<div dir="ltr"><div>Cool HW trick. :)<br></div>Are those 'sxtw' ops free? <br><br>I have to look at the HW manuals again, but I don't think x86-64 has that capability.<br></div><div class="gmail_extra"><br><br>
<div class="gmail_quote">On Thu, Jun 26, 2014 at 11:23 AM, James Molloy <span dir="ltr"><<a href="mailto:james.molloy@arm.com" target="_blank">james.molloy@arm.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div link="blue" vlink="purple" lang="EN-GB"><div><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Hi Sanjay,<u></u><u></u></span></p><p class="MsoNormal">
<span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">The behaviour I’m talking about I’ve actually pinned down to CodeGenPrepare not working too well with ISA’s that don’t have a good scaled load. I have a patch to fix it that is going through performance testing now.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Your testcase seems specific to x86 – for aarch64 we get the rather spiffy:<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">_Z3fooPii: // @_Z3fooPii<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">// BB#0: // %entry<u></u><u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> add w8, w1, #1 // =1<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> add w9, w1, #2 // =2<u></u><u></u></span></p><p class="MsoNormal">
<span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> ldr w8, [x0, w8, sxtw #2]<u></u><u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> ldr w9, [x0, w9, sxtw #2]<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> add w8, w9, w8<u></u><u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> str w8, [x0, w1, sxtw #2]<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> ret<u></u><u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">The sext can be matched as part of the addressing mode for AArch64 – maybe it’s something in codegenprepare for x86 going awry?<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Cheers,<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">James<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p><div style="border:none;border-left:solid blue 1.5pt;padding:0cm 0cm 0cm 4.0pt">
<div><div style="border:none;border-top:solid #b5c4df 1.0pt;padding:3.0pt 0cm 0cm 0cm"><p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"" lang="EN-US">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"" lang="EN-US"> Sanjay Patel [mailto:<a href="mailto:spatel@rotateright.com" target="_blank">spatel@rotateright.com</a>] <br>
<b>Sent:</b> 26 June 2014 18:11<br><b>To:</b> Manjunath DN<br><b>Cc:</b> James Molloy; <a href="mailto:llvmdev@cs.uiuc.edu" target="_blank">llvmdev@cs.uiuc.edu</a></span></p><div><div class="h5"><br><b>Subject:</b> Re: [LLVMdev] Contributing the Apple ARM64 compiler backend<u></u><u></u></div>
</div><p></p></div></div><div><div class="h5"><p class="MsoNormal"><u></u> <u></u></p><div><div><p class="MsoNormal">>> We've also seen similar instances where multiple registers are used to compute very similar<br>
>> addresses (such as x+0 and x+4!) and this increases register pressure.<br><br>I don't have an ARM enabled build of the tools to test with, but I suspect what I'm seeing here:<br><a href="http://llvm.org/bugs/show_bug.cgi?id=20134" target="_blank">http://llvm.org/bugs/show_bug.cgi?id=20134</a><u></u><u></u></p>
</div><p class="MsoNormal"> <u></u><u></u></p><div><p class="MsoNormal">...would also be bad on AArch64.<u></u><u></u></p></div><div><p class="MsoNormal"><u></u> <u></u></p><div><p class="MsoNormal">On Wed, Jun 25, 2014 at 8:58 PM, Manjunath DN <<a href="mailto:manjunath.dn@gmail.com" target="_blank">manjunath.dn@gmail.com</a>> wrote:<u></u><u></u></p>
<div><div><p class="MsoNormal">HI James,<u></u><u></u></p></div><div><p class="MsoNormal">Thanks for your reply and hints on what can be done for the Aarch64 backend optimization for llvm<u></u><u></u></p></div><div><p class="MsoNormal">
We have SPEC license and v8 hardware. So I will start looking into it<u></u><u></u></p></div><div><p class="MsoNormal">warm regards<u></u><u></u></p></div><div><p class="MsoNormal">Manjunath<u></u><u></u></p></div><div><p class="MsoNormal">
<u></u><u></u></p></div></div><div><div><div><p class="MsoNormal" style="margin-bottom:12.0pt"><u></u> <u></u></p><div><p class="MsoNormal">On Wed, Jun 25, 2014 at 8:42 PM, James Molloy <<a href="mailto:james.molloy@arm.com" target="_blank">james.molloy@arm.com</a>> wrote:<u></u><u></u></p>
<p class="MsoNormal" style="margin-bottom:12.0pt">Hi Manjunath,<br><br>At the time of writing that status we had only done our initial analysis.<br>This was done without real hardware and attempted to identify poor code<br>
sequences but we were unable to quantify how much effect this would actually<br>have.<br><br>Since then we've done more analysis using Cortex-A57 and Cortex-A53 on an<br>internal development platform.<br><br>For SPEC, we are between 10% and 0% behind GCC on 9 benchmarks, and 25%<br>
ahead on one benchmark. Most benchmarks are less than 5% behind GCC.<br><br>Because of the licencing of SPEC, I have to be quite restricted in what I<br>say and I can't give any numbers - sorry about that.<br><br>We are focussing on Cortex-A57, and the things we've identified so far are:<br>
* The CSEL instruction behaves worse than the equivalent branch structure<br>in at least one benchmark. In an out of order core, select-like instructions<br>are going to be slower than their branched equivalent if the branch is<br>
predictable due to CSEL having two dependencies.<br><br> * Redundant calculations inside if conditions. We've seen:<br> 1. "if (a[x].b < c[y].d || a[x].e > c[y].f)" - the calculations of a[x]<br>and c[y] are repeated, when they are common. We've also seen similar<br>
