[llvm-dev] Request suggestions about how to remove redundencies caused by SCEV expansion fundementally

[Andrew Trick via llvm-dev]

> On Aug 24, 2016, at 1:20 PM, Wei Mi <wmi at google.com> wrote:
> 
> On Wed, Aug 24, 2016 at 12:01 PM, Sanjoy Das
> <sanjoy at playingwithpointers.com> wrote:
>> Hi Wei,
>> 
>> Wei Mi wrote:
>>> Sanjoy and Andy, thanks a lot for your suggestions.
>>> 
>>> On Wed, Aug 24, 2016 at 8:53 AM, Andrew Trick<atrick at apple.com>  wrote:
>>>> 
>>>>> On Aug 23, 2016, at 11:30 PM, Sanjoy Das<sanjoy at playingwithpointers.com>
>>>>> wrote:
>>>>> 
>>>>> Hi Wei,
>>>>> 
>>>>> I've not seen GCC's SCEV so I cannot make a comparative comment here
>>>>> (maybe Chris, Andy or Dan can chime in here), but I personally am in
>>>>> the "make the cleanup passes smarter" camp.  We can also try to make
>>>>> SCEV expansion smarter -- not by putting more things in SCEVExpander
>>>>> (it is already complex enough!), but by splitting out a dedicated
>>>>> SCEVSimplifier that you invoke on code generated from SCEVExpander to
>>>>> strength reduce it.  SCEVSimplifier can then internally use routines
>>>>> in SCEV, so that it is "as smart as" SCEV in most cases.
>>>>> 
>>>>> — Sanjoy
>>> 
>>> Combine the efforts of making the cleanup passes smarter and
>>> simplifying the expanded code to make it more IR canonical looks like
>>> a promising way. But I may not understand SCEVSimplifier you suggested
>>> here correctly: The input of SCEVSimplifier is IR or SCEV? The code
>>> generated by SCEVExpander is IR. If the input of SCEVSimplifier is IR,
>>> how does it use SCEV routine internally?
>> 
>> It accepts a ScalarEvolution instance that it uses internally to check
>> for equivalences, so the interface is:
>> 
>> class Simplifier {
>> public:
>>  Simplifier(ScalarEvolution &SE);
>>  Value *simplify(Value *);
>> };
>> 
>> or something like that.  Once we have this separation between
>> expansion and strength reduction, we can be more aggressive (than we
>> can reasonably be within SCEVExpander) about teaching Simplifier SCEV
>> based CSE rules.
> 
> To rephrase it and see if I understand it correctly:
> 
> Existing Value A;
> 
> Expanded Value B (B is identical with A, but in a different form);
> 
> So we can convert both A and B to SCEV again and compare their
> equivalence and use the result to guide strength reduction. It is like
> to enhance existing cleanup pass using the SCEV CSE rules.
> 
> Or the idea of simplify is to transform B back to the form close to A
> with SCEV knowledge?
> 
> It sounds like an interesting idea.


I would need to see how this solves the bugs you posted earlier, like
https://llvm.org/bugs/show_bug.cgi?id=24920 <https://llvm.org/bugs/show_bug.cgi?id=24920>

-Andy
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