[LLVMdev] [Polly] Aliasing problems escalation (WAS: Re: [DragonEgg] [Polly] Should we expect DragonEgg to produce identical LLVM IR for identical GIMPLE?)

[Dmitry Mikushin]

Update: in the case discussed in *this* thread, GCC optimization also
helps: with -O2 GCC pulls out dimensions loads from loops. Polly still has
a problem due to bitcast, but this is another issue.

- D.

2013/2/10 Dmitry Mikushin <dmitry at kernelgen.org>

> Hi Duncan,
>
> Got it. So, it looks like in [case] it is really sufficient to have -O1
> for GCC to remove loads (not really remove, but rather pull outside of
> loops, which is sufficient).
>
> And in this case - GCC -O1 does not help.
>
> Still, is the [case] sufficient for you to track the missing GCC's info
> and make it accessible to LLVM? Meanwhile, I think we will use GCC -O1,
> however, I guess, it would likely introduce other issues in turn. Oh.
>
> [case]
> http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-February/059313.html
>
> Thanks,
> - D.
>
> 2013/1/16 Duncan Sands <baldrick at free.fr>
>
>> Hi Dmitry,
>>
>>
>> On 15/01/13 22:22, Dmitry Mikushin wrote:
>>
>>> Hi Duncan,
>>>
>>> You mean - what happens if DragonEgg is invoked with GCC optimization? I
>>> tried
>>> -O3 and -fplugin-arg-dragonegg-llvm-**ir-optimize=1, but nothing
>>> changed.
>>>
>>
>> no, I meant if you run gcc with optimization and don't use dragonegg at
>> all.
>> If I understand right, your problem is that the LLVM optimizers don't
>> remove
>> some loads, and this blocks polly.  If the GCC optimizers are able to
>> remove
>> the loads, then that means that the gcc optimizers have access to some
>> info
>> that the LLVM optimizers don't have (because removing the loads is invalid
>> given the information currently in the IR).  We would then have to just
>> have
>> to teach dragonegg to extract that information and pass it on to the LLVM
>> optimizers.
>>
>> Ciao, Duncan.
>>
>>
>>> - D.
>>>
>>> 2013/1/8 Duncan Sands <baldrick at free.fr <mailto:baldrick at free.fr>>
>>>
>>>
>>>     On 04/01/13 15:58, Duncan Sands wrote:
>>>
>>>         PS: Another possibility is to do link-time optimization, since
>>> at that
>>>         point the
>>>         optimizers are capable of finding out if that global is used
>>> anywhere
>>>         else or
>>>         not.
>>>
>>>
>>>     By the way, do the GCC optimizers get this, i.e. remove the loads?
>>>
>>>     Ciao, Duncan.
>>>
>>>
>>>
>>
>
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