llvm dev - Jan 2015 - [LLVMdev] creating a vreg in eliminateFrameIndex()

Thanks Jon and Hal for the helpful pointers.  By returning true from
requiresRegisterScavenging() and requiresFrameIndexScavenging(), LLVM
handled all the scavenging effort.  That is nearly painless for the
target, so why do some targets seem to do scavenging on their own?

When the scavenged register is loaded with a simple immediate, is it
safe to search the BB and replace other uses of the same immediate
with the reg?  In my case this gives a code size reduction.  The
search would stop on finding another use of the scavenged register.

Thanks,
-steve

On Thu, Jan 29, 2015 at 1:15 PM, Jonathan Roelofs
<jroelofs.lists at gmail.com> wrote:
>
>
> On 1/29/15 2:00 PM, Steve King wrote:
>>
>> Hello LLVM,
>> The ARM target sometimes adds an instruction with a virtual register
>> in eliminateFrameIndex():
>>
>>
>>
https://github.com/llvm-mirror/llvm/blob/master/lib/Target/ARM/ARMBaseRegisterInfo.cpp
>>
>> This looks late for a virtual register to appear.  Where is this vreg
made
>> real?
>
> The register scavenger should take care of such emergency post-RA register
> allocation. For thumb1, this happens in
> Thumb1RegisterInfo::saveScavengerRegister():
>
http://llvm.org/docs/doxygen/html/structllvm_1_1Thumb1RegisterInfo.html#ac73131381e7f5533f8e1e601f8e1c9ca
>
>
> Cheers,
>
> Jon
>>
>>
>> Thanks,
>> -steve
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>
> --
> Jon Roelofs
> jonathan at codesourcery.com
> CodeSourcery / Mentor Embedded
>

----- Original Message -----
> From: "Steve King" <steve at metrokings.com>
> To: jonathan at codesourcery.com
> Cc: llvmdev at cs.uiuc.edu
> Sent: Friday, January 30, 2015 10:52:19 AM
> Subject: Re: [LLVMdev] creating a vreg in eliminateFrameIndex()
> 
> Thanks Jon and Hal for the helpful pointers.  By returning true from
> requiresRegisterScavenging() and requiresFrameIndexScavenging(), LLVM
> handled all the scavenging effort.  That is nearly painless for the
> target, so why do some targets seem to do scavenging on their own?
Some of the targets pre-date the introduction of that feature.
> 
> When the scavenged register is loaded with a simple immediate, is it
> safe to search the BB and replace other uses of the same immediate
> with the reg?  In my case this gives a code size reduction.  The
> search would stop on finding another use of the scavenged register.
Are you asking whether that would be safe to do on all targets?

 -Hal
> 
> Thanks,
> -steve
> 
> On Thu, Jan 29, 2015 at 1:15 PM, Jonathan Roelofs
> <jroelofs.lists at gmail.com> wrote:
> >
> >
> > On 1/29/15 2:00 PM, Steve King wrote:
> >>
> >> Hello LLVM,
> >> The ARM target sometimes adds an instruction with a virtual
> >> register
> >> in eliminateFrameIndex():
> >>
> >>
> >>
https://github.com/llvm-mirror/llvm/blob/master/lib/Target/ARM/ARMBaseRegisterInfo.cpp
> >>
> >> This looks late for a virtual register to appear.  Where is this
> >> vreg made
> >> real?
> >
> > The register scavenger should take care of such emergency post-RA
> > register
> > allocation. For thumb1, this happens in
> > Thumb1RegisterInfo::saveScavengerRegister():
> >
http://llvm.org/docs/doxygen/html/structllvm_1_1Thumb1RegisterInfo.html#ac73131381e7f5533f8e1e601f8e1c9ca
> >
> >
> > Cheers,
> >
> > Jon
> >>
> >>
> >> Thanks,
> >> -steve
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >>
> >
> > --
> > Jon Roelofs
> > jonathan at codesourcery.com
> > CodeSourcery / Mentor Embedded
> >
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> 
-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory

On Fri, Jan 30, 2015 at 8:59 AM, Hal Finkel <hfinkel at anl.gov>
wrote:
>> When the scavenged register is loaded with a simple immediate, is it
>> safe to search the BB and replace other uses of the same immediate
>> with the reg?  In my case this gives a code size reduction.  The
>> search would stop on finding another use of the scavenged register.
>
> Are you asking whether that would be safe to do on all targets?
No, just a poor question -- I had in mind only my out-of-tree target.
I take the answer as "it depends on your target".