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<p class=MsoNormal><span style='color:#1F497D'>Hi all,<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'>Marc pointed out to me that this
might be related to my question about floating-point equality, however I still
think there’s a valid optimization opportunity here.<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'>The Intel documents specify that
the maxss instruction returns the second operand if either operand is a NaN. I
believe this is exactly the intended behavior of (x > y) ? x : y where >
is an ordered compare. If either x or y is NaN the > returns false so y is returned,
the second argument.<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'>I wrote a little test app where
I compared the results of (x > y) ? x : y with some inline assembly using
maxss for all combinations of 0.0, 1.0 and NaN as inputs for x and y, and they
were identical.<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'>Cheers,<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'>Nicolas<o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p>
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<div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0cm 0cm 0cm'>
<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"'>
llvmdev-bounces@cs.uiuc.edu [mailto:llvmdev-bounces@cs.uiuc.edu] <b>On Behalf
Of </b>Nicolas Capens<br>
<b>Sent:</b> Tuesday, 27 May, 2008 11:50<br>
<b>To:</b> 'LLVM Developers Mailing List'<br>
<b>Subject:</b> [LLVMdev] Min and max<o:p></o:p></span></p>
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<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Hi all,<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>I’m trying to implement a floating-point
‘min’ and ‘max’ operation using select. For
‘min’ I get the expected x86 assembly minss instruction, but for
‘max’ I get a branch instead of maxss.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>The corresponding C syntax code looks like this:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>float z = (x > y) ? x : y;<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Any clues?<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Could someone maybe explain to me the basics of LLVM’s
target specific optimizations and code generation? I’d love to analyze
things like this myself but I don’t know where to start.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Thanks,<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Nicolas Capens<o:p></o:p></p>
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