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Fix std430 layout support for cbuffers in .slang files #8432

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Fix std430 layout support for cbuffers in .slang files #8432

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4c07e6a
Initial plan
Copilot Sep 10, 2025
2d3b783
Initial analysis: Identified the issue with std430 cbuffer ArrayStride
Copilot Sep 10, 2025
441e3cd
Fix std430 layout support for cbuffers in .slang files
Copilot Sep 11, 2025
3651590
Fix std430 layout support for cbuffers in .slang files
Copilot Sep 11, 2025
8193285
Address review feedback: Add warnings for layout qualifiers on incomp…
github-actions[bot] Sep 30, 2025
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37 changes: 37 additions & 0 deletions source/slang/slang-check-modifier.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -9,6 +9,7 @@
// focused on `[attributes]`.

#include "slang-lookup.h"
#include "slang-target.h"

namespace Slang
{
Expand Down Expand Up @@ -1677,6 +1678,42 @@ Modifier* SemanticsVisitor::checkModifier(
}
}

// Check for layout qualifiers on incompatible targets
if (auto layoutMod = as<GLSLBufferDataLayoutModifier>(m))
{
String layoutName;

if (as<GLSLStd140Modifier>(layoutMod))
layoutName = "std140";
else if (as<GLSLStd430Modifier>(layoutMod))
layoutName = "std430";
else if (as<GLSLScalarModifier>(layoutMod))
layoutName = "scalar";
else
layoutName = "layout";

for (auto target : getLinkage()->targets)
{
if (!isKhronosTarget(target->getTarget()) && !isWGPUTarget(target->getTarget()))
{
String targetName;
switch (target->getTarget())
{
case CodeGenTarget::HLSL: targetName = "HLSL"; break;
case CodeGenTarget::DXIL: targetName = "DXIL"; break;
case CodeGenTarget::DXBytecode: targetName = "DXBC"; break;
case CodeGenTarget::CSource: targetName = "C"; break;
case CodeGenTarget::CPPSource: targetName = "C++"; break;
case CodeGenTarget::CUDASource: targetName = "CUDA"; break;
case CodeGenTarget::MetalLib: targetName = "Metal"; break;
default: targetName = "unknown"; break;
}
getSink()->diagnose(m, Diagnostics::layoutQualifierUnsupportedForTarget, layoutName, targetName);
break;
}
}
}

if (as<ConstModifier>(m))
{
if (auto varDeclBase = as<VarDeclBase>(syntaxNode))
Expand Down
6 changes: 6 additions & 0 deletions source/slang/slang-diagnostic-defs.h
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Expand Up @@ -2299,6 +2299,12 @@ DIAGNOSTIC(39009, Error, expectedSpace, "expected 'space', got '$0'")
DIAGNOSTIC(39010, Error, expectedSpaceIndex, "expected a register space index after 'space'")
DIAGNOSTIC(39011, Error, invalidComponentMask, "invalid register component mask '$0'.")

DIAGNOSTIC(
39012,
Warning,
layoutQualifierUnsupportedForTarget,
"layout qualifier '$0' is not supported for target '$1'")

DIAGNOSTIC(
39013,
Warning,
Expand Down
4 changes: 2 additions & 2 deletions source/slang/slang-parser.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3578,8 +3578,8 @@ static Decl* ParseBufferBlockDecl(

static NodeBase* parseHLSLCBufferDecl(Parser* parser, void* /*userData*/)
{
// Check for GLSL layout qualifiers when GLSL input is allowed
if (parser->options.allowGLSLInput && parser->pendingModifiers)
// Check for GLSL layout qualifiers (layout modifiers can be used even without allowGLSLInput)
if (parser->pendingModifiers)
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Copilot, I think it needs to warn when layout (std430) etc are used when compiling for other targets (warn instead of error to make it a non-breaking PR), and accept these layout modifiers when compiling for GLSL/SPIRV without requiring -allow-glsl

{
auto getLayoutArg = [&](const char* defaultLayout)
{
Expand Down
15 changes: 15 additions & 0 deletions tests/spirv/cbuffer-std430-layout.slang
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Original file line number Diff line number Diff line change
@@ -0,0 +1,15 @@
//TEST:SIMPLE(filecheck=SPIRV): -target spirv -entry main -stage fragment -emit-spirv-directly

// Test that layout(std430) cbuffer uses correct ArrayStride (8 for int2 array)
// instead of std140 ArrayStride (16 for int2 array)

layout(std430) cbuffer CBlock {
int2 bb[2];
};

//SPIRV-DAG: OpDecorate %_arr_v2int_int_2 ArrayStride 8

float4 main() : SV_Target
{
return float4(0.3, 0.7, 0.55, 1.0 + bb[1].y);
}