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</o:shapelayout></xml><![endif]--></head><body lang=EN-IE link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'>Instead of suppressing the integer promotion rules which are part of the ISO C/C++ Standards, we wrote a new pass that analyses the IR to see if the input values and output value were of an integer type that was narrower than the promoted types used in the IR, and if we could prove that the outcome would be identical if the type was unpromoted, then we reduced the IR to use the narrower form.<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'>In our case the motive was to enhance vectorisation because our vector ALU can work with 8-, 16- and 32-bit integers natively, and handling ‘</span><span style='font-family:"Courier New";color:black;mso-fareast-language:EN-US'>vXi8</span><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'>’ vectors ended was actually being promoted to multiple ‘</span><span style='font-family:"Courier New";color:black;mso-fareast-language:EN-US'>v4i32</span><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'>’ vectors requiring 4 times as many instructions as were necessary, or worse still, fully scalarized.<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'>This pass was presented by my colleague Stephen Rogers in a “Lighting Talk” at the October 2015 LLVM Conference in San Jose and titled “</span><span style='font-family:"Book Antiqua",serif;color:black;mso-fareast-language:EN-US'>Integer Vector Optimizations and “Usual Arithmetic Conversions”</span><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'>”. I can’t find the paper or slides on the LLVM Meetings page, perhaps these are not archived for Lightning Talks (?), but as they are not large I have attached them here.<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'>This approach allowed us to gain the optimisations that are possible with our architecture which supports 8-, 16- and 32-bit native integer computations (scalar and vector), while also respecting the ISO C and C++ Standards. I am a lot more nervous of a front-end switch for this, as it will lead to non-compliant programs, and in the presence of overloading and template-instantiation it could also lead to very different programs, and would recommend that we do not add a front-end switch which alters the semantics of the language in this way.<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'>It is my intention to publish this pass if it is of general interest, and since it is target independent there are no particular blocking issue for me (Patents, IP, etc.) to doing so. I do have to catch-up on the HEAD revision to ensure that it still works correctly, but it was working perfectly at SVN #262824 and it will be a month before I have enough time to catch up on the HEAD revision as we are busy with a product release that takes precedence.<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'>All the best,<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'> MartinO<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><b><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri",sans-serif'>From:</span></b><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri",sans-serif'> cfe-dev [mailto:cfe-dev-bounces@lists.llvm.org] <b>On Behalf Of </b>David Majnemer via cfe-dev<br><b>Sent:</b> 27 May 2016 19:55<br><b>To:</b> Dilan Manatunga <manatunga@gmail.com><br><b>Cc:</b> clang developer list <cfe-dev@lists.llvm.org>; Norman Rink <norman.rink@tu-dresden.de>; cfe-dev-request@lists.llvm.org<br><b>Subject:</b> Re: [cfe-dev] Disable integer promotion (Dilan Manatunga via cfe-dev)<o:p></o:p></span></p><p class=MsoNormal><o:p> </o:p></p><div><p class=MsoNormal>You could set IntWidth to 16 or 8 in clang, not unlike what MSP430 does:<o:p></o:p></p><div><p class=MsoNormal><a href="https://github.com/llvm-mirror/clang/blob/3317d0fa0bd1f5c5adc14bcc6adc2a38acc9064b/lib/Basic/Targets.cpp#L6823">https://github.com/llvm-mirror/clang/blob/3317d0fa0bd1f5c5adc14bcc6adc2a38acc9064b/lib/Basic/Targets.cpp#L6823</a><o:p></o:p></p></div></div><div><p class=MsoNormal><o:p> </o:p></p><div><p class=MsoNormal>On Fri, May 27, 2016 at 10:32 AM, Dilan Manatunga via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p><blockquote style='border:none;border-left:solid #CCCCCC 1.0pt;padding:0cm 0cm 0cm 6.0pt;margin-left:4.8pt;margin-right:0cm'><div><p class=MsoNormal>I need disabling this feature because I am researching architectures where 8-bit or 16-bit adds are preferred to 32-bit. So, integer promotion kinda mucks everything up. I was hoping there was a way in