Introduce CDataLayout & -fvk-use-c-layout (#8136)

Closes #8112. ~~The issue asks for a "C layout", but in this PR I use
the term "CPU layout" because this naming was pre-existing in the
codebase as `kCPULayoutRulesImpl_`. The primary purpose of this layout
is to match CPU-side struct definitions with the shader side. I'm open
to better naming suggestions, though.~~

Edit: switched back to using `CDataLayout` & `-fvk-use-c-layout`, as the
CPU target depends on the object layout rules of existing CPU layout
rules, but they're incompatible with actual shaders. So a new
`kCLayoutRulesImpl_` was needed anyway.

---------

Co-authored-by: Ellie Hermaszewska <ellieh@nvidia.com>

1 files changed, 16 insertions, 4 deletions

diff --git a/source/slang/slang-emit-spirv.cpp b/source/slang/slang-emit-spirv.cpp
index 6b42896d0..d6f4ac176 100644
--- a/source/slang/slang-emit-spirv.cpp
+++ b/source/slang/slang-emit-spirv.cpp
@@ -1847,10 +1847,22 @@ struct SPIRVEmitContext : public SourceEmitterBase, public SPIRVEmitSharedContex
                         IRSizeAndAlignment sizeAndAlignment;
                         uint32_t stride;
 
-                        getNaturalSizeAndAlignment(
-                            m_targetProgram->getOptionSet(),
-                            valueType,
-                            &sizeAndAlignment);
+                        if (auto layout = valueType->findDecoration<IRSizeAndAlignmentDecoration>())
+                        {
+                            auto rule = IRTypeLayoutRules::get(layout->getLayoutName());
+                            getSizeAndAlignment(
+                                m_targetProgram->getOptionSet(),
+                                rule,
+                                valueType,
+                                &sizeAndAlignment);
+                        }
+                        else
+                        {
+                            getNaturalSizeAndAlignment(
+                                m_targetProgram->getOptionSet(),
+                                valueType,
+                                &sizeAndAlignment);
+                        }
                         uint64_t valueSize = sizeAndAlignment.size;
 
                         // Any unsized data type (e.g. struct or array) will have size of