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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple style='word-wrap: break-word;-webkit-nbsp-mode: space;-webkit-line-break: after-white-space'><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Hi Jacob, <o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Thanks for quick feedback. See my comments below, <o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Cheers, <o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Zino <o:p></o:p></span></p><div><div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> Jakob Stoklund Olesen [mailto:stoklund@2pi.dk] <br><b>Sent:</b> Monday, January 23, 2012 7:18 PM<br><b>To:</b> Zino Benaissa<br><b>Cc:</b> llvm-commits@cs.uiuc.edu; rajav@codeaurora.org<br><b>Subject:</b> Re: [llvm-commits] Tuning LLVM Greedy Register Allocator to optimize for code size when targeting ARM Thumb 2 instruction set<o:p></o:p></span></p></div></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal><o:p> </o:p></p><div><div><p class=MsoNormal>On Jan 23, 2012, at 5:11 PM, Zino Benaissa wrote:<o:p></o:p></p></div><p class=MsoNormal><br><br><o:p></o:p></p><div><div><p class=MsoNormal><b><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'>Description:</span></b><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'><o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'>This contribution extends LLVM greedy Register Allocator to optimize for code size when LLVM compiler targets ARM Thumb 2 instruction set. This heuristic favors<span class=apple-converted-space><b><i><span style='color:#1F497D'> </span></i></b></span>assigning register R0 through R7 to operands used in instruction that can be encoded in 16 bits (16-bit is allowed only if R0-7 are used). Operands that appear most frequently in a function (and in instructions that qualify) get R0-7 register.<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'>This heuristic is turned on by default and has impact on generated code only if -mthumb compiler switch is used. To turn this heuristic off use -disable-favor-r0-7 feature flag.<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'> <o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif"'>This patch modifies:<span class=apple-converted-space> </span><br>1) The LLVM greedy register allocator located in LLVM/CodeGen directory: To add the new code size heuristic.<br>2) The ARM-specific flies located in LLVM/Target/ARM directory: To add the function that determines<span class=apple-converted-space><span style='color:#1F497D'> </span></span>which instruction can be encoded in 16-bits and a fix to enable the compiler to emit CMN instruction in 16-bits encoding.<span class=apple-converted-space> </span><br>3) The LLVM test suite: fix test/CodeGen/Thumb2/thumb2-cmn.ll test.<o:p></o:p></span></p></div></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Hi Zino,<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Thanks for working on this interesting patch.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Please submit the CMN-related changes as an independent patch.<o:p></o:p></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoListParagraph style='margin-left:0in;text-indent:0in;mso-list:l0 level1 lfo1'><![if !supportLists]><span style='font-size:11.0pt;font-family:Wingdings;color:#1F497D'><span style='mso-list:Ignore'>Ĝ<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Sure, I can do that.<o:p></o:p></span></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>If you don't mind, I would like you to run a couple of experiments to better understand why this change improves some benchmarks.<o:p></o:p></p></div><div><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoListParagraph style='margin-left:0in;text-indent:0in;mso-list:l0 level1 lfo1'><![if !supportLists]><span style='font-size:11.0pt;font-family:Wingdings;color:#1F497D'><span style='mso-list:Ignore'>Ĝ<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Sure, please let me know what you find. <o:p></o:p></span></p><p class=MsoListParagraph style='margin-left:0in'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p></div><div><p class=MsoNormal>First of all, is the regHasSizeImpact() hook necessary? Do you get significantly different results if you pretend this function always returns 2?<o:p></o:p></p></div><div><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoListParagraph style='margin-left:0in;text-indent:0in;mso-list:l0 level1 lfo1'><![if !supportLists]><span style='font-size:11.0pt;font-family:Wingdings;color:#1F497D'><span style='mso-list:Ignore'>Ĝ<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>For my experiments, precision is quite important to maximize code size gains. <o:p></o:p></span></p><p class=MsoListParagraph style='margin-left:0in'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p></div><div><p class=MsoNormal>Second, what happens if you use a 'flatter' spill weight? Instead of your patch, in LiveIntervals::getSpillWeight replace this:<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><div><p class=MsoNormal> float lc = std::pow(1 + (100.0 / (loopDepth + 10)), (double)loopDepth);<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>with this:<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div></div><div><div><p class=MsoNormal> float lc = std::pow(1 + (100.0 / (loopDepth + 10))/5, (double)loopDepth);<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>And in CalcSpillWeights.h, replace the number 25 in normalizeSpillWeight() with 250. Does that give you similar results?<o:p></o:p></p></div><div><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoListParagraph style='margin-left:0in;text-indent:0in;mso-list:l0 level1 lfo1'><![if !supportLists]><span style='font-size:11.0pt;font-family:Wingdings;color:#1F497D'><span style='mso-list:Ignore'>Ĝ<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>I did not try this experiment. However by doing so, aren’t you increasing chances for spills to be inserted in hot blocks (like inner loops)? <o:p></o:p></span></p><p class=MsoListParagraph style='margin-left:0in'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p></div><div><p class=MsoNormal>Thanks,<o:p></o:p></p></div><div><p class=MsoNormal>/jakob<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div></div></div></div></body></html>