clang to disable it, instead of having to implement an LLVM pass to coalesce unnecessary promotions. <o:p></o:p></p><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Thanks for catching the IR mistake. Should have double checked that. This should be the correct version:<o:p></o:p></p></div><div><div><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana",sans-serif'>nt8_t a = 1;<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana",sans-serif'>int8_t b = 2;<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana",sans-serif'>int8_t c = a + b<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana",sans-serif'><o:p> </o:p></span></p></div><div><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana",sans-serif'>The LLVM IR will be:<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana",sans-serif'>%x = sext i8 %a to i32<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana",sans-serif'>%y = sext i8 %b to i32<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana",sans-serif'>%z = add nsw i32 %x, %y<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana",sans-serif'>%c = trunc i32 %z to i8<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana",sans-serif'><o:p> </o:p></span></p></div><div><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana",sans-serif'>Instead, it would simply compile to:<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana",sans-serif'>$c = add nsw i8 %z, $y<o:p></o:p></span></p></div></div><div><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana",sans-serif'><o:p> </o:p></span></p></div><div><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana",sans-serif'>-Dilan<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Verdana",sans-serif'><o:p> </o:p></span></p></div></div><div><div><p class=MsoNormal><o:p> </o:p></p><div><div><p class=MsoNormal>On Fri, May 27, 2016 at 5:30 AM Norman Rink via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p></div><blockquote style='border:none;border-left:solid #CCCCCC 1.0pt;padding:0cm 0cm 0cm 6.0pt;margin-left:4.8pt;margin-right:0cm'><p class=MsoNormal>Hi Dilan,<br><br>I would like to second your request for an option to disable integer<br>promotion. What do you need it for?<br><br>As far as I am aware, there is no such option and the code that implements<br>integer promotion is somewhat scattered across ³SemaExpr.cpp².<br><br>Also, I think your example code snippet contains a few ³i32²s too many. It<br>will be clearer to people what you are looking for if your code example is<br>consistent with your question.<br><br>Best,<br><br>Norman<br><br><br>>Message: 1<br>>Date: Fri, 27 May 2016 01:50:12 +0000<br>>From: Dilan Manatunga via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>><br>>To: <a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a><br>>Subject: [cfe-dev] Disable integer promotion<br>>Message-ID:<br>> <CAHpgGu4=<a href="mailto:jFC9ohQQZZMp2NMG3Hw0sE5U4-Lqrgb%2B6gcXv9SEtQ@mail.gmail.com" target="_blank">jFC9ohQQZZMp2NMG3Hw0sE5U4-Lqrgb+6gcXv9SEtQ@mail.gmail.com</a>><br>>Content-Type: text/plain; charset="utf-8"<br>><br>>Is there a way to disable integer promotion when performing math<br>>operations. For example, when compiling a statement such as this:<br>>int8_t a = 1;<br>>int8_t b = 2;<br>>int8_t c = a + b<br>><br>>The LLVM IR will be:<br>>%x = sext i32 %a to i32<br>>%y = sext i32 %b to i32<br>>%z = add nsw i32 %x, %y<br>>%c = trunc i32 %z to i16<br>><br>>Instead, it would simply compile to:<br>>$c = add nsw i32 %z, $y<br>><br>>-Dilan Manatunga<br>>-------------- next part --------------<br>>An HTML attachment was scrubbed...<br>>URL:<br>><<a href="http://lists.llvm.org/pipermail/cfe-dev/attachments/20160527/4a7920ab/att" target="_blank">http://lists.llvm.org/pipermail/cfe-dev/attachments/20160527/4a7920ab/att</a><br>>achment-0001.html><br>><br>>------------------------------<br><br><br>_______________________________________________<br>cfe-dev mailing list<br><a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a><br><a href="http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev" target="_blank">http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev</a><o:p></o:p></p></blockquote></div></div></div><p class=MsoNormal style='margin-bottom:12.0pt'><br>_______________________________________________<br>cfe-dev mailing list<br><a href="mailto:cfe-dev@lists.llvm.org">cfe-dev@lists.llvm.org</a><br><a href="http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev" target="_blank">http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev</a><o:p></o:p></p></blockquote></div><p class=MsoNormal><o:p> </o:p></p></div></div></body></html>