<html xmlns:v="urn:schemas-microsoft-com:vml" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" xmlns="http://www.w3.org/TR/REC-html40"><head><meta http-equiv=Content-Type content="text/html; charset=us-ascii"><meta name=Generator content="Microsoft Word 15 (filtered medium)"><style><!--
/* Font Definitions */
@font-face
{font-family:"Cambria Math";
panose-1:2 4 5 3 5 4 6 3 2 4;}
@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}
@font-face
{font-family:Consolas;
panose-1:2 11 6 9 2 2 4 3 2 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0in;
margin-bottom:.0001pt;
font-size:11.0pt;
font-family:"Calibri",sans-serif;}
a:link, span.MsoHyperlink
{mso-style-priority:99;
color:#0563C1;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{mso-style-priority:99;
color:#954F72;
text-decoration:underline;}
span.EmailStyle17
{mso-style-type:personal-compose;
font-family:"Calibri",sans-serif;
color:windowtext;}
.MsoChpDefault
{mso-style-type:export-only;
font-family:"Calibri",sans-serif;}
@page WordSection1
{size:8.5in 11.0in;
margin:1.0in 1.0in 1.0in 1.0in;}
div.WordSection1
{page:WordSection1;}
--></style><!--[if gte mso 9]><xml>
<o:shapedefaults v:ext="edit" spidmax="1026" />
</xml><![endif]--><!--[if gte mso 9]><xml>
<o:shapelayout v:ext="edit">
<o:idmap v:ext="edit" data="1" />
</o:shapelayout></xml><![endif]--></head><body lang=EN-US link="#0563C1" vlink="#954F72"><div class=WordSection1><p class=MsoNormal>Hello all!<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>I’ve been mucking around in an old codebase at work looking for easy performance wins. One avenue involves replacing a switch-based variable assignment with something derived from the parity of an input variable. I was pretty surprised when I saw the generated assembly, and I’m wondering about the reasoning behind it.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>In short, it boils down to the assembly <span style='font-family:Consolas'>__builtin_parity()</span> produces. Clang 6.0.1 (and trunk on Godbolt) produces:<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal><span style='font-family:Consolas'>parity(int): # @parity(int)<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'> mov eax, edi<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'> shr eax<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'> and eax, 1431655765<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'> sub edi, eax<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'> mov eax, edi<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'> and eax, 858993459<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'> shr edi, 2<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'> and edi, 858993459<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'> add edi, eax<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'> mov eax, edi<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'> shr eax, 4<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'> add eax, edi<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'> and eax, 17764111<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'> imul eax, eax, 16843009<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'> shr eax, 24<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'> and eax, 1<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'> ret<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'><o:p> </o:p></span></p><p class=MsoNormal>While GCC 8.1.0 (and trunk on Godbolt) produces<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal><span style='font-family:Consolas'>parity(int):<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'> mov eax, edi<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'> shr edi, 16<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'> xor eax, edi<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'> xor al, ah<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'> setnp al<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'> movzx eax, al<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'> ret<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'><o:p> </o:p></span></p><p class=MsoNormal>I know a popcnt followed by an and would be better, but unfortunately some of my users don’t have computers that support the popcnt instruction, so I can’t use a newer -march flag.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Could someone explain why the difference between Clang and GCC here, and whether it should make a difference? The code in question is in a hot loop in my code, so I’d imagine the size difference could impact unrolling (and result in icache differences too), but I haven’t finished poking around with benchmarks.<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>Alex <o:p></o:p></p></div></body></html>