[clang][FMV] Fix crash with cpu_specific attribute. (#115762)

When dealing with cpu_specific GlobalDecl,
GetOrCreateMultiVersionResolver should immediately return the already
created llvm function if it exists.

Fixes #115299.

Author: labrinea

clang/lib/CodeGen/CodeGenModule.cpp

@@ -4562,15 +4562,17 @@ llvm::Constant *CodeGenModule::GetOrCreateMultiVersionResolver(GlobalDecl GD) {
     ResolverName += ".resolver";
   }
 
+  bool ShouldReturnIFunc =
+      getTarget().supportsIFunc() && !FD->isCPUSpecificMultiVersion();
+
   // If the resolver has already been created, just return it. This lookup may
   // yield a function declaration instead of a resolver on AArch64. That is
   // because we didn't know whether a resolver will be generated when we first
   // encountered a use of the symbol named after this resolver. Therefore,
   // targets which support ifuncs should not return here unless we actually
   // found an ifunc.
   llvm::GlobalValue *ResolverGV = GetGlobalValue(ResolverName);
-  if (ResolverGV &&
-      (isa<llvm::GlobalIFunc>(ResolverGV) || !getTarget().supportsIFunc()))
+  if (ResolverGV && (isa<llvm::GlobalIFunc>(ResolverGV) || !ShouldReturnIFunc))
     return ResolverGV;
 
   const CGFunctionInfo &FI = getTypes().arrangeGlobalDeclaration(GD);
@@ -4583,7 +4585,7 @@ llvm::Constant *CodeGenModule::GetOrCreateMultiVersionResolver(GlobalDecl GD) {
 
   // For cpu_specific, don't create an ifunc yet because we don't know if the
   // cpu_dispatch will be emitted in this translation unit.
-  if (getTarget().supportsIFunc() && !FD->isCPUSpecificMultiVersion()) {
+  if (ShouldReturnIFunc) {
     unsigned AS = getTypes().getTargetAddressSpace(FD->getType());
     llvm::Type *ResolverType =
         llvm::FunctionType::get(llvm::PointerType::get(DeclTy, AS), false);
@@ -4602,11 +4604,9 @@ llvm::Constant *CodeGenModule::GetOrCreateMultiVersionResolver(GlobalDecl GD) {
 
   llvm::Constant *Resolver = GetOrCreateLLVMFunction(
       ResolverName, DeclTy, GlobalDecl{}, /*ForVTable=*/false);
-  assert(isa<llvm::GlobalValue>(Resolver) &&
+  assert(isa<llvm::GlobalValue>(Resolver) && !ResolverGV &&
          "Resolver should be created for the first time");
   SetCommonAttributes(FD, cast<llvm::GlobalValue>(Resolver));
-  if (ResolverGV)
-    replaceDeclarationWith(ResolverGV, Resolver);
   return Resolver;
 }
 

clang/test/CodeGen/attr-cpuspecific.c

@@ -154,6 +154,12 @@ void usages(void) {
   CpuSpecificNoDispatch();
   // LINUX: @CpuSpecificNoDispatch.ifunc()
   // WINDOWS: @CpuSpecificNoDispatch()
+  //
+  // Adding another use of CpuSpecificNoDispatch reproduces the
+  // crash in https://github.com/llvm/llvm-project/issues/115299
+  CpuSpecificNoDispatch();
+  // LINUX: @CpuSpecificNoDispatch.ifunc()
+  // WINDOWS: @CpuSpecificNoDispatch()
   OrderDispatchUsageSpecific();
   // LINUX: @OrderDispatchUsageSpecific.ifunc()
   // WINDOWS: @OrderDispatchUsageSpecific()