erichkeane added inline comments.
================
Comment at: clang/lib/CodeGen/CodeGenModule.cpp:3002
false);
llvm::Constant *Resolver = GetOrCreateLLVMFunction(
MangledName + ".resolver", ResolverType, GlobalDecl{},
----------------
zsrkmyn wrote:
> erichkeane wrote:
> > zsrkmyn wrote:
> > > 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
> > I had typed a reply, but apparently it didn't submit: Ah, nvm, I see now
> > that external-weak is different from weak.
> >
> > I don't really get the linkages sufficiently to know what the right thing
> > to do is then. If we DO have a definition, I'd say weak_odr so it can be
> > merged, right? If we do NOT, could externally_available work?
> No, I think it should be `external` instead of `available_externally`. The
> later cannot used for declaration as well.
>
> So, getting back to the example, **1)** if we have `cpu_dispatch` and
> `cpu_specific` in same TU, it's okay to use `weak_odr` for `foo.resolver` as
> it is defined when `emitCPUDispatchDefinition` and it can be merged. **2)**
> If we only have `cpu_specific` in a TU and have a reference to the dispatched
> function, `foo.resolver` will be referenced without definition, and
> `external` is the proper linkage to make it work.
>
> That's why I didn't set linkage type at this line.
> No, I think it should be external instead of available_externally. The later
> cannot used for declaration as well.
Wouldn't that make it un-mergable later? Meaning, if you emitted the
declaration from one TU, and the definition from another that you'd get a link
error?
I think the rules are more subtle than that. Any time you have a
`cpu_dispatch`, the resolver is weak_odr so that it can be merged later. The
presence of `cpu_specific` shouldn't matter.
For 2, I think you're mostly correct, as long as the linker can still merge
them.
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D67058/new/
https://reviews.llvm.org/D67058
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