fix(ir): Fix vertex index detection inside control flow - #8
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- vertexIndex reads nested inside if/for/assign/return were invisible to UsesVertexIndex, so backends emitted shaders referencing vertexIndex without the @Builtin(vertex_index)/[[vertex_id]]/gl_VertexID declaration the reference needed — naga rejects the resulting WGSL outright - replace type-switch walkers in stmtMatches/exprMatches with interface methods (StmtCF.exprs/nestedStmts, Expr.children) so a new CF or Expr variant missing these methods is a compile error instead of a silently-false walker result - remove the hand-rolled vertexIndex walker in lower/lower_vertex.go, replacing it with ir.StageUsesVertexIndexBuiltin which routes through the exhaustive shared walker the same way UsesDerivatives already does for dpdx/dpdy/fwidth - add regression tests: IR walker unit tests for the walker, an end-to-end lowering test through the compiler, a new conformance material with golden outputs for all six backends, and a validation test proving naga rejects the old invalid WGSL
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Summary
The previous vertex-index detection logic only scanned top-level IR statements, missing reads nested inside control flow (e.g., an
ifreassignment). This caused emitted shaders to omit the per-backend vertex-index binding while still referencing it, resulting in invalid code. Refactored the IR's usage walkers to dispatch through interface methods, making exhaustive control-flow cases enforceable at compile time.Changes
ExprandStmtCFinterfaces to requirechildren()andexprs()/nestedStmts()methods, replacing brittle type switches in usage walkers with safe interface dispatch.irStageUsesVertexIndexvertex-index walker with the new exhaustiveir.StageUsesVertexIndexBuiltin, correctly detecting references insideif/for/assignstatements.ir/uses_test.goandlower/lower_test.goto verify derivatives and vertex-index are found inside control flow.validate/validate_test.gotest to prove the previous output failed Naga validation.Testing
go test ./ir ./lower ./validate -run 'TestUsesDerivativesFindsCallInsideReturnCF|TestStageUsesVertexIndexBuiltinFindsRefInsideIf|TestLowerMeshAuthoredVertexUsesVertexIndexInsideControlFlow'