<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Mar 29, 2021, at 2:55 PM, Björn Pettersson A 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=""><meta charset="UTF-8" class=""><div class="WordSection1" style="page: WordSection1; caret-color: rgb(0, 0, 0); font-family: Helvetica; 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; text-decoration: none;"><div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">I was about to say that TargetRegisterInfo::shouldCoalesce can be overridden by the target to prevent coalescing. But it looks like MachineCSE is doing coalescing/propagation without considering that target hook.<o:p class=""></o:p></div><div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">Maybe there should be a similar interface (unless shouldCoalesce can be used) to allow a target to prevent coalescing in certain circumstances also in MachineCSE.<o:p class=""></o:p></div><div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><o:p class=""> </o:p></div><div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">It would of course be nice if the register allocator could undo the contrain+coalescing, i.e. splitting the intervals, and then also relax the register classes if possible. Possibly avoiding spill.</div></div></div></blockquote><div><br class=""></div><div>That’s exactly what the greed allocator does :).</div><br class=""><blockquote type="cite" class=""><div class=""><div class="WordSection1" style="page: WordSection1; caret-color: rgb(0, 0, 0); font-family: Helvetica; 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; text-decoration: none;"><div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">No idea if that already is attempted or even feasible.<o:p class=""></o:p></div><div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><o:p class=""> </o:p></div><div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">/Björn<o:p class=""></o:p></div><div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><o:p class=""> </o:p></div><div style="border-style: none none none solid; border-left-width: 1.5pt; border-left-color: blue; padding: 0cm 0cm 0cm 4pt;" class=""><div class=""><div style="border-style: solid none none; border-top-width: 1pt; border-top-color: rgb(225, 225, 225); padding: 3pt 0cm 0cm;" class=""><div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><b class="">From:</b><span class="Apple-converted-space"> </span>llvm-dev <<a href="mailto:llvm-dev-bounces@lists.llvm.org" class="">llvm-dev-bounces@lists.llvm.org</a>><span class="Apple-converted-space"> </span><b class="">On Behalf Of<span class="Apple-converted-space"> </span></b>David Callahan via llvm-dev<br class=""><b class="">Sent:</b><span class="Apple-converted-space"> </span>den 28 mars 2021 21:05<br class=""><b class="">To:</b><span class="Apple-converted-space"> </span>LLVM Dev Mailing list <<a href="mailto:llvm-dev@lists.llvm.org" class="">llvm-dev@lists.llvm.org</a>><br class=""><b class="">Subject:</b><span class="Apple-converted-space"> </span>[llvm-dev] Register constraints for implicit register usage<o:p class=""></o:p></div></div></div><div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><o:p class=""> </o:p></div><div class=""><p class="MsoNormal" style="margin: 0cm 0cm 8pt; font-size: 11pt; font-family: Calibri, sans-serif; line-height: 15.546667098999023px;"><span style="" class="">I am working on a machine with a special register class which here I will call Xrc containing registers X0..Xk. <o:p class=""></o:p></span></p><p class="MsoNormal" style="margin: 0cm 0cm 8pt; font-size: 11pt; font-family: Calibri, sans-serif; line-height: 15.546667098999023px;"><span style="" class="">Some instructions implicitly reference X0. I am modeling this with a pseudo instruction that makes that reference explicit and created a register class X0rc which just contains X0. This seems to work fine and SelectionDAG introduces copies between the two register classes where I would expect them. <o:p class=""></o:p></span></p><p class="MsoNormal" style="margin: 0cm 0cm 8pt; font-size: 11pt; font-family: Calibri, sans-serif; line-height: 15.546667098999023px;"><span style="" class="">The problem I face is that MachineCSE::PerformTrivialCopyPropagation eliminates these copies and effectively changes the register class for the output register of unconstrained instructions to be X0rc. This is done so aggressively that I end up with multiple overlapping live ranges and so values are constantly being spilled and reloaded. Also, it creates situations where multiple operands of an instruction are bound to this class and so register allocation fails. <o:p class=""></o:p></span></p><p class="MsoNormal" style="margin: 0cm 0cm 8pt; font-size: 11pt; font-family: Calibri, sans-serif; line-height: 15.546667098999023px;"><span style="" class="">What is the preferred way to handle this kind of situation? <o:p class=""></o:p></span></p><p class="MsoNormal" style="margin: 0cm 0cm 8pt; font-size: 11pt; font-family: Calibri, sans-serif; line-height: 15.546667098999023px;"><span style="" class="">Thanks <o:p class=""></o:p></span></p><p class="MsoNormal" style="margin: 0cm 0cm 8pt; font-size: 11pt; font-family: Calibri, sans-serif; line-height: 15.546667098999023px;"><span style="" class="">david <o:p class=""></o:p></span></p><div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt;" class=""><o:p class=""> </o:p></span></div></div></div></div><span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; 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; text-decoration: none; float: none; display: inline !important;" class="">_______________________________________________</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; 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; text-decoration: none;" class=""><span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; 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; text-decoration: none; float: none; display: inline !important;" class="">LLVM Developers mailing list</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; 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; text-decoration: none;" class=""><span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; 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; text-decoration: none; float: none; display: inline !important;" class=""><a href="mailto:llvm-dev@lists.llvm.org" class="">llvm-dev@lists.llvm.org</a></span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; 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; text-decoration: none;" class=""><span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; 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; text-decoration: none; float: none; display: inline !important;" class=""><a href="https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev" class="">https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev</a></span></div></blockquote></div><br class=""></body></html>