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Arsenault, Matthew via llvm-dev

2019-Nov-14 08:20 UTC

[llvm-dev] imm COPY generated by PHI elim not propagated

In this case the load imm is foldable into the copy, once converted to a mov.
Directly folding this would be 4 v_mov_b32 instead of 5 produced currently

-Matt

On 11/14/19, 07:20, "llvm-dev on behalf of Quentin Colombet via
llvm-dev" <llvm-dev-bounces at lists.llvm.org on behalf of llvm-dev at
lists.llvm.org> wrote:
    
    Hi Ryan,
    
    Unless you can fold your immediate directly in an instruction, it is
actually not profitable to propagate them. Indeed you will end up with a bunch
of load imm instead of reusing a register that already hold this value.
    
    The way it works right now is, if holding this value in a register is too
expensive, i.e., it triggers a spill, then we rematerialize the immediate
instead of holding a register for it.
    
    Cheers,
    -Quentin
    
    > On Nov 13, 2019, at 7:36 AM, Ryan Taylor via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
    >
    > I have some code such that:
    >
    > vgpr1 = mov 0
    > branch bb
    > bb:
    > PHI vgpr2 = vgpr1, ….
    > PHI vgpr3 = vgpr1, ….
    > PHI vgpr4 = vgpr1, ….
    > PHI vgpr5 = vgpr1, ….
    >
    > PHI node elimination is generating copies for all these PHIs (and
hoisting them) as such:
    >
    > vgpr1 = 0
    > vgpr20 = COPY vgpr1 // old vgpr2
    > vgpr30 = COPY vgpr1 // old vgpr3
    > vgpr40 = COPY vgpr1 // old vgpr4
    > vgpr 50 = COPY vgprt1 // old vgpr5
    >
    > I expect the zero to get propagated in a later phase but it's not.
I was looking at adding immediate folding to the register coalescer but this
doesn't really seem like the right place. Any suggestions?
    >
    > I'm sort of surprised that other targets haven't run into this
issue.
    >
    > -Ryan
    >
    >
    > _______________________________________________
    > LLVM Developers mailing list
    > llvm-dev at lists.llvm.org
    > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
    
    _______________________________________________
    LLVM Developers mailing list
    llvm-dev at lists.llvm.org
    https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

Quentin Colombet via llvm-dev

2019-Nov-14 17:15 UTC

head link

[llvm-dev] imm COPY generated by PHI elim not propagated

That sounds like the folding could be done when you expand the copy in expand
pseudo after regalloc.
> On Nov 14, 2019, at 12:20 AM, Arsenault, Matthew <Matthew.Arsenault at
amd.com> wrote:
> 
> In this case the load imm is foldable into the copy, once converted to a
mov. Directly folding this would be 4 v_mov_b32 instead of 5 produced currently
> 
> -Matt
> 
> On 11/14/19, 07:20, "llvm-dev on behalf of Quentin Colombet via
llvm-dev" <llvm-dev-bounces at lists.llvm.org on behalf of llvm-dev at
lists.llvm.org> wrote:
> 
>    Hi Ryan,
> 
>    Unless you can fold your immediate directly in an instruction, it is
actually not profitable to propagate them. Indeed you will end up with a bunch
of load imm instead of reusing a register that already hold this value.
> 
>    The way it works right now is, if holding this value in a register is
too expensive, i.e., it triggers a spill, then we rematerialize the immediate
instead of holding a register for it.
> 
>    Cheers,
>    -Quentin
> 
>> On Nov 13, 2019, at 7:36 AM, Ryan Taylor via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
>> 
>> I have some code such that:
>> 
>> vgpr1 = mov 0
>> branch bb
>> bb:
>> PHI vgpr2 = vgpr1, ….
>> PHI vgpr3 = vgpr1, ….
>> PHI vgpr4 = vgpr1, ….
>> PHI vgpr5 = vgpr1, ….
>> 
>> PHI node elimination is generating copies for all these PHIs (and
hoisting them) as such:
>> 
>> vgpr1 = 0
>> vgpr20 = COPY vgpr1 // old vgpr2
>> vgpr30 = COPY vgpr1 // old vgpr3
>> vgpr40 = COPY vgpr1 // old vgpr4
>> vgpr 50 = COPY vgprt1 // old vgpr5
>> 
>> I expect the zero to get propagated in a later phase but it's not.
I was looking at adding immediate folding to the register coalescer but this
doesn't really seem like the right place. Any suggestions?
>> 
>> I'm sort of surprised that other targets haven't run into this
issue.
>> 
>> -Ryan
>> 
>> 
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 
>    _______________________________________________
>    LLVM Developers mailing list
>    llvm-dev at lists.llvm.org
>    https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 
>

Ryan Taylor via llvm-dev

2019-Nov-15 19:16 UTC

head link

[llvm-dev] imm COPY generated by PHI elim not propagated

This would require getting the reaching definition which requires live
intervals analysis.


On Thu, Nov 14, 2019 at 12:15 PM Quentin Colombet <qcolombet at apple.com>
wrote:
> That sounds like the folding could be done when you expand the copy in
> expand pseudo after regalloc.
>
> > On Nov 14, 2019, at 12:20 AM, Arsenault, Matthew <
> Matthew.Arsenault at amd.com> wrote:
> >
> > In this case the load imm is foldable into the copy, once converted to
a
> mov. Directly folding this would be 4 v_mov_b32 instead of 5 produced
> currently
> >
> > -Matt
> >
> > On 11/14/19, 07:20, "llvm-dev on behalf of Quentin Colombet via
> llvm-dev" <llvm-dev-bounces at lists.llvm.org on behalf of
> llvm-dev at lists.llvm.org> wrote:
> >
> >    Hi Ryan,
> >
> >    Unless you can fold your immediate directly in an instruction, it
is
> actually not profitable to propagate them. Indeed you will end up with a
> bunch of load imm instead of reusing a register that already hold this
> value.
> >
> >    The way it works right now is, if holding this value in a register
is
> too expensive, i.e., it triggers a spill, then we rematerialize the
> immediate instead of holding a register for it.
> >
> >    Cheers,
> >    -Quentin
> >
> >> On Nov 13, 2019, at 7:36 AM, Ryan Taylor via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >>
> >> I have some code such that:
> >>
> >> vgpr1 = mov 0
> >> branch bb
> >> bb:
> >> PHI vgpr2 = vgpr1, ….
> >> PHI vgpr3 = vgpr1, ….
> >> PHI vgpr4 = vgpr1, ….
> >> PHI vgpr5 = vgpr1, ….
> >>
> >> PHI node elimination is generating copies for all these PHIs (and
> hoisting them) as such:
> >>
> >> vgpr1 = 0
> >> vgpr20 = COPY vgpr1 // old vgpr2
> >> vgpr30 = COPY vgpr1 // old vgpr3
> >> vgpr40 = COPY vgpr1 // old vgpr4
> >> vgpr 50 = COPY vgprt1 // old vgpr5
> >>
> >> I expect the zero to get propagated in a later phase but it's
not. I
> was looking at adding immediate folding to the register coalescer but this
> doesn't really seem like the right place. Any suggestions?
> >>
> >> I'm sort of surprised that other targets haven't run into
this issue.
> >>
> >> -Ryan
> >>
> >>
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> llvm-dev at lists.llvm.org
> >> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
> >    _______________________________________________
> >    LLVM Developers mailing list
> >    llvm-dev at lists.llvm.org
> >    https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
> >
>
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