[HLSL][SPIR-V] Handle SV_Postion builtin in PS

Keenuts · Keenuts · commit 84fd2cd7f42a · 2025-06-10T14:41:17.000+02:00
This commit is using the same mechanism as vk::ext_builtin_input to
implement the SV_Position semantic input.
The HLSL signature is not yet ready for DXIL, hence this commit only
implements the SPIR-V side.

This is incomplete as it doesn't allow the semantic on hull/domain and
other shaders, but it's a first step to validate the overall input/output
semantic logic.
diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
@@ -4879,6 +4879,13 @@ def HLSLResourceBinding: InheritableAttr {
   }];
 }
 
+def HLSLSV_Position : HLSLAnnotationAttr {
+  let Spellings = [HLSLAnnotation<"sv_position">];
+  let Subjects = SubjectList<[ParmVar, Field]>;
+  let LangOpts = [HLSL];
+  let Documentation = [HLSLSV_PositionDocs];
+}
+
 def HLSLPackOffset: HLSLAnnotationAttr {
   let Spellings = [HLSLAnnotation<"packoffset">];
   let LangOpts = [HLSL];
diff --git a/clang/include/clang/Basic/AttrDocs.td b/clang/include/clang/Basic/AttrDocs.td
@@ -8529,6 +8529,20 @@ The full documentation is available here: https://docs.microsoft.com/en-us/windo
   }];
 }
 
+def HLSLSV_PositionDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+The ``SV_Position`` semantic, when applied to an input parameter in a pixel
+shader, contains the location of the pixel center (x, y) in screen space.
+This semantic can be applied to the parameter, or a field in a struct used
+as input parameter.
+This attribute is supported as input in pixel, hull, domain and mesh shaders.
+This attribute is supported as output in vertex, geometry and domain shaders.
+
+The full documentation is available here: https://docs.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl-semantics
+  }];
+}
+
 def HLSLGroupSharedAddressSpaceDocs : Documentation {
   let Category = DocCatVariable;
   let Content = [{
diff --git a/clang/include/clang/Sema/SemaHLSL.h b/clang/include/clang/Sema/SemaHLSL.h
@@ -125,6 +125,7 @@ class SemaHLSL : public SemaBase {
   void handleSV_DispatchThreadIDAttr(Decl *D, const ParsedAttr &AL);
   void handleSV_GroupThreadIDAttr(Decl *D, const ParsedAttr &AL);
   void handleSV_GroupIDAttr(Decl *D, const ParsedAttr &AL);
+  void handleSV_PositionAttr(Decl *D, const ParsedAttr &AL);
   void handlePackOffsetAttr(Decl *D, const ParsedAttr &AL);
   void handleShaderAttr(Decl *D, const ParsedAttr &AL);
   void handleResourceBindingAttr(Decl *D, const ParsedAttr &AL);
@@ -146,6 +147,7 @@ class SemaHLSL : public SemaBase {
 
   // Diagnose whether the input ID is uint/unit2/uint3 type.
   bool diagnoseInputIDType(QualType T, const ParsedAttr &AL);
+  bool diagnosePositionType(QualType T, const ParsedAttr &AL);
 
   bool CanPerformScalarCast(QualType SrcTy, QualType DestTy);
   bool ContainsBitField(QualType BaseTy);
diff --git a/clang/lib/CodeGen/CGHLSLRuntime.cpp b/clang/lib/CodeGen/CGHLSLRuntime.cpp
@@ -384,6 +384,28 @@ static Value *buildVectorInput(IRBuilder<> &B, Function *F, llvm::Type *Ty) {
   return B.CreateCall(F, {B.getInt32(0)});
 }
 
