<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Hi James,<div class=""><br class=""></div><div class="">It looks like the first patch gives nearly the same results as both patches together (at least, in terms of gains and regressions on geekbench). E.g. the code generated for mandelbrot is the same.</div><div class=""><br class=""></div><div class="">Thanks,</div><div class="">Michael</div><div class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Apr 21, 2015, at 11:29 AM, James Molloy <<a href="mailto:james@jamesmolloy.co.uk" class="">james@jamesmolloy.co.uk</a>> wrote:</div><br class="Apple-interchange-newline"><div class="">Hi Michael,<br class=""><br class="">Jingue pointed out that my second patch did pessimize some code, so do you still see the regressions with my first patch (the reassoc one)?<br class=""><br class="">Cheers,<br class=""><br class="">James<br class=""><div class="gmail_quote">On Tue, 21 Apr 2015 at 19:26, Michael Zolotukhin <<a href="mailto:mzolotukhin@apple.com" class="">mzolotukhin@apple.com</a>> wrote:<br class=""><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word" class="">Hi James,<div class=""><br class=""></div><div class="">Thank you for taking your time to work on it! I tried the patches you posted and they did fix the problem on twofish. However, with them there are other regressions - and while you are at it, could you please take a look at the generated code? Probably, you can come up with another one-line fix.</div><div class=""><br class=""></div><div class="">So, one regression is on Geekbench/Mandlerbrot, here is the ‘fast’ and ‘slow’ disassembly files (fast - from reference compiler, slow - reference compiler+D9148+D9149).</div><div class=""><br class=""></div><div class=""></div></div><div style="word-wrap:break-word" class=""><div class=""></div></div><div style="word-wrap:break-word" class=""><div class=""></div><div class=""><br class=""></div><div class="">I can see that we generate less ‘madd’ instructions, but in some cases we end up with less-optimal sequences, including ‘mul', ‘add' and ‘or' instead. Also, in some cases we now generate 2 ‘str' instead of 1 ‘stp'.</div><div class=""><br class=""></div><div class="">Do you have any ideas what might go wrong this time?</div><div class=""><br class=""></div><div class="">Thanks,</div><div class="">Michael</div></div><div style="word-wrap:break-word" class=""><div class=""><br class=""><div class=""><blockquote type="cite" class=""><div class="">On Apr 21, 2015, at 6:55 AM, James Molloy <<a href="mailto:james@jamesmolloy.co.uk" target="_blank" class="">james@jamesmolloy.co.uk</a>> wrote:</div><br class=""></blockquote><blockquote type="cite" class=""><div class=""><div dir="ltr" class="">See:<br class=""><br class=""><a href="http://reviews.llvm.org/D9148" target="_blank" class="">http://reviews.llvm.org/D9148</a><br class=""><div class=""><a href="http://reviews.llvm.org/D9149" target="_blank" class="">http://reviews.llvm.org/D9149</a><br class=""></div><div class=""><br class=""></div><div class="">Cheers,</div><div class=""><br class=""></div><div class="">James</div></div><br class=""><div class="gmail_quote">On Tue, 21 Apr 2015 at 12:06 James Molloy <<a href="mailto:james@jamesmolloy.co.uk" target="_blank" class="">james@jamesmolloy.co.uk</a>> wrote:<br class=""><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr" class="">Hi Michael, Gerolf,<br class=""><br class=""><div class="">The main goal of this optimization was to reduce the number of multiply-adds emitted when complex GEPs are used. With regards to SPEC, this is particularly prevalent in sjeng and gobmk (although we don't just care about SPEC and there were plenty of other places we caught this idiom).</div><div class=""><br class=""></div><div class="">The pass is actively reducing the number of multiply-adds in gobmk:</div><div class=""><br class=""></div><div class="">$ llc engine.ll -O3 -mllvm -aarch64-gep-opt=false -o - |& grep madd | wc -l</div><div class=""> 297</div><div class=""><br class=""></div><div class="">$ llc engine.ll -O3 -mllvm -aarch64-gep-opt=true -o - |& grep madd | wc -l</div><div class=""> 158</div><div class=""><br class=""></div><div class="">So it demonstrably is making at least some code better.</div><div class=""><br class=""></div><div class="">I was able to reproduce your twofish example. Twofish, like most crypto algorithms, has a pattern repeated many times so the problem could just be down to one pattern producing one more instruction, which it turns out is exactly the case.</div><div class=""><br class=""></div><div class="">The problem is that with gep-opt enabled, we're associating shifts and constants the other way around.