Flag to prevent packing of cbuffer elements in HLSL backend. (#3993)

4 files changed, 24 insertions, 3 deletions

diff --git a/source/slang/slang-emit-c-like.cpp b/source/slang/slang-emit-c-like.cpp
index 38708ae5e..25dff6083 100644
--- a/source/slang/slang-emit-c-like.cpp
+++ b/source/slang/slang-emit-c-like.cpp
@@ -2066,6 +2066,21 @@ bool CLikeSourceEmitter::isSingleElementConstantBuffer(IRInst* cbufferType)
     return true;
 }
 
+bool CLikeSourceEmitter::shouldForceUnpackConstantBufferElements(IRInst* cbufferType)
+{
+    if (getTargetReq()->getTarget() != CodeGenTarget::HLSL)
+        return false;
+    if (!getTargetProgram()->getOptionSet().getBoolOption(CompilerOptionName::NoHLSLPackConstantBufferElements))
+        return false;
+    auto type = as<IRUniformParameterGroupType>(cbufferType);
+    if (!type)
+        return false;
+    auto structType = as<IRStructType>(type->getElementType());
+    if (!structType)
+        return false;
+    return true;
+}
+
 void CLikeSourceEmitter::defaultEmitInstExpr(IRInst* inst, const EmitOpInfo& inOuterPrec)
 {
     EmitOpInfo outerPrec = inOuterPrec;
@@ -2195,8 +2210,9 @@ void CLikeSourceEmitter::defaultEmitInstExpr(IRInst* inst, const EmitOpInfo& inO
         {
             auto prec = getInfo(EmitOp::Postfix);
             needClose = maybeEmitParens(outerPrec, prec);
-            auto skipBase = isD3DTarget(getTargetReq()) &&
-                hasExplicitConstantBufferOffset(ii->getBase()->getDataType());
+            bool skipBase = (isD3DTarget(getTargetReq()) &&
+                hasExplicitConstantBufferOffset(ii->getBase()->getDataType())) ||
+                shouldForceUnpackConstantBufferElements(ii->getBase()->getDataType());
             if (!skipBase)
             {
                 auto base = ii->getBase();
diff --git a/source/slang/slang-emit-c-like.h b/source/slang/slang-emit-c-like.h
index ee7ce413a..64aa5f945 100644
--- a/source/slang/slang-emit-c-like.h
+++ b/source/slang/slang-emit-c-like.h
@@ -251,6 +251,7 @@ public:
     void emitType(IRType* type, NameLoc const& nameAndLoc);
     bool hasExplicitConstantBufferOffset(IRInst* cbufferType);
     bool isSingleElementConstantBuffer(IRInst* cbufferType);
+    bool shouldForceUnpackConstantBufferElements(IRInst* cbufferType);
 
     //
     // Expressions
diff --git a/source/slang/slang-emit-hlsl.cpp b/source/slang/slang-emit-hlsl.cpp
index 9fcdef106..adca798e6 100644
--- a/source/slang/slang-emit-hlsl.cpp
+++ b/source/slang/slang-emit-hlsl.cpp
@@ -221,9 +221,10 @@ void HLSLSourceEmitter::_emitHLSLParameterGroup(IRGlobalParam* varDecl, IRUnifor
     _emitHLSLRegisterSemantic(LayoutResourceKind::ConstantBuffer, &containerChain, varDecl);
 
     auto elementType = type->getElementType();
-    if (hasExplicitConstantBufferOffset(type))
+    if (shouldForceUnpackConstantBufferElements(type) || hasExplicitConstantBufferOffset(type))
     {
         // If the user has provided any explicit `packoffset` modifiers,
+        // or the user has explicitly requested for cbuffer fields to be unpacked,
         // we have to unwrap the struct and emit the fields directly.
         emitStructDeclarationsBlock(as<IRStructType>(elementType), true);
         m_writer->emit("\n");
diff --git a/source/slang/slang-options.cpp b/source/slang/slang-options.cpp
index 13a11f10b..8a5c52e0e 100644
--- a/source/slang/slang-options.cpp
+++ b/source/slang/slang-options.cpp
@@ -520,6 +520,8 @@ void initCommandOptions(CommandOptions& options)
         { OptionKind::NoMangle, "-no-mangle", nullptr, "Do as little mangling of names as possible." },
         { OptionKind::NoHLSLBinding, "-no-hlsl-binding", nullptr, "Do not include explicit parameter binding semantics in the output HLSL code,"
                                                                   "except for parameters that has explicit bindings in the input source." },
+        { OptionKind::NoHLSLPackConstantBufferElements, "-no-hlsl-pack-constant-buffer-elements", nullptr,
+        "Do not pack elements of constant buffers into structs in the output HLSL code." },
         { OptionKind::ValidateUniformity, "-validate-uniformity", nullptr, "Perform uniformity validation analysis." },
         { OptionKind::AllowGLSL, "-allow-glsl", nullptr, "Enable GLSL as an input language." },
     };
@@ -1691,6 +1693,7 @@ SlangResult OptionsParser::_parse(
             case OptionKind::DumpAst:
             case OptionKind::IncompleteLibrary:
             case OptionKind::NoHLSLBinding:
+            case OptionKind::NoHLSLPackConstantBufferElements:
                 linkage->m_optionSet.set(optionKind, true); break;
                 break;
             case OptionKind::MatrixLayoutRow: