<div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote">On Wed, May 9, 2018 at 10:39 AM, Hiroshi Yamauchi <span dir="ltr"><<a href="mailto:yamauchi@google.com" target="_blank">yamauchi@google.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div style="font-family:arial,helvetica,sans-serif"><br></div><br><div class="gmail_quote"><span class="gmail-"><div dir="ltr">On Tue, May 8, 2018 at 11:15 AM Daniel Berlin <<a href="mailto:dberlin@dberlin.org" target="_blank">dberlin@dberlin.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote">On Tue, May 8, 2018 at 10:38 AM, Hiroshi Yamauchi via llvm-dev <span dir="ltr"><<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr">(<div style="color:rgb(34,34,34);font-size:13px;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;text-decoration-style:initial;text-decoration-color:initial;font-family:arial,helvetica,sans-serif;display:inline">​I came across this issue in the context of</div><span style="color:rgb(34,34,34);font-family:sans-serif;font-size:13px;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"> </span><a href="https://reviews.llvm.org/D46336" style="color:rgb(17,85,204);font-family:sans-serif;font-size:13px;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" target="_blank">D46336</a><span style="color:rgb(34,34,34);font-family:sans-serif;font-size:13px;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">.</span><div style="font-family:arial,helvetica,sans-serif;display:inline">​ ​</div>Thanks, Sanjay, for starting this discussion.)<br><div><br></div><div>If <div style="font-family:arial,helvetica,sans-serif;display:inline">​we will</div> move <span style="color:rgb(34,34,34);font-family:sans-serif;font-size:13px;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><div style="font-family:arial,helvetica,sans-serif;display:inline">​reassociation, </div></span>or keep additional ones<div style="font-family:arial,helvetica,sans-serif;display:inline">​,​</div> out of instcombine, <div style="font-family:arial,helvetica,sans-serif;display:inline">​open questions for me would be</div><div style="font-family:arial,helvetica,sans-serif;display:inline">​​:</div><br><br>1. Since -reassociate isn't a fixed point pass,</div></div></blockquote><div><br></div><div>This is fixable, fwiw, without fixpointing it.</div></div></div></div></blockquote><div><br></div></span><div><div style="font-family:arial,helvetica,sans-serif">How?</div></div></div></div></blockquote><div><br></div><div>Depends on specifically which part you would like to know about ;)</div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div class="gmail_quote"><span class="gmail-"><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div> we might need to repeat "-instcombine -reassociate" multiple times to <div style="font-family:arial,helvetica,sans-serif;display:inline">​fold</div> down to what we want (relating to <a href="https://reviews.llvm.org/D46336#1087082" target="_blank">my comment here</a>). I assumed this isn't not what we want to do<div style="font-family:arial,helvetica,sans-serif;display:inline">​? My impression is we don't do a fixed-point with passes?</div></div></div></blockquote><div><br></div><div>Well, i mean there is no practical difference between passes that we fixpoint externally and fixpoint internally.</div></div></div></div></blockquote></span><div><div style="font-family:arial,helvetica,sans-serif">​​</div><div style="font-family:arial,helvetica,sans-serif"><span style="color:rgb(34,34,34);font-family:arial,helvetica,sans-serif;font-size:13px;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">I had the following in mind: Does the pass manager support fixpointing externally? Is there any performance difference? Are people okay with that in general?</span></div><div style="font-family:arial,helvetica,sans-serif"><br></div><div style="font-family:arial,helvetica,sans-serif">But if <span style="font-family:sans-serif">there is no practical difference</span>, I don't see any problem with that :)</div></div><span class="gmail-"><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div> <br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div> <br></div></div></blockquote><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div>2. <div style="font-family:arial,helvetica,sans-serif;display:inline">​Since -<span style="color:rgb(34,34,34);font-family:sans-serif;font-size:13px;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">reassociate needs to come up with one operand order (at least currently as the only <span style="color:rgb(34,34,34);font-family:sans-serif;font-size:13px;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">reassociate </span>pass), would there exist a single, unique operand order that would enable all <span style="color:rgb(34,34,34);font-family:arial,helvetica,sans-serif;font-size:13px;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><span style="color:rgb(34,34,34);font-family:sans-serif;font-size:13px;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">reassociative/commutative</span> foldings that we want? </span></span></div></div></div></blockquote><div><br></div><div>In what way?</div><div>Are you asking whether there is a single reassociation order that makes all foldings occur in the same operation or something?<br>I don't feel like i understand what you are asking.<br></div></div></div></div></blockquote><div><br></div></span><div><div><span style="font-family:arial,helvetica,sans-serif">Does this rephrase help: with the motivating examples (like and-of-shifts or bit check patterns) from the above differentials in mind, can we come up with a single <span style="color:rgb(34,34,34);font-family:sans-serif;font-size:13px;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">reassociation order that solves all those and all the others that may come up in the future? Would we need different <span style="color:rgb(34,34,34);font-family:sans-serif;font-size:13px;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">reassociation orders to fold different patterns?</span></span></span></div></div><span class="gmail-"><div></div></span></div></div></blockquote><div><br></div><div>It doesn't quite help.</div><div>When stated that generally, there can be no such ordering at all, that's easy to prove.  It is a statically undecidable problem.</div><div><br></div><div>There is however, a different question and answer to a few related problems that maybe you are really asking?<br>1. Is there a way to determine and apply the a maximal or nearly-maximal set of folds/graph transforms that could be applied to a given set of code in a sane and principled way -> yes</div><div><br></div><div>(see, e.g., <a href="http://www.cs.cornell.edu/~ross/publications/eqsat/">http://www.cs.cornell.edu/~ross/publications/eqsat/</a>)</div><div><br></div><div>2. Is there a way to determine all expressions in the program as it exists that are equivalent or equivalent under constant time constant folding/reassociation, in a reasonable time bound -> yes</div><div><br></div><div>(not a single easy link, happy to talk about it)</div><div><br></div><div>Your original question is basically equivalent to</div><div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:small;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial">Is there a way to determine all expressions in the program as it exists that are equivalent or could be made equivalent through any type of folding that one can think up?</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:small;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial">The answer to that is "no", it's provable that this is not statically decidable, so the time bound doesn't matter :)</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:small;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial"><br></div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:small;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial">You have to limit the possible folding/evaluation you apply in various ways to make this decidable, and then further limit it to make the time bound reasonable.</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:small;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial"><br></div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:small;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial">This all quickly devolves into herbrand equivalence and it's variations.</div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:small;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial"><br></div><div style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:small;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial"><br></div></div></div></div></div>