llvm dev - Apr 2011 - [LLVMdev] Explicit register usage in LLVM assembly

Hello!
     Is there a way to force explicit register usage (e.g. %r15 in amd64 
architecture) in
LLVM assembly code? I was proposed in #llvm channel at irc.oftc.net to 
use inline
assembly but i find it rather impractical in my case. Is there any other 
way?

Thanx,
~y.

Hello Yiannis,

You could write a custom backend that doesn't allocate %r15 for general
usage.
 The normal way to do this is to set up a custom calling convention for all 
functions that keeps a sentinel in %r15 so that it always holds the sentinel. 
 This is how new operating systems are supported with custom ABIs.  The only 
problem is that you cannot be assured that %r15 stays put between function calls
such as for an interrupt.

If you had asked "Is there another EASY way to force explicit register
usage?"
the answer would have been "No."


--Sam Crow


----- Original Message ----
> From: Yiannis Tsiouris <yiannis.tsiouris at gmail.com>
> To: llvmdev at cs.uiuc.edu
> Cc: Chris Stavrakakis <hydralisk.r at gmail.com>
> Sent: Sat, April 2, 2011 9:39:13 AM
> Subject: [LLVMdev] Explicit register usage in LLVM assembly
> 
> Hello!
>      Is there a way to force explicit register usage (e.g.  %r15 in amd64 
> architecture) in
> LLVM assembly code? I was proposed in  #llvm channel at irc.oftc.net to 
> use inline
> assembly but i find it rather impractical in my case. Is  there any other 
> way?
> 
> Thanx,
> ~y.
> _______________________________________________
> LLVM  Developers mailing list
> LLVMdev at cs.uiuc.edu           http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>

On 04/02/2011 06:26 PM, Samuel Crow wrote:
> Hello Yiannis,
>
> You could write a custom backend that doesn't allocate %r15 for general
usage.
>  The normal way to do this is to set up a custom calling convention for all
> functions that keeps a sentinel in %r15 so that it always holds the
sentinel.
>  This is how new operating systems are supported with custom ABIs.  The
only
> problem is that you cannot be assured that %r15 stays put between function
calls
> such as for an interrupt.
>
> If you had asked "Is there another EASY way to force explicit register
usage?"
> the answer would have been "No."
>
>
> --Sam Crow
>
>
> ----- Original Message ----
>   
>> From: Yiannis Tsiouris <yiannis.tsiouris at gmail.com>
>> To: llvmdev at cs.uiuc.edu
>> Cc: Chris Stavrakakis <hydralisk.r at gmail.com>
>> Sent: Sat, April 2, 2011 9:39:13 AM
>> Subject: [LLVMdev] Explicit register usage in LLVM assembly
>>
>> Hello!
>>      Is there a way to force explicit register usage (e.g.  %r15 in
amd64
>> architecture) in
>> LLVM assembly code? I was proposed in  #llvm channel at irc.oftc.net to
>> use inline
>> assembly but i find it rather impractical in my case. Is  there any
other
>> way?
>>
>> Thanx,
>> ~y.
>>     
Hello Sam,
  I am not sure i made myself clear about what i want. I was wondering
if there
is a way to communicate to LLVM's register allocator and define which
registers are pre-coloured and should not be used for allocation.


~y.