<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="">Is annotating "known" libcall with "norecurse" something that is just "work in-progress", or are we blocked because we may have a user-defined strlen for example?<div class="">What about intrinsics?</div><div class=""><br class=""></div><div class="">-- </div><div class="">Mehdi</div><div class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Mar 21, 2016, at 5:34 PM, Chandler Carruth <<a href="mailto:chandlerc@google.com" class="">chandlerc@google.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class="">I think the conceptual issues have largely been sorted out, it is mostly that it is *much* harder to deduce norecurse than it might seem like superficially.</div><br class=""><div class="gmail_quote"><div dir="ltr" class="">On Mon, Mar 21, 2016 at 4:02 PM Mehdi Amini <<a href="mailto:mehdi.amini@apple.com" class="">mehdi.amini@apple.com</a>> wrote:<br class=""></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word" class=""><div class=""><blockquote type="cite" class=""><div class="">On Mar 21, 2016, at 3:57 PM, Sanjin Sijaric via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank" class="">llvm-dev@lists.llvm.org</a>> wrote:</div><br class=""><div class=""><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class=""><div style="margin:0in 0in 0.0001pt;font-size:11pt;font-family:Calibri,sans-serif" class="">Hi,<u class=""></u><u class=""></u></div><div style="margin:0in 0in 0.0001pt;font-size:11pt;font-family:Calibri,sans-serif" class=""><u class=""></u> <u class=""></u></div><div style="margin:0in 0in 0.0001pt;font-size:11pt;font-family:Calibri,sans-serif" class="">GlobalOpt may not consider demoting globals to locals in the “main” function when C is used. It used to consider “main” specifically prior to commit r253168 , for both C and C++. Since r253168, the check for the norecurse attribute may prevent “main” from being considered. This happens because the Function Attributes pass will not add the norecurse attribute to functions that have calls to library functions that aren’t themselves marked with the norecurse attribute, such as putchar. Even a call to llvm.lifetime.start, for example, will prevent a function from being considered as non-recursive as llvm.lifetime.start isn’t marked with the “norecurse” attribute.<u class=""></u><u class=""></u></div><div style="margin:0in 0in 0.0001pt;font-size:11pt;font-family:Calibri,sans-serif" class=""><u class=""></u> <u class=""></u></div><div style="margin:0in 0in 0.0001pt;font-size:11pt;font-family:Calibri,sans-serif" class="">We have a C workload that benefits from this demotion with LTO, as some hot functions get inlined into main.<u class=""></u><u class=""></u></div><div style="margin:0in 0in 0.0001pt;font-size:11pt;font-family:Calibri,sans-serif" class=""><u class=""></u> <u class=""></u></div><div style="margin:0in 0in 0.0001pt;font-size:11pt;font-family:Calibri,sans-serif" class="">The comment in tools/clang/lib/CodeGen/CodeGenFunctions.cpp explains the reason for marking “main” with the norecurse attribute in C++:<u class=""></u><u class=""></u></div><div style="margin:0in 0in 0.0001pt;font-size:11pt;font-family:Calibri,sans-serif" class=""><u class=""></u> <u class=""></u></div><div style="margin:0in 0in 0.0001pt;font-size:11pt;font-family:Calibri,sans-serif" class="">// If we're in C++ mode and the function name is "main", it is guaranteed<u class=""></u><u class=""></u></div><div style="margin:0in 0in 0.0001pt;font-size:11pt;font-family:Calibri,sans-serif" class="">// to be norecurse by the standard (3.6.1.3 "The function main shall not be<u class=""></u><u class=""></u></div><div style="margin:0in 0in 0.0001pt;font-size:11pt;font-family:Calibri,sans-serif" class="">// used within a program").<u class=""></u><u class=""></u></div><div style="margin:0in 0in 0.0001pt;font-size:11pt;font-family:Calibri,sans-serif" class=""><u class=""></u> <u class=""></u></div><div style="margin:0in 0in 0.0001pt;font-size:11pt;font-family:Calibri,sans-serif" class="">No such restriction exists in the C standard, as far as I can tell.</div></div></div></blockquote><div class=""><br class=""></div></div></div><div style="word-wrap:break-word" class=""><div class=""><div class="">This seem to be you problem.</div></div></div><div style="word-wrap:break-word" class=""><div class=""><br class=""><blockquote type="cite" class=""><div class=""><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class=""><div style="margin:0in 0in 0.0001pt;font-size:11pt;font-family:Calibri,sans-serif" class=""><u class=""></u><u class=""></u></div><div style="margin:0in 0in 0.0001pt;font-size:11pt;font-family:Calibri,sans-serif" class=""><u class=""></u> <u class=""></u></div><div style="margin:0in 0in 0.0001pt;font-size:11pt;font-family:Calibri,sans-serif" class="">Is there anything that can be done to alleviate this restriction in C?</div></div></div></blockquote><div class=""><br class=""></div></div></div><div style="word-wrap:break-word" class=""><div class=""><div class="">Except if we had a source-level attribute, or a clang command line flag, I don't see how.</div><div class="">Write your main in C++ maybe?</div></div></div><div style="word-wrap:break-word" class=""><div class=""><div class=""><br class=""></div><blockquote type="cite" class=""><div class=""><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class=""><div style="margin:0in 0in 0.0001pt;font-size:11pt;font-family:Calibri,sans-serif" class=""> Can we make the Function Attributes pass more aggressive, for example? </div></div></div></blockquote><div class=""><br class=""></div></div></div><div style="word-wrap:break-word" class=""><div class=""><div class="">Are you suggesting to break C semantics or I misunderstand what you mean?</div></div></div><div style="word-wrap:break-word" class=""><div class=""><div class=""><br class=""></div><br class=""><blockquote type="cite" class=""><div class=""><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class=""><div style="margin:0in 0in 0.0001pt;font-size:11pt;font-family:Calibri,sans-serif" class=""> Or mark certain library functions as “norecurse”, although I don’t see how this can be guaranteed.</div></div></div></blockquote><div class=""><br class=""></div></div></div><div style="word-wrap:break-word" class=""><div class=""></div>This is pretty recent and we don't have a good support for libcalls and norecurse. I think Chandler found also some conceptual issue to get it to work properly.<div class="">CC: Chandler+James who probably remember more than I do now.</div></div><div style="word-wrap:break-word" class=""><div class=""><br class=""><div class=""><br class=""></div><div class="">-- </div><div class="">Mehdi</div><div class=""><br class=""></div></div></div></blockquote></div>
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