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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:"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'><o:p> </o:p></span></p><div><div><p class=MsoNormal>On Oct 26, 2012, at 10:08 PM, Hal Finkel <<a href="mailto:hfinkel@anl.gov">hfinkel@anl.gov</a>> wrote:<o:p></o:p></p></div><p class=MsoNormal><br><br><o:p></o:p></p><p class=MsoNormal><span style='font-size:13.5pt;font-family:"Helvetica","sans-serif"'>To be fair, I think we expected this. I see two general solutions:<br>1. Cover all such cases with target hooks</span><o:p></o:p></p><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>This is possible. <o:p></o:p></p></div><p class=MsoNormal><br><br><o:p></o:p></p><p class=MsoNormal><span style='font-size:13.5pt;font-family:"Helvetica","sans-serif"'>2. When the cost of some operation is requested, actually compile it (make a small selection DAG and run instruction selection), and then use the result (we could just count the number of machine instructions, for example). We'd need to cache the results for this to be practical.</span><o:p></o:p></p><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>I don't think that this is a good approach. The compile time for this kind of mechanism would be very high. <o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>We can build an off-line tool that would compile IR to assembly. We can use this (off-line) tool to generate cost-tables. <o:p></o:p></p></div><div><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>I've been thinking of looking at something along those lines (obviously partly ARM specific, but large bits of the code would be target independent). One of the other issues is that, from experience on x86, using the TSC to time pieces of code you really need to be running a noticeable number of "iterations" not just because of the TSC not serialising, but because you're really trying to estimate the cost of an instruction in the context of lots of executions (eg, it might be stored in DecoderQueue for tight loops and not need decoding ,etc). All this suggests it's ill-suited to be running at run-time. (Note this is for estimating the cost of individual instructions: trying differently machine-code variants on your entire function kernel at run-time might be significant enough that the cost is worth it.) The obvious issue is that IR might be compiled differently in the off-line system (particularly if it doesn't have surrounding instructions to affect decisions) compared to when it's compiled in-situ. Not sure there is a good solution for that.<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'>Dave<o:p></o:p></span></p></div></div></div></body></html>