[llvm-dev] Store unswitch

[James Molloy via llvm-dev]

It's basically ready to commit; the reviewers were fairly happy with it. It
needs rebasing on top of NewGVN and any bugs that shakes out fixed, but
that's about it.

I want to get around to it soon-ish, but I've wanted that for a while!
On Wed, 26 Apr 2017 at 16:50, Hongbin Zheng <etherzhhb at gmail.com> wrote:

> Hi James,
>
> I have an ad-hoc solution in mind to solve this problem.
> But if it can be solved under the framework of GVN"Sink", it is even
> better.
>
> any plan on your https://reviews.llvm.org/D24805?
>
> Thanks
> Hongbin
>
>
> On Wed, Apr 26, 2017 at 2:13 AM, James Molloy <james at jamesmolloy.co.uk>
> wrote:
>
>> Hi,
>>
>> Yes, I can see why that would not work.
>>
>> The sinking algorithm in SimplifyCFG isn't particularly clever. In
>> particular it can't reason about memory ordering and aliasing. In
>> unswitch1(), it can identify that the stores correlate because the
>> correlating stores appear in the same relative source order. In unswitch2()
>> they have been permuted, and the algorithm cannot deal with this. This
>> requires some kind of value numbering to do efficiently.
>>
>> The GVNSink pass that I really should get around to updating should solve
>> this, eventually!
>>
>> James
>>
>>
>> On Wed, 26 Apr 2017 at 08:19 Hongbin Zheng via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> Looks like this do not work:
>>>
>>> // Type your code here, or load an example.
>>> int a[10];
>>>
>>> void unswitch2(int i, int x, int y0, int y1) {
>>>   if (x) {
>>>     a[i] = y0;
>>>     a[i + 1] = y1;
>>>   } else {
>>>     a[i + 1] = y0;
>>>     a[i] = y1;
>>>   }
>>> }
>>>
>>> https://godbolt.org/g/Ldd5qV
>>>
>>> On Tue, Apr 25, 2017 at 10:22 PM, Hongbin Zheng <etherzhhb at gmail.com>
>>> wrote:
>>>
>>>> Thanks,
>>>>
>>>> Looks like inst combine do the job
>>>>
>>>> On Tue, Apr 25, 2017 at 9:36 PM, Davide Italiano <davide at freebsd.org>
>>>> wrote:
>>>>
>>>>> On Tue, Apr 25, 2017 at 9:24 PM, Hongbin Zheng via llvm-dev
>>>>> <llvm-dev at lists.llvm.org> wrote:
>>>>> > Hi,
>>>>> >
>>>>> > Is there a pass in LLVM that can optimize:
>>>>> >
>>>>> > if (x)
>>>>> >   a[i] = y0;
>>>>> > else
>>>>> >   a[i] = y1;
>>>>> >
>>>>> > to
>>>>> >
>>>>> > a[i] = x ? y0 : y1?
>>>>> >
>>>>> > I tried opt (3.9) with -O3 but looks like such an optimization do not
>>>>> > happened.
>>>>> >
>>>>>
>>>>> The same IR at -O3 for both cases on this example.
>>>>> https://godbolt.org/g/Tk2MM8
>>>>>
>>>>> --
>>>>> Davide
>>>>>
>>>>> "There are no solved problems; there are only problems that are more
>>>>> or less solved" -- Henri Poincare
>>>>>
>>>>
>>>>
>>> _______________________________________________
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>>> llvm-dev at lists.llvm.org
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>>>
>>
>
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