[LLVMdev] portable way to modify the FP register?

[James Molloy]

Hi,

That seems sensible, but:

> for the moment we're only considering X86-64 targets and we're forcing
> the FP to be materialized (ie, -disable-fp-elim) to ensure we can be
> independent of what happens to SP.

Does it therefore seem surprising that there is no way to do this portably? ;)

James Molloy
Graduate Compiler Engineer, ARM Ltd.


> -----Original Message-----
> From: Salvador Abreu [mailto:spa at di.uevora.pt]
> Sent: 13 December 2010 21:47
> To: James Molloy
> Cc: Salvador Abreu; llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] portable way to modify the FP register?
>
> Hi again,
>
> On Dec 13, 2010, at 18:03, James Molloy wrote:
>
> > To add to what Anton said, what if you're on a non-x86 target, or --
> no-frame-pointer is set?
>
> for the moment we're only considering X86-64 targets and we're forcing
> the FP to be materialized (ie, -disable-fp-elim) to ensure we can be
> independent of what happens to SP.
>
> btw, I can't think of any current ISA/ABI which doesn't have a frame
> pointer register...
>
> > Also, I'm intrigued as to how you guarantee that in the intervening
> time between the coroutine/generator function bar() returning one
> result and the next invocation of bar(), foo() doesn't call another
> function and overwrite bar()'s stack frame?
>
> the stack frame allocated by bar (called by foo) did not get released,
> and foo needs to use fp-based addressing.  bar's stack frame now
> occupies the top of the stack and this will be in effect when the
> subsequent functions get called, which will create new stack frames on
> top of bar's.
>
> we're defining a new calling convention which expects certain calls'
> frames to linger (similar to coroutining)
>
> cheers,
> Salvador
>
> >
> > Cheers,
> >
> > James Molloy
> > Graduate Compiler Engineer, ARM Ltd.
> >
> >
> >> -----Original Message-----
> >> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-
> bounces at cs.uiuc.edu]
> >> On Behalf Of Salvador Abreu
> >> Sent: 13 December 2010 15:57
> >> To: llvmdev at cs.uiuc.edu
> >> Subject: [LLVMdev] portable way to modify the FP register?
> >>
> >> Hi,
> >>
> >> I'd like to know if there's a portable way of modifying the FP
> >> register, in LLVM.
> >>
> >> Suppose foo() calls bar(); bar emits a result but remains active and
> >> needs to be able to produce more results - a bit like a coroutine.
> We
> >> want bar to restore FP to what it was inside foo before transferring
> >> control back to foo (bar's stack frame is not disposed of).
> >>
> >> We'd make sure that foo only accesses local variables via FP (SP
> >> addressing is disabled).
> >>
> >> What we'd like to do is something like (abstracted syntax and
> semantics
> >> :-)
> >>  (FP) = @llvm.frameaddress(1) ;
> >>
> >> So, is there a portable way to change FP?
> >>
> >> --
> >> Salvador Abreu       --  http://www.di.uevora.pt/~spa/
> >> Departamento de Informatica
> >> Universidade de Evora - PORTUGAL
> >>
> >>
> >>
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
> > -- IMPORTANT NOTICE: The contents of this email and any attachments
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> >
>
> --
> Salvador Abreu       --  http://www.di.uevora.pt/~spa/
> Departamento de Informatica
> Universidade de Evora - PORTUGAL
>


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