[llvm-dev] How to force unused external routine declaration into object

Fri May 8 10:00:52 PDT 2020

yeah, don't think there's anything builtin to handle that - likely you'll
have to roll your own

On Fri, May 8, 2020 at 7:37 AM John Reagan <john.reagan at vmssoftware.com>
wrote:

> Yes, a external declaration with no uses.  No calls to it.  No address
> taken. Just the external declaration and I want it to say around and
> make it into the ELF symbol table as an undefined external reference.
>
> Our BLISS language behaves that way and we have code that relies on that
> behavior as a way to get the linker to include a particular routine into
> the final image.  On our other platforms, our old backend had to invent
> a special attribute to indicate "you must put this in the ELF symbol
> table regardless".
>
> I can easily create a compiler-generated static and initialize it with
> the address.  That will do what I want at the slight expense of a static
> location (memory is cheap) and a relocation for the linker to process
> (it is already doing a ton anyway, what's one more).  It isn't that
> common so I'm not too worried.  I was just wondering if I was missing it.
>
> Thanks
>
> On 5/7/2020 9:58 PM, David Blaikie wrote:
> >
> >
> > On Thu, May 7, 2020 at 6:47 PM John Reagan via llvm-dev
> > <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> >
> >     I had thought about "used", but not aware of the @llvm.used, et al.
> >
> >     I wrote some test programs with __attribute__((used)) but that felt
> like
> >     something you'd put on function definitions to force code to be
> >     generated regardless.
> >
> >
> > Isn't that what you're asking for?
> >
> > Though I'm slightly confused by your original statement - /LLVM/ (the
> > middle/backend, compiler - generating object files) wouldn't discard an
> > external-linkage function with no calls, because the call might be in
> > another module. Oh, a declaration?
> >
> > You have a declaration (without a definition) of a function in an
> > llvm::Module and you want some remnant of that to appear in the object
> > file? Yeah, I don't think that's supported - how's that done on other
> > compilers/what's the prior art you're trying to emulate here?
> >
> >
> >
> >     In the worst case, I'll do some metadata hack (I've ready had to do
> that
> >     for BLISS' GLOBAL BIND feature).
> >
> >     Thanks for the response.  I'll let folks know what I find out.
> >
> >     P.S. we're about to enter field test for OpenVMS after 2+ years of
> work
> >     on the system and compilers.  I can now boot it under VBox on my W10
> >     system here at home.
> >
> >     On 5/7/2020 4:16 PM, Robinson, Paul wrote:
> >     >
> >     >
> >     >> -----Original Message-----
> >     >> From: llvm-dev <llvm-dev-bounces at lists.llvm.org
> >     <mailto:llvm-dev-bounces at lists.llvm.org>> On Behalf Of John Reagan
> >     >> via llvm-dev
> >     >> Sent: Wednesday, May 6, 2020 9:29 PM
> >     >> To: via llvm-dev <llvm-dev at lists.llvm.org
> >     <mailto:llvm-dev at lists.llvm.org>>
> >     >> Subject: [llvm-dev] How to force unused external routine
> >     declaration into
> >     >> object
> >     >>
> >     >> I'm defining an external function in the IR that has no uses at
> >     all.  No
> >     >> calls, no address taken, nada.
> >     >>
> >     >> Such an unused declaration seems to be just dropped on the floor
> >     as not
> >     >> needed.  Seems reasonable in most cases.
> >     >>
> >     >> However, one of my OpenVMS compilers (BLISS) has a language rule
> that
> >     >> expects such definitions to get into the ELF symbol table as a
> way to
> >     >> compel the linker to include certain object modules.
> >     >>
> >     >> With our backend for our older targets, we had a
> "required_om_entry"
> >     >> function attribute that told our backend to put it out regardless.
> >     >>
> >     >> I was looking at the list of function attributes and don't see
> >     anything
> >     >> that would accomplish this.
> >     >>
> >     >> I could just create a bogus variable and initialize it with the
> >     >> function's value, but that seems unsavory.
> >     >>
> >     >> I'm still using a way old version but I'll adapt as needed.
> >     >>
> >     >> Any suggestions?  Did I miss something?
> >     >
> >     > Hi John,
> >     >
> >     > This sounds like __attribute__((used)) which Clang turns into
> >     > entries in the @llvm.used list.  There's also @llvm.compiler.used
> >     > and I'm not clear what the difference is, but hopefully that's
> >     > enough of a pointer that you can get it to work for Bliss.
> >     >
> >     > --paulr
> >     >
> >     >>
> >     >> John (safely at home with sufficient food & wine)
> >     >> _______________________________________________
> >     >> LLVM Developers mailing list
> >     >> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
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> >
>
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