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Expand double vector dot products to FMul/FMad instead of emitting invalid Dot2/3/4 intrinsics #8

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Expand double vector dot products to FMul/FMad instead of emitting invalid Dot2/3/4 intrinsics #8

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b41944b
Initial plan
Copilot Mar 13, 2026
14224ea
Fix mul with double vectors producing invalid DXIL dot intrinsic
Copilot Mar 13, 2026
45f72a9
Expand test to cover both mul and dot with double vectors
Copilot Mar 13, 2026
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7 changes: 7 additions & 0 deletions lib/HLSL/HLOperationLower.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2590,6 +2590,13 @@ Value *ExpandDot(Value *arg0, Value *arg1, unsigned vecSize, hlsl::OP *hlslOP,

Value *TranslateFDot(Value *arg0, Value *arg1, unsigned vecSize,
hlsl::OP *hlslOP, IRBuilder<> &Builder) {
// Dot2/3/4 DXIL operations only support half and float, not double.
// For double vectors, expand the dot product using FMul and FMad.
if (arg0->getType()->getScalarType()->isDoubleTy()) {
return ExpandDot(arg0, arg1, vecSize, hlslOP, Builder,
DXIL::OpCode::FMad);
}

switch (vecSize) {
case 2:
return TrivialDotOperation(OP::OpCode::Dot2, arg0, arg1, hlslOP, Builder);
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29 changes: 29 additions & 0 deletions tools/clang/test/HLSLFileCheck/hlsl/intrinsics/mul/mul_double.hlsl
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@@ -0,0 +1,29 @@
// RUN: %dxc -T cs_6_0 -E main -DDIM=4 %s | FileCheck %s
// RUN: %dxc -T cs_6_0 -E main -DDIM=3 %s | FileCheck %s
// RUN: %dxc -T cs_6_0 -E main -DDIM=2 %s | FileCheck %s

// Verify that mul of double vectors does not produce invalid DXIL dot
// intrinsics (Dot2/3/4 only support half and float). Instead, the dot
// product should be expanded using FMul and FMad.

// CHECK-NOT: call double @dx.op.dot2.f64
// CHECK-NOT: call double @dx.op.dot3.f64
// CHECK-NOT: call double @dx.op.dot4.f64
// CHECK: fmul fast double
// CHECK: call double @dx.op.tertiary.f64(i32 46,

#if DIM == 4
typedef double4 DVec;
#elif DIM == 3
typedef double3 DVec;
#else
typedef double2 DVec;
#endif

RWStructuredBuffer<DVec> In;
RWStructuredBuffer<double> Out;

[numthreads(1, 1, 1)]
void main() {
Out[0] = mul(In[0], In[1]);
}