llvm dev - Dec 2015 - Allowing virtual registers after register allocation

> On Dec 10, 2015, at 10:49 AM, Derek Schuff <dschuff at google.com>
wrote:
> 
> 
> 
> On Thu, Dec 10, 2015 at 10:13 AM Quentin Colombet <qcolombet at
apple.com <mailto:qcolombet at apple.com>> wrote:
> 
> I am tempted to think no, we don’t, but I don’t know the use cases.
> What post-RA passes with want to run with virtual regs?
> 
> The immediate one that precipitated this mail was PrologEpilogInserter.
> However currently the only other pass we have disabled in
WebAssemblyTargetMachine is MachineCopyPropagation.
> Several passes (post-RA MachineLICM, StackSlotColoring) already only run if
RA runs.
> Everything else is running today. Currently that's ShrinkWrap,
BranchFolder, ExpandPostRAPseudos, PostRAScheduler, GCMachineCodeAnalysis,
MachineBlockPlacement, FuncletLayout, and StackMapLiveness. All of these run
after our register coloring pass.
I don’t know for the other passes, but I don’t think it makes sense to teach
PrologEpilogInserter to work on virtual registers, since part of its job is to
get rid of any virtual registers created when lowering the frame.
I.e., that would indirectly mean that we would need to teach the scavenger how
to recycle virtual registers!

The bottom line is, it feels wrong to me.

Cheers,
Q.
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----- Original Message -----
> From: "Quentin Colombet" <qcolombet at apple.com>
> To: "Derek Schuff" <dschuff at google.com>
> Cc: "Hal Finkel" <hfinkel at anl.gov>, llvm-dev at
lists.llvm.org
> Sent: Thursday, December 10, 2015 1:11:19 PM
> Subject: Re: [llvm-dev] Allowing virtual registers after register
allocation
> 
> On Dec 10, 2015, at 10:49 AM, Derek Schuff < dschuff at google.com >
> wrote:
> 
> On Thu, Dec 10, 2015 at 10:13 AM Quentin Colombet <
> qcolombet at apple.com > wrote:
> 
> I am tempted to think no, we don’t, but I don’t know the use cases.
> What post-RA passes with want to run with virtual regs?
> 
> 
> The immediate one that precipitated this mail was
> PrologEpilogInserter.
> However currently the only other pass we have disabled in
> WebAssemblyTargetMachine is MachineCopyPropagation.
> Several passes (post-RA MachineLICM, StackSlotColoring) already only
> run if RA runs.
> Everything else is running today. Currently that's ShrinkWrap,
> BranchFolder, ExpandPostRAPseudos, PostRAScheduler,
> GCMachineCodeAnalysis, MachineBlockPlacement, FuncletLayout, and
> StackMapLiveness. All of these run after our register coloring pass.
> 
> 
> I don’t know for the other passes, but I don’t think it makes sense
> to teach PrologEpilogInserter to work on virtual registers, since
> part of its job is to get rid of any virtual registers created when
> lowering the frame.
> I.e., that would indirectly mean that we would need to teach the
> scavenger how to recycle virtual registers!
> 
I think this is exactly the part of PEI that they disable.

 -Hal
> 
> The bottom line is, it feels wrong to me.
> 
> 
> Cheers,
> Q.
-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory

On Thu, Dec 10, 2015 at 11:13 AM Hal Finkel <hfinkel at anl.gov> wrote:
>
> > I don’t know for the other passes, but I don’t think it makes sense
> > to teach PrologEpilogInserter to work on virtual registers, since
> > part of its job is to get rid of any virtual registers created when
> > lowering the frame.
> > I.e., that would indirectly mean that we would need to teach the
> > scavenger how to recycle virtual registers!
> >
>
> I think this is exactly the part of PEI that they disable.
>
>
Yes; see  http://reviews.llvm.org/D15394
If the target has no callee-saved registers and you disable scavenging,
you're basically left with.... prolog/epilog insertion and FrameIndex
elimination.
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