[HLSL] Use hidden visibility for external linkage.

Implements
https://github.com/llvm/wg-hlsl/blob/main/proposals/0026-symbol-visibility.md.

The change is to stop using the `hlsl.export` attribute. Instead,
symbols with "program linkage" in HLSL will have export linkage with
default visibility, and symbols with "external linkage" in HLSL will
have export linkage with hidden visibility.

Author: s-perron

clang/lib/CodeGen/CGHLSLRuntime.cpp

@@ -444,14 +444,6 @@ void CGHLSLRuntime::emitEntryFunction(const FunctionDecl *FD,
   }
 }
 
-void CGHLSLRuntime::setHLSLFunctionAttributes(const FunctionDecl *FD,
-                                              llvm::Function *Fn) {
-  if (FD->isInExportDeclContext()) {
-    const StringRef ExportAttrKindStr = "hlsl.export";
-    Fn->addFnAttr(ExportAttrKindStr);
-  }
-}
-
 static void gatherFunctions(SmallVectorImpl<Function *> &Fns, llvm::Module &M,
                             bool CtorOrDtor) {
   const auto *GV =

clang/lib/CodeGen/CGHLSLRuntime.h

@@ -151,7 +151,6 @@ class CGHLSLRuntime {
   void setHLSLEntryAttributes(const FunctionDecl *FD, llvm::Function *Fn);
 
   void emitEntryFunction(const FunctionDecl *FD, llvm::Function *Fn);
-  void setHLSLFunctionAttributes(const FunctionDecl *FD, llvm::Function *Fn);
 
   llvm::Instruction *getConvergenceToken(llvm::BasicBlock &BB);
 

clang/lib/CodeGen/CodeGenFunction.cpp

@@ -1274,7 +1274,6 @@ void CodeGenFunction::StartFunction(GlobalDecl GD, QualType RetTy,
     if (FD->hasAttr<HLSLShaderAttr>()) {
       CGM.getHLSLRuntime().emitEntryFunction(FD, Fn);
     }
-    CGM.getHLSLRuntime().setHLSLFunctionAttributes(FD, Fn);
   }
 
   EmitFunctionProlog(*CurFnInfo, CurFn, Args);

clang/lib/CodeGen/CodeGenModule.cpp

@@ -1655,6 +1655,11 @@ void CodeGenModule::setGlobalVisibility(llvm::GlobalValue *GV,
     return;
   }
 
+  if (Context.getLangOpts().HLSL && !D->isInExportDeclContext()) {
+    GV->setVisibility(llvm::GlobalValue::HiddenVisibility);
+    return;
+  }
+
   if (GV->hasDLLExportStorageClass() || GV->hasDLLImportStorageClass()) {
     // Reject incompatible dlllstorage and visibility annotations.
     if (!LV.isVisibilityExplicit())

llvm/lib/Target/DirectX/DXILFinalizeLinkage.cpp

@@ -24,7 +24,9 @@ static bool finalizeLinkage(Module &M) {
   for (Function &EF : M.functions()) {
     if (EF.isIntrinsic())
       continue;
-    if (EF.hasFnAttribute("hlsl.shader") || EF.hasFnAttribute("hlsl.export"))
+    if (EF.hasExternalLinkage() && EF.hasDefaultVisibility())
+      continue;
+    if (EF.hasFnAttribute("hlsl.shader"))
       continue;
     Funcs.push_back(&EF);
   }

llvm/lib/Target/SPIRV/SPIRVCallLowering.cpp

@@ -444,7 +444,8 @@ bool SPIRVCallLowering::lowerFormalArguments(MachineIRBuilder &MIRBuilder,
                    .addUse(FuncVReg);
     addStringImm(F.getName(), MIB);
   } else if (F.getLinkage() != GlobalValue::InternalLinkage &&
-             F.getLinkage() != GlobalValue::PrivateLinkage) {
+             F.getLinkage() != GlobalValue::PrivateLinkage &&
+             F.getVisibility() != GlobalValue::HiddenVisibility) {
     SPIRV::LinkageType::LinkageType LnkTy =
         F.isDeclaration()
             ? SPIRV::LinkageType::Import

llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp

@@ -3895,7 +3895,8 @@ bool SPIRVInstructionSelector::selectGlobalValue(
   if (hasInitializer(GlobalVar) && !Init)
     return true;
 
-  bool HasLnkTy = !GV->hasInternalLinkage() && !GV->hasPrivateLinkage();
+  bool HasLnkTy = !GV->hasInternalLinkage() && !GV->hasPrivateLinkage() &&
+                  !GV->hasHiddenVisibility();
   SPIRV::LinkageType::LinkageType LnkType =
       GV->isDeclarationForLinker()
           ? SPIRV::LinkageType::Import