[clang] [FMV] Emit the resolver along with the default version definition. (PR #84405)

[Alexandros Lamprineas via cfe-commits]

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@@ -4347,6 +4362,19 @@ void CodeGenModule::emitCPUDispatchDefinition(GlobalDecl GD) {
   }
 }
 
+/// Adds a declaration to the list of multi version functions if not present.
+void CodeGenModule::AddDeferredMultiVersionResolverToEmit(GlobalDecl GD) {
+  const auto *FD = cast<FunctionDecl>(GD.getDecl());
+  assert(FD && "Not a FunctionDecl?");
+
+  if (FD->isTargetVersionMultiVersion()) {
+    StringRef NamePrefix = getMangledName(GD).split('.').first;
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labrinea wrote:

Initially I was using `FD->getName()` as the key here but this wouldn't work for C++ mangled names. `getMangledName()` internally calls `getMangledNameImpl()` which returns the multi versioned name (this would be "foo.bar.baz._Mlse" in you example). The key needs to be the prefix "foo.bar.baz" so that we don't push every multi version of "foo.bar.baz" in the list `MultiVersionFuncs` to be processed by `emitMultiVersionFunctions()`. This would generate a resolver body with multiple copies of the basic blocks corresponding to each version. Hmm, I am not sure how to handle this example properly, perhaps with `rsplit()`.

https://github.com/llvm/llvm-project/pull/84405