[LLVMdev] creating a vreg in eliminateFrameIndex()

[Hal Finkel]

----- 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
> >
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory