Fix varying output structs in GLSL source (#8501)

Closes #8500.

`slang-ir-translate-global-varying-var.cpp` turns the global varying
outputs into a struct that's returned from the entry point. Currently,
there's a problem when one of the outputs is a struct. It always creates
a generic `IRTypeLayout`, even when a correct type layout already
exists. Somehow, this appears to work when the global varying outputs
aren't structs.

The crash occurs in
`slang-ir-glsl-legalize.cpp:createGLSLGlobalVaryingsImpl()`. It
correctly handles the generated outer struct, but when that contains an
inner struct, it's been given a non-struct type layout and crashes.

This PR uses the correct layout if found, instead of generating a broken
placeholder. This matches the behaviour that has already been
implemented for inputs.

Additionally, I removed a call to `addResourceUsage` from both the input
and output side. I can't see any way in which it would've affected
anything, the layout builder is never used after that call and it
doesn't retroactively modify the layout that was already created.

1 files changed, 21 insertions, 12 deletions

diff --git a/source/slang/slang-ir-translate-global-varying-var.cpp b/source/slang/slang-ir-translate-global-varying-var.cpp
index c899de653..6b8c4842b 100644
--- a/source/slang/slang-ir-translate-global-varying-var.cpp
+++ b/source/slang/slang-ir-translate-global-varying-var.cpp
@@ -184,13 +184,6 @@ struct GlobalVarTranslationContext
                 }
                 else
                 {
-                    if (!hasExistingLayout)
-                    {
-                        fieldTypeLayoutBuilder.addResourceUsage(
-                            LayoutResourceKind::VaryingInput,
-                            LayoutSize(1));
-                    }
-
                     // Start off the offset as nextOffset. If the global "in"s have existing
                     // offsetAttr's, we will add the offsets from those as well.
                     auto resInfo =
@@ -290,8 +283,27 @@ struct GlobalVarTranslationContext
                     auto key = builder.createStructKey();
                     auto ptrType = as<IRPtrTypeBase>(output->getDataType());
                     builder.createStructField(resultType, key, ptrType->getValueType());
-                    IRTypeLayout::Builder fieldTypeLayout(&builder);
-                    IRVarLayout::Builder varLayoutBuilder(&builder, fieldTypeLayout.build());
+
+                    IRTypeLayout::Builder fieldTypeLayoutBuilder(&builder);
+                    IRTypeLayout* fieldTypeLayout = nullptr;
+                    bool hasExistingLayout = false;
+                    if (auto existingLayoutDecoration =
+                            output->findDecoration<IRLayoutDecoration>())
+                    {
+                        if (auto existingVarLayout =
+                                as<IRVarLayout>(existingLayoutDecoration->getLayout()))
+                        {
+                            fieldTypeLayout = existingVarLayout->getTypeLayout();
+                            hasExistingLayout = true;
+                        }
+                    }
+
+                    if (!hasExistingLayout)
+                    {
+                        fieldTypeLayout = fieldTypeLayoutBuilder.build();
+                    }
+
+                    IRVarLayout::Builder varLayoutBuilder(&builder, fieldTypeLayout);
                     varLayoutBuilder.setStage(entryPointDecor->getProfile().getStage());
                     if (auto semanticDecor = output->findDecoration<IRSemanticDecoration>())
                     {
@@ -301,9 +313,6 @@ struct GlobalVarTranslationContext
                     }
                     else
                     {
-                        fieldTypeLayout.addResourceUsage(
-                            LayoutResourceKind::VaryingOutput,
-                            LayoutSize(1));
                         if (auto layoutDecor = findVarLayout(output))
                         {
                             if (auto offsetAttr =