[LLVMdev] creating a vreg in eliminateFrameIndex()

[Steve King]

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
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>
> --
> Jon Roelofs
> jonathan at codesourcery.com
> CodeSourcery / Mentor Embedded
>