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<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 generate code containing an ordered float
compare for equality, and select. The resulting code however has an unordered
compare and some Boolean logic that I think could be eliminated. In C syntax the
code looks like this:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal style='text-indent:36.0pt;text-autospace:none'><span
style='font-size:10.0pt;font-family:"Courier New"'>float x, y;<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt;text-autospace:none'><span
style='font-size:10.0pt;font-family:"Courier New"'>int a, b, c<o:p></o:p></span></p>
<p class=MsoNormal style='text-autospace:none'><span style='font-size:10.0pt;
font-family:"Courier New"'><o:p> </o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt;text-autospace:none'><span
style='font-size:10.0pt;font-family:"Courier New"'>if(x == y) //
Rotate the integers<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt;text-autospace:none'><span
style='font-size:10.0pt;font-family:"Courier New"'>{<o:p></o:p></span></p>
<p class=MsoNormal style='text-autospace:none'><span style='font-size:10.0pt;
font-family:"Courier New"'> int
t;<o:p></o:p></span></p>
<p class=MsoNormal style='text-autospace:none'><span style='font-size:10.0pt;
font-family:"Courier New"'><o:p> </o:p></span></p>
<p class=MsoNormal style='text-autospace:none'><span style='font-size:10.0pt;
font-family:"Courier New"'> t
= a;<o:p></o:p></span></p>
<p class=MsoNormal style='text-autospace:none'><span style='font-size:10.0pt;
font-family:"Courier New"'> a
= b;<o:p></o:p></span></p>
<p class=MsoNormal style='text-autospace:none'><span style='font-size:10.0pt;
font-family:"Courier New"'> b
= c;<o:p></o:p></span></p>
<p class=MsoNormal style='text-autospace:none'><span style='font-size:10.0pt;
font-family:"Courier New"'> c
= t;<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt'><span style='font-size:10.0pt;
font-family:"Courier New"'>}</span><o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>This is the resulting x86 assembly code:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal style='text-indent:36.0pt;text-autospace:none'><span
style='font-size:8.0pt;font-family:"Courier New";color:gray'>movss
xmm0,dword ptr [ecx+4] <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt;text-autospace:none'><span
lang=NL-BE style='font-size:8.0pt;font-family:"Courier New";color:gray'>ucomiss
xmm0,dword ptr [ecx+8] <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt;text-autospace:none'><span
lang=NL-BE style='font-size:8.0pt;font-family:"Courier New";color:gray'>sete
al <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt;text-autospace:none'><span
style='font-size:8.0pt;font-family:"Courier New";color:gray'>setnp
dl <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt;text-autospace:none'><span
style='font-size:8.0pt;font-family:"Courier New";color:gray'>test
dl,al <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt;text-autospace:none'><span
style='font-size:8.0pt;font-family:"Courier New";color:gray'>mov
edx,edi <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt;text-autospace:none'><span
style='font-size:8.0pt;font-family:"Courier New";color:gray'>cmovne
edx,ecx <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt;text-autospace:none'><span
style='font-size:8.0pt;font-family:"Courier New";color:gray'>cmovne
ecx,esi <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt'><span style='font-size:8.0pt;
font-family:"Courier New";color:gray'>cmovne
esi,edi</span><o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>While I’m pleasantly surprised that my branch does get
turned into several select operations as intended (cmov - conditional move –
in x86), I’m confused why it uses the ucomiss instruction (unordered
compare and set flags). I only used IRBuilder::CreateFCmpOEQ. It also appears
to invert the conditional, for no clear reason. I think it could be rewritten
as follows:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal style='text-indent:36.0pt;text-autospace:none'><span
style='font-size:8.0pt;font-family:"Courier New";color:gray'>movss
xmm0,dword ptr [ecx+4] <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt;text-autospace:none'><span
style='font-size:8.0pt;font-family:"Courier New";color:gray'>comiss
xmm0,dword ptr [ecx+8] <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt;text-autospace:none'><span
style='font-size:8.0pt;font-family:"Courier New";color:gray'>mov
edx,edi <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt;text-autospace:none'><span
style='font-size:8.0pt;font-family:"Courier New";color:gray'>cmove
edx,ecx <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt;text-autospace:none'><span
style='font-size:8.0pt;font-family:"Courier New";color:gray'>cmove
ecx,esi <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt'><span style='font-size:8.0pt;
font-family:"Courier New";color:gray'>cmove esi,edi</span><o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Compared to the original C syntax code this looks pretty
straightforward. Curiously, when I replace the compare-for-equality with
something like a less-than, it does generate such compact code (using <span
style='font-size:8.0pt;font-family:"Courier New";color:gray'>comiss</span> and <span
style='font-size:8.0pt;font-family:"Courier New";color:gray'>cmova</span>). And
the not-equal case looks like this:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal style='text-indent:36.0pt;text-autospace:none'><span
style='font-size:8.0pt;font-family:"Courier New";color:gray'>movss
xmm0,dword ptr [ecx+4] <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt;text-autospace:none'><span
style='font-size:8.0pt;font-family:"Courier New";color:gray'>ucomiss
xmm0,dword ptr [ecx+8] <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt;text-autospace:none'><span
style='font-size:8.0pt;font-family:"Courier New";color:gray'>mov
esi,ecx <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt;text-autospace:none'><span
style='font-size:8.0pt;font-family:"Courier New";color:gray'>cmove
esi,edx <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt;text-autospace:none'><span
style='font-size:8.0pt;font-family:"Courier New";color:gray'>cmovne
ecx,eax <o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt'><span style='font-size:8.0pt;
font-family:"Courier New";color:gray'>cmove
edx,eax</span><o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>So this generates compact code but with an unordered compare.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Anyway, it looks like the compare-for-equality case in
particular is missing an optimization opportunity. It’s no big deal to me
but I thought someone here might be interested…<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Cheers,<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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