[0045] - Matrix Layout Attributes

Motivation

HLSL allows per-declaration matrix layout overrides via the row_major and column_major keywords. So far in clang the matrix layout is only controlable globally via the -fmatrix-memory-layout command-line flag (LangOptions::DefaultMatrixMemoryLayout). We need to add the parsing and semantic analysis, and codegen infrastructure needed to support per-declaration layout, matching DXC behavior.

The plan

1. Keyword Registration (TokenKinds.def)

We need to do the same thing we did for KEYWORD(cbuffer, KEYHLSL). Add row_major and column_major as KEYWORD(..., KEYHLSL) entries. This will limit the keywords to HLSL while allowing them to remain identifers in C/C++ and other langugage modes.

2. Attribute Definition (Attr.td)

A new HLSLMatrixLayout inheritable attribute is needed with:

• It must cover both keywords.
• The Spelling should be a CustomKeyword because the token kinds are explicitly defined in TokenKinds.def.
• We should still add LangOpts: [HLSL] to restricts the attribute to HLSL mode.

3. Parsing (ParseDecl.cpp)

Now that we have a tok::kw_row_major and tok::kw_column_major we need to consume the token the same way we consume tok::kw_groupshared. Ie we need to call ParseHLSLQualifiers(). We also need to modify isTypeSpecifierQualifier because the matrix layout attributes are a type-specifier.

4. Semantic Analysis (SemaHLSL.cpp, SemaDeclAttr.cpp)

A new SemaHLSL::handleMatrixLayoutAttr handler is needed so that ProcessDeclAttribute can handle the ParsedAttr::AT_HLSLMatrixLayout case. The handler should performs three validations:

• Type check: The declaration’s type must be a ConstantMatrixType.
• Conflict check: If the declaration already has an HLSLMatrixLayoutAttr with a different spelling (e.g., row_major column_major float3x3), an error is emitted.
• Duplicate check: If the declaration already has an HLSLMatrixLayoutAttr with the same spelling (e.g., row_major row_major float3x3), a warning is emitted.

5. Diagnostics (DiagnosticSemaKinds.td)

We need some new diagnostics:

• Limit the attribute to only matrix types"
• Make sure we can’t apply conflicting attributes (eg. row and column major)

6. Tests (Parser and Sema)

Tests we need:

• Sema Errors on scalars, vectors, arrays, structs. Conflict and duplicate diagnostics.
• AST dump with correct spelling for variables, fields, and typedefs.
• Verifies row_major/column_major are not keywords in C++ mode.

7. Update All global layout check to check attribute first.

The curent behavior checks getLangOpts().getDefaultMatrixMemoryLayout(). This needs to be updated to first check for an HLSLMatrixLayoutAttr on the relevant declaration before falling back to the global default. A helper function like isMatrixRowMajor(const Expr *E) would centralize this logic.

Codegen Files (where layout affects IR)

Below are the areas in Codgen that needed to be updated.

Sema Files (semantic checks)

Below are the areas in Sema that need to be updated.

8. Tests (Codgen and Init Order)

We need tests that match the stated purposes in the above two tables.