+static void addBuiltinDecoration(llvm::GlobalVariable *GV, unsigned BuiltIn) {
+  LLVMContext &Ctx = GV->getContext();
+  IRBuilder<> B(GV->getContext());
+  MDNode *Operands =
+      MDNode::get(Ctx, {ConstantAsMetadata::get(B.getInt32(11)),
+                        ConstantAsMetadata::get(B.getInt32(BuiltIn))});
+  MDNode *Decoration = MDNode::get(Ctx, {Operands});
+  GV->addMetadata("spirv.Decorations", *Decoration);
+}
+
+static llvm::Value *createSPIRVBuiltinLoad(IRBuilder<> &B, llvm::Module &M,
+                                           llvm::Type *Ty, const Twine &Name,
+                                           unsigned BuiltInID) {
+  auto *GV = new llvm::GlobalVariable(
+      M, Ty, /* isConstant= */ true, llvm::GlobalValue::ExternalLinkage,
+      /* Initializer= */ nullptr, Name, /* insertBefore= */ nullptr,
+      llvm::GlobalVariable::GeneralDynamicTLSModel,
+      /* AddressSpace */ 7, /* isExternallyInitialized= */ true);
+  addBuiltinDecoration(GV, BuiltInID);
+  return B.CreateLoad(Ty, GV);
+}
+
 llvm::Value *CGHLSLRuntime::emitInputSemantic(IRBuilder<> &B,
                                               const ParmVarDecl &D,
                                               llvm::Type *Ty) {
@@ -407,6 +429,13 @@ llvm::Value *CGHLSLRuntime::emitInputSemantic(IRBuilder<> &B,
     llvm::Function *GroupIDIntrinsic = CGM.getIntrinsic(getGroupIdIntrinsic());
     return buildVectorInput(B, GroupIDIntrinsic, Ty);
   }
+  if (D.hasAttr<HLSLSV_PositionAttr>()) {
+    if (getArch() == llvm::Triple::spirv)
+      return createSPIRVBuiltinLoad(B, CGM.getModule(), Ty, "sv_position",
+                                    /* BuiltIn::Position */ 0);
+    llvm_unreachable(
+        "Shader signature for semantics not implemented for DXIL.");
+  }
   assert(false && "Unhandled parameter attribute");
   return nullptr;
 }
@@ -626,16 +655,8 @@ void CGHLSLRuntime::initializeBufferFromBinding(const HLSLBufferDecl *BufDecl,
 
 void CGHLSLRuntime::handleGlobalVarDefinition(const VarDecl *VD,
                                               llvm::GlobalVariable *GV) {
-  if (auto Attr = VD->getAttr<HLSLVkExtBuiltinInputAttr>()) {
-    LLVMContext &Ctx = GV->getContext();
-    IRBuilder<> B(GV->getContext());
-    MDNode *Operands = MDNode::get(
-        Ctx, {ConstantAsMetadata::get(
-                  B.getInt32(/* Spirv::Decoration::BuiltIn */ 11)),
-              ConstantAsMetadata::get(B.getInt32(Attr->getBuiltIn()))});
-    MDNode *Decoration = MDNode::get(Ctx, {Operands});
-    GV->addMetadata("spirv.Decorations", *Decoration);
-  }
+  if (auto Attr = VD->getAttr<HLSLVkExtBuiltinInputAttr>())
+    addBuiltinDecoration(GV, Attr->getBuiltIn());
 }
 
 llvm::Instruction *CGHLSLRuntime::getConvergenceToken(BasicBlock &BB) {
diff --git a/clang/lib/Parse/ParseHLSL.cpp b/clang/lib/Parse/ParseHLSL.cpp
@@ -289,6 +289,7 @@ void Parser::ParseHLSLAnnotations(ParsedAttributes &Attrs,
   case ParsedAttr::AT_HLSLSV_GroupID:
   case ParsedAttr::AT_HLSLSV_GroupIndex:
   case ParsedAttr::AT_HLSLSV_DispatchThreadID:
+  case ParsedAttr::AT_HLSLSV_Position:
     break;
   default:
     llvm_unreachable("invalid HLSL Annotation");
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -7557,6 +7557,9 @@ ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D, const ParsedAttr &AL,
   case ParsedAttr::AT_HLSLWaveSize:
     S.HLSL().handleWaveSizeAttr(D, AL);
     break;
+  case ParsedAttr::AT_HLSLSV_Position:
+    S.HLSL().handleSV_PositionAttr(D, AL);
+    break;
   case ParsedAttr::AT_HLSLVkExtBuiltinInput:
     S.HLSL().handleVkExtBuiltinInputAttr(D, AL);
     break;
diff --git a/clang/lib/Sema/SemaHLSL.cpp b/clang/lib/Sema/SemaHLSL.cpp
@@ -764,6 +764,13 @@ void SemaHLSL::CheckSemanticAnnotation(
       return;
     DiagnoseAttrStageMismatch(AnnotationAttr, ST, {llvm::Triple::Compute});
     break;
+  case attr::HLSLSV_Position:
+    // TODO: allow use on other shader types & output once the overall semantic
+    // logic is implemented.
+    if (ST == llvm::Triple::Pixel)
+      return;
+    DiagnoseAttrStageMismatch(AnnotationAttr, ST, {llvm::Triple::Pixel});
+    break;
   default:
     llvm_unreachable("Unknown HLSLAnnotationAttr");
   }
@@ -1147,6 +1154,26 @@ void SemaHLSL::handleSV_DispatchThreadIDAttr(Decl *D, const ParsedAttr &AL) {
                  HLSLSV_DispatchThreadIDAttr(getASTContext(), AL));
 }
 
