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>

2 files changed, 25 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);
         }
     }
 }
diff --git a/tests/spirv/sv-vertex-id-preserve-params.slang b/tests/spirv/sv-vertex-id-preserve-params.slang
new file mode 100644
index 000000000..6bf4bfb89
--- /dev/null
+++ b/tests/spirv/sv-vertex-id-preserve-params.slang
@@ -0,0 +1,13 @@
+//TEST:SIMPLE(filecheck=SPIRV): -entry VertexMain -stage vertex -target spirv -preserve-params
+//TEST:SIMPLE(filecheck=SPIRV): -entry VertexMain -stage vertex -target spirv -emit-spirv-directly -preserve-params
+
+// Test for https://github.com/shader-slang/slang/issues/7689
+// SV_VertexID combined with -preserve-params should not crash
+
+// SPIRV: OpEntryPoint
+
+[shader("vertex")]
+float4 VertexMain(uint vertexId: SV_VertexID, uint instanceId: SV_InstanceID) : SV_Position
+{
+    return float4(0.0);
+}
\ No newline at end of file