[llvm] r263521 - [MachO] Add MachO alt-entry directive support.

[Lang Hames via llvm-commits]

Hi Rafael,

Why was it decided that the .alt_entry syntax was needed?


The majority of the MachO assembly users felt it was clearer syntax. This
should be MachO specific though - I'll take a look and either change where
this is parsed, or issue an error if it's parsed for a non-MachO file.

I think this is a odd. You are handling "foo" and "foo + 0" differently.


I believe 'foo + 0' is special-cased to behave the same as 'foo

On Tue, Mar 15, 2016 at 10:40 AM, Rafael Espíndola <
rafael.espindola at gmail.com> wrote:

> I don't think this was ready for commit.
>
> Why was it decided that the .alt_entry syntax was needed?
>
> > +
> > +    if (MAI->hasAltEntry() && isa<MCBinaryExpr>(Expr))
> > +      OutStreamer->EmitSymbolAttribute(Name, MCSA_AltEntry);
> > +
>
> I think this is a odd. You are handling "foo" and "foo + 0"
> differently. Given what you handle "a = ..." in MC, why do you need to
> change codegen at all?
>
> >    DirectiveKindMap[".symbol_resolver"] = DK_SYMBOL_RESOLVER;
> > +  DirectiveKindMap[".alt_entry"] = DK_ALT_ENTRY;
> >    DirectiveKindMap[".private_extern"] = DK_PRIVATE_EXTERN;
> >    DirectiveKindMap[".reference"] = DK_REFERENCE;
> >    DirectiveKindMap[".weak_definition"] = DK_WEAK_DEFINITION;
>
> Now you get a crash when using .alt_entry on non MachO. It should be
> an error. That is, you should not be parsing this in generic code,
> only on MachO specific code.
>
> Probably most importantly:
>
> As noted in the review this introduces a different bug with regards to
> ld and llvm-mc not agreeing about which symbols are atoms. Given
>
> bar:
>         .alt_entry foo
> foo:
> call bar
>
> llvm-mc produces a relocation. Given
>
> bar:
> foo = .
> call bar
>
> we get no relocation. In both cases the linker thinks foo is not an atom.
>
> Cheers,
> Rafael
>
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