<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 Oct 1, 2014, at 12:00 PM, Quentin Colombet <<a href="mailto:qcolombet@apple.com" class="">qcolombet@apple.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><blockquote type="cite" class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 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=""><br class="Apple-interchange-newline">On Oct 1, 2014, at 11:07 AM, Serge Pavlov <<a href="mailto:sepavloff@gmail.com" class="">sepavloff@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class="">Hi Quentin,<br class=""><div class="gmail_extra"><br clear="all" class=""><div class="">2014-10-02 0:21 GMT+07:00 Quentin Colombet<span class="Apple-converted-space"> </span><span dir="ltr" class=""><<a href="mailto:qcolombet@apple.com" target="_blank" class="">qcolombet@apple.com</a>></span>:<br class=""></div><div class="gmail_quote"><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex;"><div class="" style="word-wrap: break-word;"><div class=""><div class=""><span class=""><blockquote type="cite" class=""><div class=""><div dir="ltr" class=""><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex;"><div class="" style="word-wrap: break-word;"><div class=""><div class=""><div class=""><div class=""><div class=""><div class=""><blockquote type="cite" class=""><div class=""><span class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; float: none; display: inline !important;">The constant hoisting pass does this kind of things. Should we try to teach it to handle this kind of cases?</span></div></blockquote></div></div></div></div></div></div></div></blockquote><div class="">That would be interesting. However this pass is x86 specific and can use processor features (subregister structure, loading 64-bit value with 32-bit move). Can theses features be used by constant hoisting? </div></div></div></div></div></blockquote><div class=""><br class=""></div></span><div class="">Maybe. This pass has a bunch of target hooks if I remember correctly. Juergen would know better :).</div></div></div></div></blockquote></div></div></div></div></blockquote></div></blockquote><div><br class=""></div><div>The constant hoisting pass only uses a simple target hook that returns the estimated cost of the immediate for a given instruction. This is mostly guessing, because at the LLVM IR level it is sometimes difficult to predict which instructions will be generated.</div><div><br class=""></div><div>There is also the problem that the immediates are not only coalesced inside a single basic block, but also across basic blocks (which is the whole point of the constant hoisting pass) and then hoisted to a common dominator. This increases register pressure and we currently don’t track register pressure during constant hoisting. That is why we limit this to expensive immediates only.</div><div><br class=""></div><div>The pass could be extended to not hoist all constants and do something similar just on the basic block level, but this overall optimization seems to be a very X86 specific code size optimization and there are even more constants generated during SelectionDAG which you wouldn’t be able to catch at LLVM IR level.</div><br class=""><blockquote type="cite" class=""><div class=""><blockquote type="cite" class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 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 dir="ltr" class=""><div class="gmail_extra"><div class="gmail_quote"><div class=""><br class=""></div><div class="">Ok, looking forward to advice:)</div><div class=""> </div><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex;"><div class="" style="word-wrap: break-word;"><div class=""><div class=""><span class=""><blockquote type="cite" class=""><div class=""><div dir="ltr" class=""><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex;"><div class="" style="word-wrap: break-word;"><div class=""><div class=""><div class=""><div class=""><div class=""><div class=""><blockquote type="cite" class=""><div class=""><span class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; float: none; display: inline !important;">Moreover, this may be beneficial for code size, but I guess it is generally not beneficial for performances. Therefore, I believe this should be done for functions with the Os or Oz attributes only.</span></div></blockquote></div></div></div></div></div></div></div></blockquote><div class="">Just curious, why? Moves from register must be faster than move from memory.</div></div></div></div></div></blockquote><div class=""><br class=""></div></span><div class="">Yes, but those are moves from immediate, which does not require memory at all.</div></div></div></div></blockquote><div class=""><br class=""></div><div class="">On the other hand it loads memory bus.</div></div></div></div></div></blockquote><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 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;" class=""><br class=""></div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 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;" class="">You mean by its encoding size or am I missing something?</div><br class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 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;"><blockquote type="cite" class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 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 dir="ltr" class=""><div class="gmail_extra"><div class="gmail_quote"><div class=""> </div><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex;"><div class="" style="word-wrap: break-word;"><div class=""><div class=""><div class="">My performance concerns are:</div><div class="">- Register pressure, like Rafael mentioned.</div><div class="">- Additional scheduling dependencies.</div><div class=""><br class=""></div><div class="">Going back to your example:</div><div class="">This yields two independent chain of computation that can be scheduled independently. Moreover, you need just one register to realize this sequence.</div><div class=""><span class=""> mov $0, 0x4(%esi)<br class=""> mov $0, 0x8(%esi)<br class=""><br class=""></span>The two sequences of computations have now to wait for the first mov immediate. Moreover, this sequence requires 2 registers.<span class=""><br class=""> mov $0, %eax<br class=""> mov %eax, 0x4(%esi)<br class=""> mov %eax, 0x8(%esi)</span></div><div class=""><br class=""></div></div></div></div></blockquote><div class=""><br class=""></div><div class="">Looks reasonable, thank you for explanation.</div><div class=""> </div><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex;"><div class="" style="word-wrap: break-word;"><div class=""><div class=""><div class=""></div><span class=""><br class=""><blockquote type="cite" class=""><div class=""><div dir="ltr" class=""><div class="gmail_extra"><div class="gmail_quote"><div class="">Both gcc and icc use moves from register when compiling with optimization. </div></div></div></div></div></blockquote><div class=""><br class=""></div></span><div class="">Sure. What I am saying is that generally speaking, trading an immediate to register copy against a register to register copy does not sound like beneficial to me. </div><div class=""><br class=""></div><div class="">Except from code size improvements, what kind of improvements are you seeing?</div></div></div></div></blockquote><div class=""> </div><div class="">The main goal was code size. In fact the main interest is optimization of memset, the problem is described in <a href="http://llvm.org/bugs/show_bug.cgi?id=5124" class="">http://llvm.org/bugs/show_bug.cgi?id=5124</a>. </div></div></div></div></div></blockquote><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 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;" class=""><br class=""></div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 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;" class="">Thanks for the context.</div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 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;" class="">This seems to confirm that, at least at first, we should do that just for Os and Oz functions (i.e., look for function attribute: OptimizeForSize and MinSize<span class="" style="background-color: rgb(255, 255, 255);">).</span></div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 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;" class=""><span class="" style="background-color: rgb(255, 255, 255);"><br class=""></span></div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 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;" class=""><span class="" style="background-color: rgb(255, 255, 255);">At this point, I’ll wait for Juergen's feedbacks on constant hoisting before doing anything.</span></div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 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;" class=""><span class="" style="background-color: rgb(255, 255, 255);">Based on his answer, we will see how to move forward.</span></div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 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;" class=""><span class="" style="background-color: rgb(255, 255, 255);"><br class=""></span></div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 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;" class=""><span class="" style="background-color: rgb(255, 255, 255);">Thanks again,</span></div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 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;" class=""><span class="" style="background-color: rgb(255, 255, 255);">-Quentin</span></div><br class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 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;"><blockquote type="cite" class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: 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 dir="ltr" class=""><div class="gmail_extra"><div class="gmail_quote"><div class=""><br class=""></div><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex;"><div class="" style="word-wrap: break-word;"><div class=""><div class=""><div class="">Also, how big are those improvements?</div><div class=""><br class=""></div></div></div></div></blockquote><div class=""><br class=""></div><div class="">Compilation of PHP distribution with and without this pass shows size reduction about 0.3%.</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""> --Serge</div></div></div></div></blockquote></div></blockquote></div><br class=""></body></html>