zsrkmyn added inline comments.
================
Comment at: clang/lib/CodeGen/CodeGenModule.cpp:3002
false);
llvm::Constant *Resolver = GetOrCreateLLVMFunction(
MangledName + ".resolver", ResolverType, GlobalDecl{},
----------------
erichkeane wrote:
> zsrkmyn wrote:
> > zsrkmyn wrote:
> > > erichkeane wrote:
> > > > zsrkmyn wrote:
> > > > > erichkeane wrote:
> > > > > > This Resolver should have the same linkage as below.
> > > > > Actually, I wanted to set linkage here at the first time, but failed.
> > > > > When compiling code with cpu_specific but no cpu_dispatch, we cannot
> > > > > set it as LinkOnceODR or WeakODR. E.g.:
> > > > >
> > > > > ```
> > > > > $ cat specific_only.c
> > > > > __declspec(cpu_specific(pentium_iii))
> > > > > int foo(void) { return 0; }
> > > > > int usage() { return foo(); }
> > > > >
> > > > > $ clang -fdeclspec specific_only.c
> > > > >
> > > > > Global is external, but doesn't have external or weak linkage!
> > > > >
> > > > > i32 ()* ()* @foo.resolver
> > > > >
> > > > > fatal error: error in backend: Broken module found, compilation
> > > > > aborted!
> > > > > ```
> > > > >
> > > > > This is found by lit test test/CodeGen/attr-cpuspecific.c, in which
> > > > > 'SingleVersion()' doesn't have a cpu_dispatch declaration.
> > > > The crash message is complaining it isn't external/weak. However,
> > > > WeakODR should count, right? Can you look into it a bit more to see
> > > > what it thinks is broken?
> > > No, actually I've tried it earlier with the example I mentioned in my
> > > last comment, but WeakODR still makes compiler complaining. I think it's
> > > `foo.resolver` that cannot be declared with as WeakODR/LinkOnceODR
> > > without definition. But I'm really not familiar with these rules.
> > According to the `Verifier::visitGlobalValue()` in Verify.cpp, an
> > declaration can only be `ExternalLinkage` or `ExternalWeakLinkage`. So I
> > still believe we cannot set the resolver to
> > `LinkOnceODRLinkage/WeakODRLinkage` here, as they are declared but not
> > defined when we only have `cpu_specified` but no `cpu_dispatch` in a TU as
> > the example above.
> That doesn't seem right then. IF it allows ExternalWeakLinkage I'd expect
> WeakODR to work as well, since it is essentially the same thing.
I think we should have a double check. It is said "It is illegal for a function
declaration to have any linkage type other than `external` or `extern_weak`" at
the last line of section Linkage Type in the reference manual [1]. I guess
`weak_odr` is not designed for declaration purpose and should be only used by
definition.
[1] https://llvm.org/docs/LangRef.html#linkage-types
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https://reviews.llvm.org/D67058/new/
https://reviews.llvm.org/D67058
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