[PATCH] Loop Rerolling Pass

[Andrew Trick]

On Oct 16, 2013, at 9:18 AM, Hal Finkel <hfinkel at anl.gov> wrote:

> ----- Original Message -----
>> 
>> On 15 October 2013 22:11, Hal Finkel < hfinkel at anl.gov > wrote:
>> 
>> 
>> 
>> 
>> 
>> 
>> I made use of SCEV everywhere that I could (I think). SCEV is used to
>> analyze the induction variables, and then at the end to help with
>> the rewriting. I don't think that I can use SCEV everywhere,
>> however. For one thing, I need to check for equivalence of
>> instructions, with certain substitutions, for instructions (like
>> function calls) that SCEV does not interpret.
>> 
>> 
>> Hi Hal,
>> 
>> 
>> This is probably my lack of understanding of all that SCEV does than
>> anything else.
>> 
>> 
>> My comment was to the fact that you seem to be investigating specific
>> cases (multiply, adding, increment size near line 240), which SCEV
>> could get that as an expression, and possibly making it slightly
>> easier to work with. I'll let other SCEV/LV experts to chime in,
>> because I basically don't know what I'm talking about, here. ;)
> 
> Okay, I see what you mean. The code in this block:
>    if (Inc == 1) {
>      // This is a special case: here we're looking for all uses (except for
>      // the increment) to be multiplied by a common factor. The increment must
>      // be by one.
>      if (I->first->getNumUses() != 2)
>        continue;
> 
> This code does not use SCEV because, IMHO, there is no need. It is looking for a very particular instruction pattern where the induction variable has only two uses: one which increments it by one (SCEV has already been used to determine that the increment is 1), and the other is a multiply by a small constant. It is to catch cases like this:
> 
> for (int i = 0; i < 500; ++i) {
>  foo(3*i);
>  foo(3*i+1);
>  foo(3*i+2);
> }
> 
> And so, aside from the increment, all uses of the IV are via the multiply. If we find this pattern, then instead of attempting to classify all IV uses as functions of i, i+1, i+2, ... we attempt to classify all uses of the multiplied IV that way.
> 

I think the more general SCEV-based way to do this would be to recursively walk the def-use chains starting at phis, looking past simple arithmetic until reaching an IV users (see IVUsers::AddUsersIfInteresting). Then you group the users by their IV operand's SCEV expression. If the SCEVs advance by the same constant, then you have your unrolled iterations and it doesn't matter how the induction variable was computed. LSRInstance::CollectChains does something similar.

-Andy

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20131016/10be2990/attachment.html>