<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=""><div class=""><br class=""></div><br class=""><div class=""><br class=""></div><div class="">It would help most if you had key examples/benchmarks where if conversion will provide (additional) performance benefits. Literature is a limited guide. For example, in hindsight many early papers on Itanium oversold the potential benefits - which is not necessarily their fault: branch predictors got better over time, global scheduling techniques improved over time etc. In the specific example of the Mahlke paper if-conversion turned out to improve global scheduling in their compiler. With a better global scheduler the result is different.</div><div class=""><br class=""></div><div class="">When you scope the design it will be helpful when the pass can be invoked on demand for a region by loop transformations and in the codegen. Specific example where if conversion enables e.g. vectorization or can eliminate targeted data dependent branches (on top of what can be done today) is where I think the benefits will be. The more specific examples you have the less likely you’ll get lost in the morass of compiler changes that get triggered by the ‘advanced’ conversion schemes.</div><div class=""><br class=""></div><div class="">-Gerolf</div><div><blockquote type="cite" class=""><div class="">On Mar 18, 2015, at 11:20 AM, Evan Cheng <<a href="mailto:evan.cheng@apple.com" class="">evan.cheng@apple.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div style="font-family: Helvetica; font-size: 18px; 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=""><blockquote type="cite" class=""><div class=""><br class="Apple-interchange-newline">On Mar 18, 2015, at 10:12 AM, Xiang Gao <<a href="mailto:qasdfgtyuiop@gmail.com" class="">qasdfgtyuiop@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class="">OK, Let me describe. There is nothing wrong with if-conversion in LLVM. The algorithm implemented in LLVM can handle the if(???){do something} and if(???){do something}else{do something else} case very well. But it can handle complicated case like when there are a lot of gotos in the program. The more systematic way to do if-conversion is based on Hyperblock<span class="Apple-converted-space"> </span><a href="http://www.eecs.umich.edu/eecs/about/articles/2015/p45-mahlke.pdf" class="">[Scott A. Mahlke et al 1992]</a>, which is set of basic blocks with one entrance and one or more exits. The algorithms now implemented in LLVM is very easy to understand, and very easy to implement. But it is not general, and it might not generate code with best efficient.</div></div></blockquote><div class=""><br class=""></div></div><div style="font-family: Helvetica; font-size: 18px; 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="">Questions:</div><div style="font-family: Helvetica; font-size: 18px; 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: 18px; 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 are not going to get good performance out of hyperblock without global scheduling. Are you planning to adopt the MI schedule to handle HB? Will it be predicate aware?</div><div style="font-family: Helvetica; font-size: 18px; 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="">* What architecture are you targeting? HB makes more sense for ISA with lots of predicated registers.</div><div style="font-family: Helvetica; font-size: 18px; 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: 18px; 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="">Evan</div><div style="font-family: Helvetica; font-size: 18px; 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=""><blockquote type="cite" class=""><div class=""><div dir="ltr" class=""><div class=""><br class=""></div><div class="">As we know, if-conversion is a double-edged sword, it might increase the efficiency, it might also make it worse. For example, if the program to be compiled have a code like:</div><div class="">if (<a>) {</div><div class=""> execute 1000 statements</div><div class="">} else {</div><div class=""> execute 1 statements</div><div class="">}</div><div class="">In the case when <a> is almost always true, it is very efficient because it avoid branching with only executing one more statement as punishment. The improvements of performance from eliminating the branching is always larger than 1 cpu cycle. However, in the case when <a> is almost always false, doing if-conversion is a stupid decision because the improvements of eliminating the branch is very limited here, but the punish, that 1000 more statements have to be executed everytime, really hurt.</div><div class=""><br class=""></div><div class="">There are studies on how to make a balance in whether to do if-conversion or not. There are both static policy<span class="Apple-converted-space"> </span><a href="http://impact.crhc.illinois.edu/shared/papers/micro-97-framework.pdf" class="">[David I. August et al 1997]</a><span class="Apple-converted-space"> </span>and dynamic policy<span class="Apple-converted-space"> </span><a href="http://www.cs.virginia.edu/kim/docs/pact00.pdf" class="">[Kim M. Hazelwood and Thomas Conte 2000]</a>. What I want to do for GSoC is to implement these approaches into LLVM.</div></div><div class="gmail_extra"><br clear="all" class=""><div class=""><div class="gmail_signature">Xiang Gao</div></div><br class=""><div class="gmail_quote">2015-03-17 18:14 GMT-04:00 John Criswell<span class="Apple-converted-space"> </span><span dir="ltr" class=""><<a href="mailto:jtcriswel@gmail.com" target="_blank" class="">jtcriswel@gmail.com</a>></span>:<br class=""><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 bgcolor="#FFFFFF" text="#000000" class=""><div class="">Dear Xiang,<br class=""><br class="">Can you briefly describe the limitations with the current if-conversion in LLVM and how you plan to improve it? I haven't read your thesis, and I (and others) are unlikely to have time to do so.<br class=""><br class="">Regards,<br class=""><br class="">John Criswell<div class=""><div class="h5"><br class=""><br class="">On 3/16/15 4:23 PM, Xiang Gao wrote:<br class=""></div></div></div><blockquote type="cite" class=""><div class=""><div class="h5"><div dir="ltr" class="">Hi,<div class=""><br class=""></div><div class="">Are you guys interested in implementing if-conversion as a GSoC 2015 project? Last year, I did a literature review about approaches of if-conversion and the if-conversion in LLVM. This was the undergraduate thesis of my bachelor degree. It seems that, the if-conversion used in LLVM is a very simple approach instead of following the literature. So I want to implement the approaches in the literature in LLVM and get support from Google for this. Are you guys interested?</div><div class=""><br class=""></div><div class="">Best,</div><div class=""><br class=""><div class=""><div class="">Xiang Gao</div></div></div></div><br class=""><fieldset class=""></fieldset><br class=""></div></div><pre class="">_______________________________________________
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