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Thanks for the info and the pointer Michael, that's very useful!</div>
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Cheers.<br>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> Michael Kruse <llvmdev@meinersbur.de><br>
<b>Sent:</b> 28 March 2020 09:32<br>
<b>To:</b> Sjoerd Meijer <Sjoerd.Meijer@arm.com><br>
<b>Cc:</b> llvm-dev@lists.llvm.org <llvm-dev@lists.llvm.org><br>
<b>Subject:</b> Re: [llvm-dev] canonical form loops</font>
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<div class="PlainText">The topic came up before, e.g. <a href="https://reviews.llvm.org/D60565#1484984">
https://reviews.llvm.org/D60565#1484984</a><br>
<br>
> 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.<br>
<br>
The general approach in LLVM is to rely on SCEV for analyzing loops<br>
instead of custom handling. As a consequence, any loop structure that<br>
is recognized by SCEV will (/should) not profit from rewriting.<br>
<br>
Michael<br>
<br>
<br>
<br>
<br>
Am Do., 26. März 2020 um 15:56 Uhr schrieb Sjoerd Meijer via llvm-dev<br>
<llvm-dev@lists.llvm.org>:<br>
><br>
> Hello,<br>
><br>
> 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.<br>
> 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.<br>
><br>
> Cheers,<br>
> Sjoerd.<br>
><br>
> [1] <a href="https://llvm.org/docs/LoopTerminology.html">https://llvm.org/docs/LoopTerminology.html</a><br>
><br>
><br>
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