<html><head><meta http-equiv="Content-Type" content="text/html charset=windows-1252"><meta http-equiv="Content-Type" content="text/html charset=windows-1252"><meta http-equiv="Content-Type" content="text/html charset=windows-1252"><meta http-equiv="Content-Type" content="text/html charset=windows-1252"><meta http-equiv="Content-Type" content="text/html charset=windows-1252"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><br><div><div>On Aug 13, 2013, at 2:39 PM, Eli Friedman <<a href="mailto:eli.friedman@gmail.com">eli.friedman@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div dir="ltr">On Tue, Aug 13, 2013 at 2:30 PM, Jim Grosbach <span dir="ltr"><<a href="mailto:grosbach@apple.com" target="_blank">grosbach@apple.com</a>></span> wrote:<br><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; position: static; z-index: auto;">Author: grosbach<br>
Date: Tue Aug 13 16:30:58 2013<br>
New Revision: 188315<br>
<br>
URL: <a href="http://llvm.org/viewvc/llvm-project?rev=188315&view=rev" target="_blank">http://llvm.org/viewvc/llvm-project?rev=188315&view=rev</a><br>
Log:<br>
DAG: Combine (and (setne X, 0), (setne X, -1)) -> (setuge (add X, 1), 2)<br>
<br>
A common idiom is to use zero and all-ones as sentinal values and to<br>
check for both in a single conditional ("x != 0 && x != (unsigned)-1").<br>
That generates code, for i32, like:<br>
testl %edi, %edi<br>
setne %al<br>
cmpl $-1, %edi<br>
setne %cl<br>
andb %al, %cl<br>
<br>
With this transform, we generate the simpler:<br>
incl %edi<br>
cmpl $1, %edi<br>
seta %al<br>
<br>
Similar improvements for other integer sizes and on other platforms. In<br>
general, combining the two setcc instructions into one is better.<br>
<br>
<a href="rdar://14689217">rdar://14689217</a><br><br></blockquote><div><br></div><div>We already have code in InstCombiner::FoldAndOfICmps to handle this sort of thing; it looks like it isn't catching this particular case for some reason, though.</div>
<div><br></div></div></div></div></blockquote><div><br></div><div>There’s already a bunch of similar checks in the DAGCombiner, too. I mainly put it there to that we’ll be able to catch more complicate cases that simplify to this one from other DAG transformations, that are exposed from target lowerings, etc..</div><div><br></div><div>Also, the transformation depends on the wrapping behavior of the add. While we can express that in IR, it feels a bit more appropriate to do that sort of change at the DAG level.</div><div><br></div><div>Do either of you have a strong preference for InstCombine instead? This isn’t the sort of thing where we have a definitive “right place” to put the transform, really.</div><div><br></div><div>-Jim</div></div></body></html>