[llvm-dev] Inline Spiller spilling multiple duplicate copies

[Ryan Taylor via llvm-dev]

The issue is we did not have the proper store instruction in isStoreToStack
(was defaulting to %noreg) so the eliminateRedundantSpills was unable to
recognize similar spills with like regs/address.

Thanks Quentin.

On Mon, Mar 14, 2016 at 1:10 PM, Quentin Colombet <qcolombet at apple.com>
wrote:

> Hi Ryan,
>
> Could you attach the test case to reproduce?
>
> I’d like to see the full regalloc output among other things.
>
> Thanks,
> -Quentin
>
> On Mar 14, 2016, at 9:31 AM, Ryan Taylor <ryta1203 at gmail.com> wrote:
>
> Quentin,
>
>   Thanks for responding.
>
>   Yes, you need virtual regs for SSA form.
>
>   What I'm seeing as far as output are multiples of the exact same
> instruction (ie multiple storeRegToStack instructions). When I say 'exact
> same instruction', I mean literally, something like:
>
> mov %r0, 0[%sp]
> mov %r0, 0[%sp]
> mov %r0, 0[%sp]
> mov %r0, 0[%sp]
>
> and so on and so on. These instructions are redundant and need to be
> removed/eliminated but they are making it all the way to the asm.
>
> Looking at InlineSpiller.cpp in spillAroundUses it's iterating over
> instructions using "Reg".
>
> For example, adding statements to the code I get the following sequence
> when iterating over the loop in spillAroundUses I mentioned above:
>
> First iteration of loop:
> MI: %vreg627<def> = COPY %vreg628; PTRRegs:%vreg627,%vreg628
> Storing PTRRegs 2147484276
>
> Second iteration of loop:
> MI: %vreg627<def> = COPY %vreg628; PTRRegs:%vreg627,%vreg628
> Storing PTRRegs 2147484276
>
> Third iteration of loop:
> MI: %vreg628<def> = COPY %vreg627; PTRRegs:%vreg628,%vreg627
>
> Given this, I will get TWO 'mov' instructions In the same that do the same
> thing (ie, there are two "Storing PTRRegs above", like:
>
> mov %b1, 0[%sp]
> mov %b1, 0[%sp]
>
> The code is calling hoistSpill which is calling storeRegToStack and then
> marking the MI Operand as dead but never checking if it's dead in
> subsequent iterations. Also, the MRI.reg_bundle seem to be made of copies,
> so the 'dead' doesn't carry over between iterations anyways.
>
> So I'm not sure if:
> 1) The dead check should be added in the loop.
> 2) The multiple COPY instructions should not have been generated in the
> first place.
>
> Thank you, your time is appreciated. If this doesn't make sense I can try
> and elaborate.
>
>
> On Mon, Mar 14, 2016 at 12:17 PM, Quentin Colombet <qcolombet at apple.com>
> wrote:
>
>> 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> 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.
>>>
>>>
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>>
>>
>>
>
>
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