llvm dev - Jul 2017 - Lowering Select to Two Predicated Movs

Hi,

I was wondering what would be the best way to lower a select operation two
predicated movs. I looked through the ARM, MIPS, and NVPTX backends and
they all seem to lower a select to some sort of conditional move or native
select operation.

Ex.
select t3, cond, t2, t1

Becomes
cond mov t3, t2
!cond mov t3, t1

-Dilan
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On 7/7/2017 11:12 AM, Dilan Manatunga via llvm-dev
wrote:
> Hi,
>
> I was wondering what would be the best way to lower a select operation 
> two predicated movs. I looked through the ARM, MIPS, and NVPTX 
> backends and they all seem to lower a select to some sort of 
> conditional move or native select operation.
ARM lowers a select to a predicated mov.  MOVCCr is a pseudo-instruction 
which gets expanded after instruction selection.

-Eli

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux
Foundation Collaborative Project

My bad for not looking further. I'm still somewhat confused though. MOVCCr
gets expanded in the ARMExpandPseudoInsts pass, and it still seems only a
case of one instruction replacing the other.

My worry of emitting two instructions, is that a dead code pass will
eliminate the first instruction cause it thinks the second instruction is
defining the same register.

-Dilan

On Fri, Jul 7, 2017 at 11:20 AM Friedman, Eli <efriedma at codeaurora.org>
wrote:
> On 7/7/2017 11:12 AM, Dilan Manatunga via llvm-dev wrote:
> > Hi,
> >
> > I was wondering what would be the best way to lower a select operation
> > two predicated movs. I looked through the ARM, MIPS, and NVPTX
> > backends and they all seem to lower a select to some sort of
> > conditional move or native select operation.
>
> ARM lowers a select to a predicated mov.  MOVCCr is a pseudo-instruction
> which gets expanded after instruction selection.
>
> -Eli
>
> --
> Employee of Qualcomm Innovation Center, Inc.
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux
> Foundation Collaborative Project
>
>
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