llvm dev - Mar 2016 - Inline Spiller spilling multiple duplicate copies

Looks like spillAroundUses is spilling multiple duplicate copies to the
stack, for example, with some regs we get 1 storeRegToStack call, for
others we get multiple (2-6+) and then these instructions are never
eliminated.

Looking at spillAroundUses it looks like multiple duplicate COPYs are being
generated, why? One for each use?

The reg_bundle holds these multiple copies so that we are iterating over
the same exact COPY instructions multiple times, even though each one hits
the same spot in the function:

if (hoistSpill(OldLI, MI)) {
     MI->getOperand(0).setIsDead();
     DeadDefs.push_back(MI);
     continue;
}

Even when I add a check:

if (MI->getOperand(0)->isDead())
    continue;

This never checks to true, on the second, third, fourth, etc duplicate.

Not sure what I'm missing? Can someone give me a brief description of why
these multiple COPYs are being written out (storeRegToStack is being
called)?

Thanks.
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I looked at this again and it appears that while spillAroundUses sets the
register as 'dead', there is no checking to see if it's dead in
subsequent
iterations of the bundle loop.

Is this intentional?

On Mon, Mar 7, 2016 at 3:28 PM, Ryan Taylor <ryta1203 at gmail.com> wrote:
> Looks like spillAroundUses is spilling multiple duplicate copies to the
> stack, for example, with some regs we get 1 storeRegToStack call, for
> others we get multiple (2-6+) and then these instructions are never
> eliminated.
>
> Looking at spillAroundUses it looks like multiple duplicate COPYs are
> being generated, why? One for each use?
>
> The reg_bundle holds these multiple copies so that we are iterating over
> the same exact COPY instructions multiple times, even though each one hits
> the same spot in the function:
>
> if (hoistSpill(OldLI, MI)) {
>      MI->getOperand(0).setIsDead();
>      DeadDefs.push_back(MI);
>      continue;
> }
>
> Even when I add a check:
>
> if (MI->getOperand(0)->isDead())
>     continue;
>
> This never checks to true, on the second, third, fourth, etc duplicate.
>
> Not sure what I'm missing? Can someone give me a brief description of
why
> these multiple COPYs are being written out (storeRegToStack is being
> called)?
>
> Thanks.
>
>
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Hi Ryan,

> On Mar 14, 2016, at 7:49 AM, Ryan Taylor via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> I looked at this again and it appears that while spillAroundUses sets the
register as 'dead', there is no checking to see if it's dead in
subsequent iterations of the bundle loop.
> 
> Is this intentional?
> 
> On Mon, Mar 7, 2016 at 3:28 PM, Ryan Taylor <ryta1203 at gmail.com
<mailto:ryta1203 at gmail.com>> wrote:
> Looks like spillAroundUses is spilling multiple duplicate copies to the
stack, for example, with some regs we get 1 storeRegToStack call, for others we
get multiple (2-6+) and then these instructions are never eliminated.
> 
> Looking at spillAroundUses it looks like multiple duplicate COPYs are being
generated, why? One for each use?
We need a new virtual register for each use to have an independent coloring for
each of them.
If it is not what you are seeing, I think I would need more information to help
you.
For instance, the machine representation you have before and after spilling and
why this is not appropriate.

Cheers,
-Quentin
> 
> The reg_bundle holds these multiple copies so that we are iterating over
the same exact COPY instructions multiple times, even though each one hits the
same spot in the function:
> 
> if (hoistSpill(OldLI, MI)) {
>      MI->getOperand(0).setIsDead();
>      DeadDefs.push_back(MI);
>      continue;
> }
> 
> Even when I add a check:
> 
> if (MI->getOperand(0)->isDead())
>     continue;
> 
> This never checks to true, on the second, third, fourth, etc duplicate.
> 
> Not sure what I'm missing? Can someone give me a brief description of
why these multiple COPYs are being written out (storeRegToStack is being
called)?
> 
> Thanks.
> 
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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