erichkeane added a subscriber: rnk.
erichkeane added inline comments.
================
Comment at: clang/lib/CodeGen/CodeGenModule.cpp:2957
+ if (!AliasFunc) {
+ auto *IFunc = cast<llvm::GlobalIFunc>(GetOrCreateLLVMFunction(
+ AliasName, DeclTy, GD, /*ForVTable=*/false, /*DontDefer=*/true,
----------------
zsrkmyn wrote:
> erichkeane wrote:
> > erichkeane wrote:
> > > I think we want this in GetOrCreateMultiVersionResolver, so that it gets
> > > created when the ifunc does. That way you just need a
> > > "FD->isCPUDispatchMultiVersion() || isCPUSpecificMultiVersion()" check
> > > inside the supportsIFunc branch.
> > After discussing this offline, I believe this is the right function to
> > create the alias. The motivating example is:
> >
> > // TU1:
> > __attribute__((cpu_dispatch(a,b,c))) void foo(void);
> >
> > // TU2:
> > extern void foo(void);
> >
> > Currently, TU1 doesn't bother to emit the ifunc, because we've attached
> > emitting this to when this is referenced.
> >
> > We made that choice because we expected TU2 to mark 'foo' as
> > cpu_dispatch/cpu_specific in SOME way. I believe that it is harmless to
> > emit the ifunc all the time, which this is attempting to do. However, this
> > needs to change the ifunc to have LinkOnceODR linkage in
> > GetOrCreateMultiVersionResolver, otherwise this can cause linker errors.
> >
> >
> I've finished most parts. But I think we should also set the resolver to have
> LinkOnceODR Linkage. Otherwise, we cannot have the cpu_dispatch declaration
> in multiple TUs.
>
> However there's no 'weak' symbol on Windows, so even setting the resolver
> linkage to LinkOnceODR cannot solve the duplicate defined symbol problem on
> Windows. Do you have any suggestions on it? :-)
Yep, I think the resolver should also be linkonceodr (as well as the ifunc).
See where they are generated and do it there.
I can't help but think that we've solved the weak symbols issue with windows
before, but I cannot for the life of me remember. @rnk @lebedev.ri , do
either of you remember? Does LinkOnceODR not do that trickery?
Repository:
rC Clang
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D67058/new/
https://reviews.llvm.org/D67058
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