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<p class=MsoNormal>I have a very simple kernel that is generating very very bad
code.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>The basic kernel pseudo-code is as follows:<o:p></o:p></p>
<p class=MsoNormal>forloop(1 to n) {<o:p></o:p></p>
<p class=MsoNormal> forloop(0 to j) {<o:p></o:p></p>
<p class=MsoNormal> A<o:p></o:p></p>
<p class=MsoNormal>}<o:p></o:p></p>
<p class=MsoNormal>B<o:p></o:p></p>
<p class=MsoNormal>}<o:p></o:p></p>
<p class=MsoNormal>C<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>It is generating very ugly and inefficient code for a vector
system similar to the following pseudo-code:<o:p></o:p></p>
<p class=MsoNormal>if (n > 1) {<o:p></o:p></p>
<p class=MsoNormal> if (j) {<o:p></o:p></p>
<p class=MsoNormal> forloop(1
to n) {<o:p></o:p></p>
<p class=MsoNormal> forloop(0
to j) {<o:p></o:p></p>
<p class=MsoNormal> A<o:p></o:p></p>
<p class=MsoNormal> }<o:p></o:p></p>
<p class=MsoNormal> B<o:p></o:p></p>
<p class=MsoNormal> }<o:p></o:p></p>
<p class=MsoNormal> C<o:p></o:p></p>
<p class=MsoNormal> } else {<o:p></o:p></p>
<p class=MsoNormal> forloop(1
to n) {<o:p></o:p></p>
<p class=MsoNormal> B<o:p></o:p></p>
<p class=MsoNormal> }<o:p></o:p></p>
<p class=MsoNormal> C<o:p></o:p></p>
<p class=MsoNormal> } <o:p></o:p></p>
<p class=MsoNormal>} else {<o:p></o:p></p>
<p class=MsoNormal>C<o:p></o:p></p>
<p class=MsoNormal>}<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>I can understand how this would be good in a scalar system
like x86, but this is just bad on a vector system.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>The reason this is bad because if a single branch is taken
by a work-item in a hardware thread(there are 64 work-items per hw thread),
then every single work-item in a hardware thread must execute that branch. In
this specific example, instead of every thread executing A, B and C once, in the
worst case(which is also happens 100% of the time), every thread will execute C
three times, B twice and A once. This also does not take into account the cost
of managing flow control on the hardware, which is relatively expensive. This
gets worse with the more flow control I add in.<o:p></o:p></p>
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