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<p class="MsoNormal"><span lang="EN-US">Hello,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">When llvm does constant folding on a math library function call with constant argument, it seems
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">the double version of library function is called no matter the original function is single or double
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">precision. Code can be found in lib/Analysis/ConstantFolding.cpp in various functions like ConstantFoldScalarCall1().
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">There is a comment that seems explaining why this choice was made:<o:p></o:p></span></p>
<p class="MsoNormal" align="left" style="text-align:left;text-autospace:none"><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:teal;background:white;mso-highlight:white"><o:p> </o:p></span></p>
<p class="MsoNormal" align="left" style="text-align:left;text-autospace:none"><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:teal;background:white;mso-highlight:white">/// Currently APFloat versions of these functions do not exist,
so we use<o:p></o:p></span></p>
<p class="MsoNormal" align="left" style="text-align:left;text-autospace:none"><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:teal;background:white;mso-highlight:white">/// the host native double versions. Float versions are not
called<o:p></o:p></span></p>
<p class="MsoNormal" align="left" style="text-align:left;text-autospace:none"><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:teal;background:white;mso-highlight:white">/// directly but for all these it is true (float)(f((double)arg))
==<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:teal;background:white;mso-highlight:white">/// f(arg). Long double not supported yet.</span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">However we do have a float version exposed by math.h? And the assumption that converting parameter
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">to double and then converting result back to float gives the same result is not always true. For example<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">the following code has different result under –O3 (ConstantFoldFP calls log10 at compile time) and –O0 –lm:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" align="left" style="text-align:left;text-autospace:none"><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:#804000;background:white;mso-highlight:white">#include <math.h><o:p></o:p></span></p>
<p class="MsoNormal" align="left" style="text-align:left;text-autospace:none"><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:#804000;background:white;mso-highlight:white">#include <stdio.h><o:p></o:p></span></p>
<p class="MsoNormal" align="left" style="text-align:left;text-autospace:none"><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:black;background:white;mso-highlight:white"><o:p> </o:p></span></p>
<p class="MsoNormal" align="left" style="text-align:left;text-autospace:none"><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:#8000FF;background:white;mso-highlight:white">int</span><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:black;background:white;mso-highlight:white">
main</span><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:navy;background:white;mso-highlight:white">()</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:black;background:white;mso-highlight:white">
</span><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:navy;background:white;mso-highlight:white">{</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:black;background:white;mso-highlight:white"><o:p></o:p></span></p>
<p class="MsoNormal" align="left" style="text-align:left;text-autospace:none"><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:black;background:white;mso-highlight:white"> printf</span><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:navy;background:white;mso-highlight:white">(</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:gray;background:white;mso-highlight:white">"%.7f\n"</span><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:navy;background:white;mso-highlight:white">,</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:black;background:white;mso-highlight:white">
log10f</span><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:navy;background:white;mso-highlight:white">(</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:#FF8000;background:white;mso-highlight:white">0.7177198578332612</span><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:navy;background:white;mso-highlight:white">));</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:black;background:white;mso-highlight:white"><o:p></o:p></span></p>
<p class="MsoNormal" align="left" style="text-align:left;text-autospace:none"><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:black;background:white;mso-highlight:white">
</span><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:blue;background:white;mso-highlight:white">return</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:black;background:white;mso-highlight:white">
</span><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:#FF8000;background:white;mso-highlight:white">0</span><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:navy;background:white;mso-highlight:white">;</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:black;background:white;mso-highlight:white"><o:p></o:p></span></p>
<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:navy;background:white;mso-highlight:white">}</span></b><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:navy"><o:p></o:p></span></b></p>
<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Courier New";color:navy"><o:p> </o:p></span></b></p>
<p class="MsoNormal"><span lang="EN-US">Could someone explain to me the current implementation choices of folding library function calls?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Or if let me know if I misunderstood anything. Any help is appreciated.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Thanks,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Duanyang<o:p></o:p></span></p>
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