Add linkage capability based on decorations (#6900)

We currently add the linkage attribute only if there are no extry
pointer. However, there are cases where we may want an extry point with
an exported function.

This commit add the linkage capability any time it sees the linkage
attribute decoration. Note I do not add the linkage attribute all of the
time and leave it to the capability trimming pass because having this
capability with vulkan shaders is generally illegal, and I don't want
the unoptimized code to fail validation.

Fixes #6738

tools/clang/lib/SPIRV/CapabilityVisitor.cpp

@@ -387,6 +387,9 @@ bool CapabilityVisitor::visit(SpirvDecoration *decor) {
 
     break;
   }
+  case spv::Decoration::LinkageAttributes:
+    addCapability(spv::Capability::Linkage);
+    break;
   default:
     break;
   }
@@ -847,16 +850,12 @@ bool CapabilityVisitor::visit(SpirvReadClock *inst) {
 }
 
 bool CapabilityVisitor::visit(SpirvModule *, Visitor::Phase phase) {
-  // If there are no entry-points in the module (hence shaderModel is not set),
-  // add the Linkage capability. This allows library shader models to use
-  // 'export' attribute on functions, and generate an "incomplete/partial"
-  // SPIR-V binary.
-  // ExecutionModel::Max means that no entrypoints exist, therefore we should
-  // add the Linkage Capability.
+  // If there are no entry-points in the module add the Shader capability.
+  // This allows library shader models with no entry pointer and just exported
+  // function. ExecutionModel::Max means that no entrypoints exist.
   if (phase == Visitor::Phase::Done &&
       shaderModel == spv::ExecutionModel::Max) {
     addCapability(spv::Capability::Shader);
-    addCapability(spv::Capability::Linkage);
   }
 
   // SPIRV-Tools now has a pass to trim superfluous capabilities. This means we

tools/clang/test/CodeGenSPIRV/fn.export.hlsl

@@ -1,7 +1,7 @@
 // RUN: %dxc -T lib_6_3 -fspv-target-env=universal1.5 -fcgl  %s -spirv | FileCheck %s
 
-// CHECK: OpCapability Shader
-// CHECK: OpCapability Linkage
+// CHECK-DAG: OpCapability Shader
+// CHECK-DAG: OpCapability Linkage
 RWBuffer< float4 > output : register(u1);
 
 // CHECK: OpDecorate %main LinkageAttributes "main" Export

tools/clang/test/CodeGenSPIRV/fn.export.with.entrypoint.hlsl

@@ -0,0 +1,13 @@
+// RUN: %dxc -T as_6_6 -E main -fspv-target-env=vulkan1.3 -fcgl  %s -spirv | FileCheck %s
+
+// CHECK: OpCapability Linkage
+// CHECK: OpDecorate %external_function LinkageAttributes "external_function" Export
+export int external_function() {
+  return 1;
+}
+
+[numthreads(8, 8, 1)]
+void main() {
+  external_function();
+	return;
+}

tools/clang/test/CodeGenSPIRV/fn.fixfuncall-linkage.hlsl

@@ -1,7 +1,7 @@
 // RUN: %dxc -T lib_6_3 -fspv-target-env=universal1.5 -fspv-fix-func-call-arguments -O0  %s -spirv | FileCheck %s
 
-// CHECK: OpCapability Shader
-// CHECK: OpCapability Linkage
+// CHECK-DAG: OpCapability Shader
+// CHECK-DAG: OpCapability Linkage
 RWStructuredBuffer< float4 > output : register(u1);
 
 // CHECK: OpDecorate %main LinkageAttributes "main" Export