[VecY]], align 2

// Return.
// CHECK-NEXT: [[RetVal:%.*]] = load <4 x half>, ptr [[VecAddr]], align 8
// CHECK-NEXT: ret <4 x half> [[RetVal]]
half4 Shuffle(half4 vec, half a, half b) {
vec.zwxy = {a, b, 13.74, a};
return vec;
}