[llvm-dev] canonical form loops

[Zaks, Ayal (Mobileye) via llvm-dev]

Interesting, thanks for digging this up!

> As a consequence, any loop structure that is recognized
> by SCEV will (/should) not profit from rewriting.

As discussed in https://reviews.llvm.org/D68577#1742745 and PR40816 showed, there is still merit and profit in further simplifying loop induction variables, or at-least the primary one; somewhat independent of continuing to rely on SCEV for analyzing loops.


> enable-iv-rewrite=false was made the default in r139579 after finding that it
> slows down many benchmarks.

This was 8.5 years ago. Time to revisit and try to re-enable some of these iv-rewrites, with a better understanding why current downstream passes pessimize canonical iv's, if they still do?


> -----Original Message-----
> From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Michael
> Kruse via llvm-dev
> Sent: Saturday, March 28, 2020 12:33
> To: Sjoerd Meijer <Sjoerd.Meijer at arm.com>
> Cc: llvm-dev at lists.llvm.org
> Subject: Re: [llvm-dev] canonical form loops
> 
> The topic came up before, e.g. https://reviews.llvm.org/D60565#1484984
> 
> > Some canonicalization passes are designed for this. In particular,
> IndVarSimplify used to make canonical loops (i.e. start at zero, increment by
> one). r133502 introduced -disable-iv-rewrite to rely more on ScalarEvolution
> instead of "opcode/pattern matching" (cite from the commit message). -
> enable-iv-rewrite=false was made the default in r139579 after finding that it
> slows down many benchmarks. It was completely removed in r153260.
> 
> The general approach in LLVM is to rely on SCEV for analyzing loops instead
> of custom handling. As a consequence, any loop structure that is recognized
> by SCEV will (/should) not profit from rewriting.
> 
> Michael
> 
> 
> 
> 
> Am Do., 26. März 2020 um 15:56 Uhr schrieb Sjoerd Meijer via llvm-dev
> <llvm-dev at lists.llvm.org>:
> >
> > Hello,
> >
> > Quick question to see if I haven't missed anything: I would like convert
> counting down loops, i.e. loops with a constant -1 step value, to counting up
> loops, because the vectoriser is able to better deal with these loops (see e.g.
> D76838 that I was discussing today with Ayal). It looks like LoopSimplifyCFG
> and IndVarSimplify don't do this. So was just curious if I haven't missed
> anything here or in another pass I haven't yet considered. I was perhaps also
> expecting this to be the canonical form of loops, but couldn't find any
> evidence of that in [1] or in source-code.
> > The obvious follow-up question is if there would be any objections to
> adding this to e.g. LoopSimplifyCFG, and adding LoopSimplifyCFG to the
> optimisation pipeline just before the vectoriser.
> >
> > Cheers,
> > Sjoerd.
> >
> > [1] https://llvm.org/docs/LoopTerminology.html
> >
> >
> > _______________________________________________
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