</div><div class=""><br class=""></div><div class="">With gep-opt disabled (synthetic code example):</div><div class=""> add x0, x0, #72</div><div class=""> ldr x0, [x1, x0, lsl #2]</div><div class=""><br class=""></div><div class="">With gep-opt enabled:</div><div class=""> add x0, x1, x0, lsl #2</div><div class=""> ldr x0, [x0, #72]</div><div class=""><br class=""></div><div class="">This is actually fine, until another expression reuses a different shift with the same offset:</div><div class=""><br class=""></div><div class="">With gep-opt disabled:</div><div class=""> add x0, x0, #72</div><div class=""> ldr x0, [x1, x0, lsl #2]</div><div class=""> ldr x2, [x1, x0, lsl #3]</div><div class=""><br class=""></div><div class="">With gep-opt enabled:</div><div class=""> add x0, x1, x0, lsl #2</div><div class=""> add x2, x1, x0, lsl #3</div><div class=""> ldr x0, [x0, #72]</div><div class=""> ldr x2, [x0, #72]</div><div class=""><br class=""></div><div class="">Amusingly, I saw the opposite behaviour on AArch32 on a different testcase and came to the conclusion that expressions with correlated immediate offsets and identical shift amounts were more likely than expressions with correlated shift amounts and identical immediate offsets, but that's a different story.</div><div class=""><br class=""></div><div class="">Anyway, the reason this is happening is because we're not running reassociate after GEP expansion. If we run reassociate it all goes away, it's a one-line fix.</div><div class=""><br class=""></div><div class="">The two testcases that were disabled - well, they weren't really disabled, but the behaviour was changed for them. I put that down to a bug in the SeparateConstOffsetFromGEP pass, where it tries to separate a GEP with only one variadic offset:</div><div class=""><br class=""></div><div class=""> GEP i8* %base, i32 %offset</div><div class=""><br class=""></div><div class="">Nothing beneficial can come of that, so I've got, again, a single line patch to fix this. I'll push both patches upstream just as soon as I verify them.</div><div class=""><br class=""></div><div class="">So in summary, I don't see any need to disable this pass. It's doing good work, it just has one or two bugs.</div><div class=""><br class=""></div><div class="">Cheers,</div><div class=""><br class=""></div><div class="">James</div></div><br class=""><div class="gmail_quote">On Tue, 21 Apr 2015 at 05:27 Michael Zolotukhin <<a href="mailto:mzolotukhin@apple.com" target="_blank" class="">mzolotukhin@apple.com</a>> wrote:<br class=""><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hi Tim and others,<br class="">
<br class="">
So, the entire discussion is moving around the assumption that this pass is beneficial in some cases, but we don’t have such data. Please note that I don’t want to say that the assumption is wrong, but as far as I understand, only Hao observed some gains. Thus, until we hear from him, we have real slow downs versus presumable gains.<br class="">
<br class="">
As for the looking into the code later - I’m not an expert in this area, and I don’t even have a motivational example for this optimization, so I’d barely be a right person for it. I’d be glad to assist with collecting some additional data/running tests if that might help though.<br class="">
<br class="">
Thanks,<br class="">
Michael<br class="">
<br class="">
<br class="">
> On Apr 20, 2015, at 8:40 PM, Tim Northover <<a href="mailto:t.p.northover@gmail.com" target="_blank" class="">t.p.northover@gmail.com</a>> wrote:<br class="">
><br class="">
> On 20 April 2015 at 20:34, Gerolf Hoflehner <<a href="mailto:ghoflehner@apple.com" target="_blank" class="">ghoflehner@apple.com</a>> wrote:<br class="">
>> It shouldn’t come across as ‘extreme’.<br class="">
><br class="">
> The "extreme" part wasn't so much the tentative disabling as "Could<br class="">
> you and/or Hao take this on after the code is disabled/backed out?". I<br class="">
> realise it wasn't an ultimatum, but it could very easily turn into<br class="">
> "we're washing our hands of this", which I'd object to.<br class="">
><br class="">
> I assume the pass was committed with some kind of metric that it<br class="">
> improved, and now we have a counter-example. It's down to everyone to<br class="">
> decide what the best way to proceed is.<br class="">
><br class="">
> Cheers.<br class="">
><br class="">
> Tim.<br class="">
><br class="">
> _______________________________________________<br class="">
> llvm-commits mailing list<br class="">
> <a href="mailto:llvm-commits@cs.uiuc.edu" target="_blank" class="">llvm-commits@cs.uiuc.edu</a><br class="">
> <a href="http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits" target="_blank" class="">http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits</a><br class="">
<br class="">
<br class="">
_______________________________________________<br class="">
llvm-commits mailing list<br class="">
<a href="mailto:llvm-commits@cs.uiuc.edu" target="_blank" class="">llvm-commits@cs.uiuc.edu</a><br class="">
<a href="http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits" target="_blank" class="">http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits</a><br class="">
</blockquote></div></blockquote></div>
</div></blockquote></div><br class=""></div></div></blockquote></div>
</div></blockquote></div><br class=""></div></body></html>