[lld] [llvm] [clang-tools-extra] [compiler-rt] [clang] [lldb] [flang] [libc] [libcxx] [Clang] Fix linker error for function multiversioning (PR #71706)

[Tom Honermann via llvm-commits]

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@@ -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?
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tahonermann wrote:

Since `target_version` is new, I think it is reasonable to change its behavior (via a different github PR).

I suspect the `target` behavior is an artifact of how the multiversion function features evolved. My best guess is that the original implementation didn't support an indirect function facility that enabled dynamic resolution from outside a defining TU. Perhaps that was part of the motivation for adding `target_clones`.

I just discovered that gcc doesn't support the `target` attribute when compiling for C. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108964. I had been under the impression that it just didn't support overloading on the `target` attribute (but that the attribute still affected code generation). Interesting that no warning is given for an ignored attribute. Here is the example from the doc Elizabeth linked compiling as C. https://godbolt.org/z/asfrhG51G.

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