+bool SemaHLSL::diagnosePositionType(QualType T, const ParsedAttr &AL) {
+  const auto *VT = T->getAs<VectorType>();
+
+  if (!T->hasFloatingRepresentation() || (VT && VT->getNumElements() > 4)) {
+    Diag(AL.getLoc(), diag::err_hlsl_attr_invalid_type)
+        << AL << "float/float1/float2/float3";
+    return false;
+  }
+
+  return true;
+}
+
+void SemaHLSL::handleSV_PositionAttr(Decl *D, const ParsedAttr &AL) {
+  auto *VD = cast<ValueDecl>(D);
+  if (!diagnosePositionType(VD->getType(), AL))
+    return;
+
+  D->addAttr(::new (getASTContext()) HLSLSV_PositionAttr(getASTContext(), AL));
+}
+
 void SemaHLSL::handleSV_GroupThreadIDAttr(Decl *D, const ParsedAttr &AL) {
   auto *VD = cast<ValueDecl>(D);
   if (!diagnoseInputIDType(VD->getType(), AL))
diff --git a/clang/test/CodeGenHLSL/semantics/SV_Position.ps.hlsl b/clang/test/CodeGenHLSL/semantics/SV_Position.ps.hlsl
@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -triple spirv-unknown-vulkan1.3-pixel -x hlsl -emit-llvm -finclude-default-header -disable-llvm-passes -o - %s | FileCheck %s
+
+// CHECK: @sv_position = external thread_local addrspace(7) externally_initialized constant <4 x float>, !spirv.Decorations !0
+
+// CHECK: define void @main() {{.*}} {
+float4 main(float4 p : SV_Position) {
+  // CHECK: %[[#P:]] = load <4 x float>, ptr addrspace(7) @sv_position, align 16
+  // CHECK: %[[#R:]] = call spir_func <4 x float> @_Z4mainDv4_f(<4 x float> %[[#P]])
+  return p;
+}
diff --git a/clang/test/SemaHLSL/Semantics/position.ps.hlsl b/clang/test/SemaHLSL/Semantics/position.ps.hlsl
@@ -0,0 +1,7 @@
+// RUN: %clang_cc1 -triple dxil-pc-shadermodel6.0-pixel -x hlsl -finclude-default-header -o - %s -ast-dump | FileCheck %s
+
+float4 main(float4 a : SV_Position) {
+// CHECK: FunctionDecl 0x{{[0-9a-fA-F]+}} <{{.*}}> line:[[@LINE-1]]:8 main 'float4 (float4)'
+// CHECK-NEXT: ParmVarDecl 0x{{[0-9a-fA-F]+}} <{{.*}}> col:20 a 'float4':'vector<float, 4>'
+// CHECK-NEXT: HLSLSV_PositionAttr 0x{{[0-9a-fA-F]+}} <{{.*}}>
+}
diff --git a/clang/test/SemaHLSL/Semantics/position.vs.hlsl b/clang/test/SemaHLSL/Semantics/position.vs.hlsl
@@ -0,0 +1,6 @@
+// RUN: %clang_cc1 -triple dxil-pc-shadermodel6.0-vertex -x hlsl -finclude-default-header -o - %s -verify
+
+// expected-error@+1 {{attribute 'SV_Position' is unsupported in 'vertex' shaders, requires pixel}}
+float4 main(float4 a : SV_Position) {
+  return a;
+}
