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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='color:#1F497D'>Ping. Anyone?<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><div style='border:none;border-left:solid blue 1.5pt;padding:0in 0in 0in 4.0pt'><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"'> llvm-commits-bounces@cs.uiuc.edu [mailto:llvm-commits-bounces@cs.uiuc.edu] <b>On Behalf Of </b>Villmow, Micah<br><b>Sent:</b> Tuesday, November 23, 2010 1:39 PM<br><b>To:</b> llvm-commits<br><b>Subject:</b> [llvm-commits] Patch for Bug7696<o:p></o:p></span></p></div></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Reposting bug report here, any chance that it can get reviewed?<o:p></o:p></p><p class=MsoNormal>Patch that adds a boolean where a backend can specify whether to disable<o:p></o:p></p><p class=MsoNormal>certain optimizations because jumps are expensive.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>The selection dag builder has an ‘optimization’ added into the visitBr function<o:p></o:p></p><p class=MsoNormal>which makes assumptions that are not valid on all architectures. The problem is<o:p></o:p></p><p class=MsoNormal>this.<o:p></o:p></p><p class=MsoNormal>The following function<o:p></o:p></p><p class=MsoNormal>kernel void cf_test(global int* a, int b, int c, int e)<o:p></o:p></p><p class=MsoNormal>{<o:p></o:p></p><p class=MsoNormal> int d = 0;<o:p></o:p></p><p class=MsoNormal> if (!b && c < e) {<o:p></o:p></p><p class=MsoNormal> d = a + b;<o:p></o:p></p><p class=MsoNormal> }<o:p></o:p></p><p class=MsoNormal> *a = d;<o:p></o:p></p><p class=MsoNormal>}<o:p></o:p></p><p class=MsoNormal>Is transformed into something equivalent to this:<o:p></o:p></p><p class=MsoNormal>Kernel void cf_test(global int* a, int b, int c, int e)<o:p></o:p></p><p class=MsoNormal>{<o:p></o:p></p><p class=MsoNormal> Int d;<o:p></o:p></p><p class=MsoNormal> If (b) {<o:p></o:p></p><p class=MsoNormal> d = 0;<o:p></o:p></p><p class=MsoNormal> } else {<o:p></o:p></p><p class=MsoNormal> if (c < e) {<o:p></o:p></p><p class=MsoNormal> d = a + b;<o:p></o:p></p><p class=MsoNormal> } else {<o:p></o:p></p><p class=MsoNormal> d = 0;<o:p></o:p></p><p class=MsoNormal> }<o:p></o:p></p><p class=MsoNormal> }<o:p></o:p></p><p class=MsoNormal> *a = d;<o:p></o:p></p><p class=MsoNormal>}<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>by the visitBr code found in SelectionDAGBuilder::visitBr():1188.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>However, if jumps are expensive or jumps are not supported and high level flow<o:p></o:p></p><p class=MsoNormal>control needs to be reconstructed. This is extremely inefficient. For example<o:p></o:p></p><p class=MsoNormal>on AMD GPU’s, a single flow control instruction can take 40 cycles to execute,<o:p></o:p></p><p class=MsoNormal>but an bit instruction, can be executed every cycle. So obviously the<o:p></o:p></p><p class=MsoNormal>assumptions made by this block of code are inefficient on AMD hardware. <o:p></o:p></p><p class=MsoNormal>Increasing control flow has a direct impact on performance and removing the<o:p></o:p></p><p class=MsoNormal>extra ‘and’ or ‘or’ in order to short circuit the conditional evaluation does<o:p></o:p></p><p class=MsoNormal>not work for our target.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>So in order to make this type of instruction rely more on target specific<o:p></o:p></p><p class=MsoNormal>information. I’ve added a new Boolean to the TargetLoweringInfo class called<o:p></o:p></p><p class=MsoNormal>JumpIsExpensive along with accessor functions.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Please review the patch and apply if acceptable.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p></div></div></body></html>