[clang] [clang-tools-extra] [llvm] [Clang] Fix linker error for function multiversioning (PR #71706)

[Jon Roelofs via cfe-commits]

================
@@ -4114,8 +4114,26 @@ void CodeGenModule::emitMultiVersionFunctions() {
     }
 
     llvm::Constant *ResolverConstant = GetOrCreateMultiVersionResolver(GD);
-    if (auto *IFunc = dyn_cast<llvm::GlobalIFunc>(ResolverConstant))
+    if (auto *IFunc = dyn_cast<llvm::GlobalIFunc>(ResolverConstant)) {
       ResolverConstant = IFunc->getResolver();
+      // In Aarch64, default versions of multiversioned functions are mangled to
+      // their 'normal' assembly name. This deviates from other targets which
+      // append a '.default' string. As a result we need to continue appending
+      // .ifunc in Aarch64.
+      // FIXME: Should Aarch64 mangling for 'default' multiversion function and
+      // in turn ifunc function match that of other targets?
----------------
jroelofs wrote:

I have been meaning to file an issue about this. Not using a different mangling means you always get the default version when referencing a multi-versioned function outside of the TU.

Consider e.g:

```
PublicHeader.h:
 
int versioned(void);
 
void *inside_TU();
 
void *outside_TU();
 
---
 
TUA.c:
 
#include "PublicHeader.h"
 
__attribute__((target_version("simd")))
int versioned(void) { return 1; }
 
__attribute__((target_version("default")))
int versioned(void) { return 2; }
 
void *inside_TU(void) { return versioned; }
 
---
 
TUB.c:
 
#include "PublicHeader.h"
 
void *outside_TU(void) { return versioned; }
 
---
 
Check.c:
 
#include "PublicHeader.h"
 
int main() { return inside_TU() == outside_TU(); }
```

@rsandifo-arm brought up a similar case on x86:

```
__attribute__((target("avx2")))
int versioned(void) { return 1; }
 
__attribute__((target("default")))
int versioned(void) { return 2; }
 
int (*inside_TU(void))(void) { return versioned; }
```

If we fix this, we should definitely make sure both the ACLE folks, and GCC are on board, and that the fix makes sense in the context of other targets.

https://github.com/llvm/llvm-project/pull/71706