instances where multiple registers are used to compute very similar<br>addresses (such as x+0 and x+4!) and this increases register pressure.<br> 2. "if (a < 0 && b == c || a > 0 && b == d)" - the first comparison of<br>
'a' against zero is done twice, when the flag results of the first<br>comparison could be used for the second comparison.<br><br> * For a loop such as "for (i = 0; i < n; ++i)<br>{do_something_with(&x[i]);}", GCC is using &x[i] as the loop induction<br>
variable where LLVM uses i and performs the calculation &x[i] on every<br>iteration. This only creates one more add instruction but the loop we see it<br>in only has 5 or so instructions.<br><br> * The inline heuristics are way behind GCC's. If we crank the inline<br>
threshold up to 1000, we can remove a 6.5% performance regression from one<br>benchmark entirely.<br><br> * We're generating (due to SLP vectorizer and a DAG combine) loads into Q<br>registers when merging consecutive loads. This is bad, because there are no<br>
callee-saved Q registers! So if the live range crosses a function call, it<br>will have to be immediately spilled again. This can be easily fixed by<br>using load-pair instructions instead. I have a patch to fix this.<br>
<br>The list above is non-exhaustive and only contains things that we think may<br>affect multiple benchmarks or real-world code.<br><br>I've also noticed:<br> * Our inline memcpy expansion pass is emitting "LDR q0, [..]; STR q0,<br>
[..]" pairs, which is less than ideal on A53. If we switched to emitting<br>"LDP x0, x1, [..]; STP x0, x1, [..]", we'd get around 30% better inline<br>memcpy performance on A53. A57 seems to deal well with the LDR q sequence.<br>
<br>I'm sorry I'm unable to provide code samples for most of the issues found so<br>far - this is an artefact of them having come from SPEC. Trivial examples do<br>not always show the same behaviour, and as we're still investigating we<br>
haven't yet been able to reduce most of these to an anonymisable testcase.<br><br>Hope this helps, but doubt it does,<br><br>James<br><br>> -----Original Message-----<br>> From: <a href="mailto:llvmdev-bounces@cs.uiuc.edu" target="_blank">llvmdev-bounces@cs.uiuc.edu</a> [mailto:<a href="mailto:llvmdev-bounces@cs.uiuc.edu" target="_blank">llvmdev-bounces@cs.uiuc.edu</a>] On<br>
> Behalf Of Manjunath N<br>> Sent: 24 June 2014 10:45<br>> To: <a href="mailto:llvmdev@cs.uiuc.edu" target="_blank">llvmdev@cs.uiuc.edu</a><br>> Subject: Re: [LLVMdev] Contributing the Apple ARM64 compiler backend<br>
><br>><br>><br>> Eric Christopher <echristo <at> <a href="http://gmail.com" target="_blank">gmail.com</a>> writes:<br>><br>> ><br>> > > The big pain issues I see merging from ARM64 to AArch64 are:<br>
> > > 1. Apple have created a fairly complete scheduling model already<br>> for<br>> > > ARM64, and we'd have to merge the partial? model in AArch64 and<br>> theirs.<br>> We<br>> > > risk regressing performance on Apple's targets here, and we can't<br>
> determine<br>> > > ourselves whether we have or not. This is not ideal.<br>> > > 2. Porting over the DAG-to-DAG optimizations and any other<br>> > > optimizations that rely on the tablegen layout will be very tricky.<br>
> > > 3. The conditional compare pass is fairly comprehensive - we'd<br>> have<br>> to<br>> > > port that over or rewrite it and that would be a lot of work.<br>> > > 4. A very quick analysis last night indicated that ARM64 has<br>
> > > implemented just under half of the optimizations we discovered<br>> opportunities<br>> > > for in SPEC and EEMBC. That's a fairly comprehensive number of<br>> > > optimizations, and they won't all be easy to port.<br>
> Eric,<br>> You mention that there a quite a few optimization opportunities in SPEC<br>> 2000/ EEMBC.<br>> I am looking to optimize the Aarch64 backend. Could you please let me know<br>> the big optimizations possible?<br>
><br>><br>><br>> _______________________________________________<br>> LLVM Developers mailing list<br>> <a href="mailto:LLVMdev@cs.uiuc.edu" target="_blank">LLVMdev@cs.uiuc.edu</a> <a href="http://llvm.cs.uiuc.edu" target="_blank">http://llvm.cs.uiuc.edu</a><br>
> <a href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev" target="_blank">http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev</a><br><br><br><br><u></u><u></u></p></div><p class="MsoNormal"><br><br clear="all"><u></u><u></u></p>
</div></div><p class="MsoNormal"><span style="color:#888888">-- <u></u><u></u></span></p><div><p class="MsoNormal"><span style="color:#888888">=========================================<br>warm regards,<br>Manjunath DN<u></u><u></u></span></p>
</div></div><p class="MsoNormal" style="margin-bottom:12.0pt"><br>_______________________________________________<br>LLVM Developers mailing list<br><a href="mailto:LLVMdev@cs.uiuc.edu" target="_blank">LLVMdev@cs.uiuc.edu</a> <a href="http://llvm.cs.uiuc.edu" target="_blank">http://llvm.cs.uiuc.edu</a><br>
<a href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev" target="_blank">http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev</a><u></u><u></u></p></div><p class="MsoNormal"><br><br clear="all"><br>-- <br>Sanjay Patel<br>
RotateRight, LLC<br><a href="http://www.rotateright.com" target="_blank">http://www.rotateright.com</a> <u></u><u></u></p></div></div></div></div></div></div></div></blockquote></div><br><br clear="all"><br>-- <br>Sanjay Patel<br>
RotateRight, LLC<br><a href="http://www.rotateright.com">http://www.rotateright.com</a>
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