[llvm-dev] Store unswitch

[Hongbin Zheng via llvm-dev]

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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>
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