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<p class="MsoNormal">Using the following example program<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">#include <math.h><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">double f(double d){<o:p></o:p></p>
<p class="MsoNormal"> return sqrt(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">and compiling it with “clang –O3 …”, I was trying to determine what it would take to get the X86 code generator to replace the call to sqrt with a sqrtsd instruction inline.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">It turns out that it could do exactly that, were it not for the fact that in the function visitUnaryFloatCall() at line 5514 in SelectionDAGBuilder.cpp, the result of
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">!I.onlyReadsMemory()<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Is true, so the code is unable to replace the function call with an ISD::FSQRT SDNode. If I remove the above test, then the compiler will emit a sqrtsd instruction.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I am hoping that someone might be able to comment on what onlyReadsMemory is supposed to do be doing in general and why it is returning false in this case.<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">Preston<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">--<o:p></o:p></p>
<p class="MsoNormal">Preston Gurd <preston.gurd@intel.com><o:p></o:p></p>
<p class="MsoNormal"> Intel Waterloo<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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