llvm dev - Dec 2010 - [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

> So, is there a portable way to change FP?
What if there is no FP register on a given target?

-- 
With best regards, Anton Korobeynikov
Faculty of Mathematics and Mechanics, Saint Petersburg State University

Hi,

To add to what Anton said, what if you're on a non-x86 target, or
--no-frame-pointer is set?

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?

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 are
confidential and may also be privileged. If you are not the intended recipient,
please notify the sender immediately and do not disclose the contents to any
other person, use it for any purpose, or store or copy the information in any
medium.  Thank you.

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 are
confidential and may also be privileged. If you are not the intended recipient,
please notify the sender immediately and do not disclose the contents to any
other person, use it for any purpose, or store or copy the information in any
medium.  Thank you.
> 
--
Salvador Abreu       --  http://www.di.uevora.pt/~spa/
Departamento de Informatica
Universidade de Evora - PORTUGAL