<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=""><br class=""><div><blockquote type="cite" class=""><div class="">On Nov 11, 2017, at 11:03 AM, Hal Finkel via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" class="">llvm-dev@lists.llvm.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="moz-cite-prefix" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255);"><br class="Apple-interchange-newline">On 11/11/2017 12:44 PM, Amara Emerson wrote:<br class=""></div><blockquote cite="mid:1784BCDB-531F-47A1-A664-240F1038E407@apple.com" type="cite" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255);" class=""><br class=""><div class=""><blockquote type="cite" class=""><div class="">On Nov 10, 2017, at 10:04 PM, Aditya Nandakumar <<a moz-do-not-send="true" href="mailto:proaditya@gmail.com" class="">proaditya@gmail.com</a>> wrote:</div><div class=""><blockquote type="cite" class="" style="font-family: SFHello-Regular; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;"><div class=""><div bgcolor="#FFFFFF" text="#000000" class=""><br class="">The current DAGCombine, being constructed on top of SDAG, has a kind of built-in CSE and automatic DCE. How will things change, if they'll change, in this new model?<br class=""></div></div></blockquote><div class="" style="font-family: SFHello-Regular; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;">Hi Hal,</div><div class="" style="font-family: SFHello-Regular; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;"><br class=""></div><div class="" style="font-family: SFHello-Regular; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;">I suspect one option is to have a separate CSE pass, and the backends get to choose where exactly they plug in their pipeline. I think DCE should be part of the combine pass (and the legalizer is about to start doing that as well).</div></div></blockquote>For SSA form MIR there’s already the MachineCSE pass. How important the CSE/DCE is at the combine stage I don’t know. As an approximation perhaps we can get an idea by disabling the behavior in the DAGCombiner and seeing the effects.</div></blockquote><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255);" class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255); float: none; display: inline !important;" class="">My impression is that the automated CSE and DCE is very important to the current implementation. </span></div></blockquote><div><br class=""></div><div>IIRC Aditya turned off CSE in SDISel for experimentation purposes (we wanted to quantify how useful that is), and the results were surprising in the sense that it pretty much didn’t affect the quality of the generated code.</div><div><br class=""></div><div>Aditya, could you share what you found when you did this experiment?</div><div>Admittedly, this was a while and I may misremember!</div><br class=""><blockquote type="cite" class=""><div class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255); float: none; display: inline !important;" class="">There's a lot of code that depends on his happening in order to have the expected effects. Otherwise, the use-count checks won't do the right thing (because the old unused uses of things won't immediately go away).</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255);" class=""><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255);" class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255); float: none; display: inline !important;" class="">I'm not entirely sure you can just turn off the uniquing in SDAG and get a sensible result.</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255);" class=""><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255);" class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255); float: none; display: inline !important;" class=""> -Hal</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255);" class=""><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255);" class=""><blockquote cite="mid:1784BCDB-531F-47A1-A664-240F1038E407@apple.com" type="cite" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255);" class=""><div class=""><br class=""></div><div class=""><blockquote type="cite" class=""><div class="">On Nov 10, 2017, at 9:54 PM, Daniel Sanders <<a moz-do-not-send="true" href="mailto:daniel_l_sanders@apple.com" class="">daniel_l_sanders@apple.com</a>> wrote:</div><div class=""><div class="" style="font-family: SFHello-Regular;"><div class=""><div class=""><br class=""></div>My thinking on this is that (with a few exceptions that I'll get to), combine and select are basically the same thing. You match some MIR, and replace it with other MIR. The main difference being that combine doesn't have to constrain to register classes (unless it wants to) while select does.</div><div class=""><br class=""></div><div class="">With that in mind, I was thinking that it makes sense to put a lot of effort into the optimization of the tablegen-erated selection table (as has been started in Quentin's recent patch) and then re-use it for combines too. We'll need to be careful how we define GlobalISel's counterpart to SelectionDAG patterns to make it expressive enough to support combines but that's essentially a second frontend (the other being the SelectionDAG importer) on a common backend</div></div></div></blockquote><div class="">Agreed that combine and selection are similar processes. It sounds like this is something we should look at prototyping.</div><div class=""><br class=""></div><blockquote type="cite" class=""><div class=""><div class="" style="font-family: SFHello-Regular;"><div class=""><br class=""></div><div class="">Req 2 becomes simple to implement in this approach. You can either use the existing feature-bits mechanism to enable/disable combine rules as a group, or add an equivalent mechanism in tablegen to decide whether a rule makes it into the emitted table or not and have multiple tables which you can run/not-run at will. With the new coverage feedback mechanism, we could potentially organize our tables semi-automatically by highlighting combine rules that never or rarely fire in a particular pass.</div><div class=""><br class=""></div><div class="">One feature I think we ought to have that isn't on the requirements list already, is that I think we should have a means to support rules with more than one match root. For example (using SelectionDAG patterns):</div><div class=""> <span class="Apple-converted-space"> </span>(set $dst1:GPR32, (i32 (load $ptr:GPR64)))</div><div class=""> <span class="Apple-converted-space"> </span>(set $dst2:GPR32, (i32 (load (add $ptr:GPR64 4))))</div><div class="">into:</div><div class=""> <span class="Apple-converted-space"> </span>(set $tmp:GPR64, (v2s32 (load $ptr:GPR64)))</div><div class=""> <span class="Apple-converted-space"> </span>(set $dst1, (extractelt $tmp:GPR64, 0))</div><div class=""> <span class="Apple-converted-space"> </span>(set $dst2, (extractelt $tmp:GPR64, 1))</div><div class="">Or something along those lines (such as fusing div/mod together). The combiner should be smart enough to make the root the $ptr, and follow the use of $ptr into the load/add, then follow the def to the 4.</div></div></div></blockquote>This seems like a nice feature, but I wonder about the impact this will have on the speed of the matching algorithm. I don’t know enough about it to say though. IMO complex features can be done in C++ code if they’re uncommon, in preference for fast handling of the common cases. Maybe a few more use cases are needed.</div><div class=""><br class=""></div><div class="">Thanks,</div><div class="">Amara</div></blockquote><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255);" class=""><pre class="moz-signature" cols="72" style="font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255);">--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory</pre><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255); float: none; display: inline !important;" class="">_______________________________________________</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255);" class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255); float: none; display: inline !important;" class="">LLVM Developers mailing list</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255);" class=""><a href="mailto:llvm-dev@lists.llvm.org" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255);" class="">llvm-dev@lists.llvm.org</a><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255);" class=""><a href="http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255);" class="">http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev</a><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255);" class=""></div></blockquote></div><br class=""></body></html>