<html><head><meta http-equiv="Content-Type" content="text/html charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><br><div><div>On Apr 24, 2013, at 12:17 PM, Arnold Schwaighofer <<a href="mailto:aschwaighofer@apple.com">aschwaighofer@apple.com</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div style="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;">Given that we have the optimization implemented in both SimplifyCFG and the Early-If converter now, I believe we can take a pragmatic approach:<br><br>* Enable the SimplifyCFG optimization now. This will allow us not to look bad on Phoronix/hmmer and spec/hmmer in llvm 3.3 (20%!). And it seems people agree that this is generally a good thing in the current compilation pipeline.<br>and<br>* Add the Early If-converter solution. This is the future proof approach. Once we get rid of the select cannonicalization and enable the early-ifconverter we have this to fall back on.<br><br><br>What do you think?<br></div></blockquote><div><br></div><div>As long as we don't introduce target heuristics in the early SimplifyCFG passes, it shouldn't be a problem. It is certainly possible for targets to reverse-if-convert if needed.</div><div><br></div><div>Long-term I agree with Chandler. I don't like GVN and other high level opts growing dependent on selects that are only formed in a small subset of the cases in which optimization should apply. That is clearly anti-canonical. To be rehashed after ISEL is fixed...</div><div><br></div><div>-Andy</div><br><blockquote type="cite"><div style="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;">On Apr 24, 2013, at 1:47 PM, Chandler Carruth <<a href="mailto:chandlerc@google.com">chandlerc@google.com</a>> wrote:<br><br><blockquote type="cite">I generally agree with Chris's position in the here-and-now FWIW. We should continue to balance the IR-level if-conversion for canonicalization against the down sides.<br><br>However, I wanted to point out that increasingly I have a different long-term hope for this type of canonicalization that has been heavily influenced by Evan, Andy, Jakob, and Dan Gohman's problems with doing canonicalizations like this at the IR level:<br><br>On Wed, Apr 24, 2013 at 7:37 PM, Chris Lattner <<a href="mailto:clattner@apple.com">clattner@apple.com</a>> wrote:<br>On Apr 24, 2013, at 11:28 AM, Shuxin Yang <<a href="mailto:shuxin.llvm@gmail.com">shuxin.llvm@gmail.com</a>> wrote:<br><blockquote type="cite"><blockquote type="cite"><br>Because this is a canonicalization of this sort, it seems clearly good to do on IR, and early. Doing something like this at the codegen level specifically for micro-architectural reasons could also make sense, but I don't see that eliminating the usefulness of doing it early as well.<br></blockquote>Introducing a "select" at IR level dose not necessarily means CodeGen convert the "select" with predicated instruction like cmov.<br>cmov is not necessary inexpensive, for example, on Pentium 4, the latency of cmov is about 10+ cycle.<span class="Apple-converted-space"> </span><br></blockquote><br>Yes, I truly understand that. My point is that it is still a canonicalization: depending on how the user wrote the code is silly for the opposite reason: if they wrote code with ?: or max on pentium 4, codegen should convert it to an "if" if the branch is biased.<br><br>I feel like we need a better strategy long-term. Increasingly, I'm of the opinion that as we move away from the SelectionDAG's basic block limitations, we should also move away from canonicalizing to the if-converted code. These days I would rather see us add utilities to LLVM to look across basic blocks which form PHI-only CFGs as-if they were straight line code, and to have the canonical form of predicated values be PHI-only CFGs. This has a nice advantage of simplifying the IR model, and being strictly more general than selects. However, it *requires* that we don't have a basic-block granularity hard limitation in passes, and that's just not the world we live in currently. Maybe eventually.<br>_______________________________________________<br>llvm-commits mailing list<br><a href="mailto:llvm-commits@cs.uiuc.edu">llvm-commits@cs.uiuc.edu</a><br>http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits</blockquote></div></blockquote></div><br></body></html>