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</o:shapelayout></xml><![endif]--></head><body lang=EN-GB link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'>Hi David,<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'>In principal, giving more lifetime info can only improve stack slot sharing and reduce runtime stack usage, which is critical for us in the embedded world.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'>I did not test that on a wide code base yet, but we had a customer reporting an issue where llvm/clang was producing code with a stack usage significantly worse than gcc. To make things worse, in their case their code was heavily recursive, to the point where using clang was simply not an option: they are forced to use gcc </span><span style='font-size:11.0pt;font-family:Wingdings;color:#1F497D'>L</span><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'>This patch only adds lifetime markers to big enough (>32 bytes) objects, consistent with what is done for named temporaries. I do not know how this 32 bytes threshold has been choosen, but there is for sure a compile time / stack size gain trade-off to be made. My experiments have shown that for our customer case, the threshold should be lower: 16-bytes. But changing this threshold would require a separate thread on this list, as well as much more measurements.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'>The improvements I have been able to get, by visual inspection of the generated assembly code, for a single call of the hot functions were:<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'> | GCC | Clang | LT-32 | LT-16 |<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'>===+=====+=======+=======+=======+<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'>F1 | 432 | 608 | 608 | 400 |<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'>F2 | 432 | 640 | 640 | 432 |<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'>F3 | 384 | 368 | 368 | 192 |<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'>F4 | 320 | 400 | 400 | 224 |<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'>Stack size is expressed in bytes.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'>GCC version 4.8<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'>LT-32 is clang with this patch (default 32 bytes threshold for all temporaries).<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'>LT-16 is clang with this patch and a 16 bytes threshold for all temporaries.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'>I believe bootstrapping clang could be a good testcase and will be needed when we will address the real problem in a separate discussion: what threshold should we use ?<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'>Very strangely to me coming from the embedded world, I have not found how to measure a program stack usage on linux, so if you have any idea, I am glad to hear about it.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'>Cheers,<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'>Arnaud<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Courier New";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><b><span lang=EN-US style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span lang=EN-US style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> David Blaikie [mailto:dblaikie@gmail.com] <br><b>Sent:</b> 16 September 2014 01:25<br><b>To:</b> Arnaud De Grandmaison<br><b>Cc:</b> llvm cfe<br><b>Subject:</b> Re: [PATCH] Emit lifetime start/end for unnamed objects --- take 3<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><o:p> </o:p></p><div><p class=MsoNormal style='margin-left:36.0pt'>I'm hardly an expert on this stuff - but just curious: what sort of testing did you put this through? Bootstrap Clang? Were you able to gather any stats on reduced stack usage with this improvement to lifetime markers?<o:p></o:p></p></div><div><p class=MsoNormal style='margin-left:36.0pt'><o:p> </o:p></p><div><p class=MsoNormal style='margin-left:36.0pt'>On Mon, Sep 15, 2014 at 4:05 PM, Arnaud A. de Grandmaison <<a href="mailto:arnaud.degrandmaison@arm.com" target="_blank">arnaud.degrandmaison@arm.com</a>> wrote:<o:p></o:p></p><div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt'>Hi All,<o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt'> <o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt'>Please find attached a patch which teaches clang to emit lifetime.start / lifetime.end markers for unnamed temporary objects.<o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt'> <o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt'>This patch can greatly reduce the stack usage of some C++ code, where it is so easy to have short lived unnamed temporaries.<o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt'> <o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt'>As noted in the subject, this is my third attempt: my previous attempts failed to handle correctly the lifetime extended temporaries, and I have had a hard time to understand the CleanupScope. It all boiled down to the fact that the body of a function is not considered a full CleanupScope (for debug information reasons), so in the case of lifetime extended objects at the top level of the function body, with a trivial destructor + lifetime.end marker, the lifetime markers were simply not considered, firing an assert in ~CodeGenFunction. All cases are now covered by testcases.<o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt'> <o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt'>I would appreciate if someone knowledgeable with the lifetime extended temporaries & cleanup scopes could give a look to this patch.<o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt'> <o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt'>Cheers,<o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt'>--<o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt'>Arnaud A. de Grandmaison<o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt'> <o:p></o:p></p></div></div><p class=MsoNormal style='mso-margin-top-alt:0cm;margin-right:0cm;margin-bottom:12.0pt;margin-left:36.0pt'><br>_______________________________________________<br>cfe-commits mailing list<br><a href="mailto:cfe-commits@cs.uiuc.edu">cfe-commits@cs.uiuc.edu</a><br><a href="http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits" target="_blank">http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits</a><o:p></o:p></p></div><p class=MsoNormal style='margin-left:36.0pt'><o:p> </o:p></p></div></div></body></html>