[llvm-dev] Store unswitch

[James Molloy via llvm-dev]

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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