[FMV] mangling of forward-declared multi-versioned callee differs between gcc and clang

[jroelofs]

https://clang.godbolt.org/z/zeeTxd3va

__attribute__((target_version("fp16")))
void foo(void);

__attribute__((target_version("fp16fml")))
void foo(void);

__attribute__((target_version("default")))
void foo(void);

void caller() {
    foo();
}

Note how clang calls it _Z3foov.ifunc, and gcc refers to it as _Z3foov. IMO gcc's behavior is the desirable one: it lets you forward-declare foo in a header without having to promise that the implementation is multi-versioned. This is useful for libraries that have to provide a stable ABI to be able to adopt FMV in the implementation of those interfaces, without having to write an excessive number of shims.

[jroelofs]

cc @elizabethandrews @DanielKristofKiss @ilinpv

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: Jon Roelofs (jroelofs)

https://clang.godbolt.org/z/zeeTxd3va

__attribute__((target_version("fp16")))
void foo(void);

__attribute__((target_version("fp16fml")))
void foo(void);

__attribute__((target_version("default")))
void foo(void);

void caller() {
    foo();
}

Note how clang calls it _Z3foov.ifunc, and gcc refers to it as _Z3foov. IMO gcc's behavior is the desirable one: it lets you forward-declare foo in a header without having to promise that the implementation is multi-versioned. This is useful for libraries that have to provide a stable ABI to be able to adopt FMV in the implementation of those interfaces, without having to write an excessive number of shims.

[jroelofs]

Note that gcc/clang match for target_clones: https://clang.godbolt.org/z/4cKoxcjGT

[jroelofs]

I think it's because of this:

llvm-project/clang/lib/CodeGen/CodeGenModule.cpp

Lines 4392 to 4400 in b1ac052

	// Holds the name of the resolver, in ifunc mode this is the ifunc (which has
	// a separate resolver).
	std::string ResolverName = MangledName;
	if (getTarget().supportsIFunc()) {
	if (!FD->isTargetClonesMultiVersion())
	ResolverName += ".ifunc";
	} else if (FD->isTargetMultiVersion()) {
	ResolverName += ".resolver";
	}