Fix SV_VertexID and SV_InstanceID crash with -preserve-params in SPIR-V emitter (#7691)

* Initial plan

* Fix SV_VertexID and SV_InstanceID crash with -preserve-params in SPIR-V emitter

Co-authored-by: csyonghe <2652293+csyonghe@users.noreply.github.com>

* Fix transformation layer to handle unused builtin params properly

This ensures that builtin decorations are correctly transformed for unused parameters with -preserve-params, preventing the SPIR-V emitter from seeing un-translated builtins like sv_vertexid.

Co-authored-by: csyonghe <2652293+csyonghe@users.noreply.github.com>

* Format code according to project style

Co-authored-by: csyonghe <2652293+csyonghe@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: csyonghe <2652293+csyonghe@users.noreply.github.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/source/slang/slang-ir-glsl-legalize.cpp b/source/slang/slang-ir-glsl-legalize.cpp
index 1a34a9877..fc1bcfa5a 100644
--- a/source/slang/slang-ir-glsl-legalize.cpp
+++ b/source/slang/slang-ir-glsl-legalize.cpp
@@ -4006,6 +4006,13 @@ void legalizeTargetBuiltinVar(GLSLLegalizationContext& context)
                         user->replaceUsesWith(sub);
                     }
                 });
+
+            // For unused parameters (like with -preserve-params), also update the builtin
+            // decoration to ensure SPIR-V emitter sees the correct builtin
+            IRBuilder builder(varInst);
+            builder.addTargetBuiltinVarDecoration(
+                varInst,
+                IRTargetBuiltinVarName::SpvInstanceIndex);
         }
         // Repalce SV_VertexID with gl_VertexIndex - gl_BaseVertex.
         else if (builtinVarName == IRTargetBuiltinVarName::HlslVertexID)
@@ -4032,6 +4039,11 @@ void legalizeTargetBuiltinVar(GLSLLegalizationContext& context)
                         user->replaceUsesWith(sub);
                     }
                 });
+
+            // For unused parameters (like with -preserve-params), also update the builtin
+            // decoration to ensure SPIR-V emitter sees the correct builtin
+            IRBuilder builder(varInst);
+            builder.addTargetBuiltinVarDecoration(varInst, IRTargetBuiltinVarName::SpvVertexIndex);
         }
     }
 }