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<p class=MsoNormal>The selection dag builder has an ‘optimization’
added into the visitBr function which makes assumptions that are not valid on
all architectures. The problem is this.<o:p></o:p></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:black'>The following function<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:black'>kernel void cf_test(global int* a, int b, int c, int e)<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:black'>{<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:black'> int d = 0;<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:black'> if (!b && c < e) {<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:black'> d = a + b;<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:black'> }<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:black'> *a = d;<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New";
color:black'>}<o:p></o:p></span></p>
<p class=MsoNormal>Is transformed into something equivalent to this:<o:p></o:p></p>
<p class=MsoNormal>Kernel void cf_test(global int* a, int b, int c, int e)<o:p></o:p></p>
<p class=MsoNormal>{<o:p></o:p></p>
<p class=MsoNormal> Int d;<o:p></o:p></p>
<p class=MsoNormal> If (b) {<o:p></o:p></p>
<p class=MsoNormal> d = 0;<o:p></o:p></p>
<p class=MsoNormal> } else {<o:p></o:p></p>
<p class=MsoNormal> if (c < e) {<o:p></o:p></p>
<p class=MsoNormal> d = a + b;<o:p></o:p></p>
<p class=MsoNormal> } else {<o:p></o:p></p>
<p class=MsoNormal> d = 0;<o:p></o:p></p>
<p class=MsoNormal> }<o:p></o:p></p>
<p class=MsoNormal> }<o:p></o:p></p>
<p class=MsoNormal> *a = d;<o:p></o:p></p>
<p class=MsoNormal>}<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>by the visitBr code found in SelectionDAGBuilder::visitBr():1188.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>However, if jumps are expensive or jumps are not supported
and high level flow control needs to be reconstructed. This is extremely
inefficient. For example on AMD GPU’s, a single flow control instruction
can take 40 cycles to execute, but an bit instruction, can be executed every
cycle. So obviously the assumptions made by this block of code are inefficient
on AMD hardware. Increasing control flow has a direct impact on performance
and removing the extra ‘and’ or ‘or’ in order to short
circuit the conditional evaluation does not work for our target.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>So in order to make this type of instruction rely more on
target specific information. I’ve added a new Boolean to the
TargetLoweringInfo class called JumpIsExpensive along with accessor functions.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Please review the patch and apply if acceptable.<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>Micah<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
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