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<p class="MsoNormal"><span style="color:#1F497D">Hi Lijia,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">IEEE 754 standard has a strict definition on comparison predicates. In short, symbol “<=” is not the flip of “>”. There’s no such symbol in C language currently (Maybe in draft, I’m not sure), but LLVM IR does
support all predicates. So the opposition of predicates ogt (“>” is C) is ule and the opposition of predicates ole (“<=” is C) is ugt.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">That’s why you get opposite results from cmp1 and cmp2. The equivalent to cmp1 in IR is:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D;mso-fareast-language:ZH-CN">define zeroext i1 @_Z4cmp2f(float %a) {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D;mso-fareast-language:ZH-CN">entry:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D;mso-fareast-language:ZH-CN"> %cmp = fcmp ule float %a, 0.000000e+00<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D;mso-fareast-language:ZH-CN"> %not = xor i1 %cmp, -1<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D;mso-fareast-language:ZH-CN"> ret i1 %not<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D;mso-fareast-language:ZH-CN">}<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Thanks<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Pengfei<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><b>From:</b> llvm-dev <llvm-dev-bounces@lists.llvm.org> <b>On Behalf Of
</b>?? via llvm-dev<br>
<b>Sent:</b> Monday, January 4, 2021 8:39 PM<br>
<b>To:</b> llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Subject:</b> [llvm-dev] Floating point ordered and unordered comparisons<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Hi All,<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I have some questions about Floating point ordered and unordered comparisons,<o:p></o:p></p>
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<p class="MsoNormal">I did some searching and found link below<o:p></o:p></p>
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<p class="MsoNormal"><a href="http://lists.llvm.org/pipermail/llvm-dev/2013-July/063571.html">http://lists.llvm.org/pipermail/llvm-dev/2013-July/063571.html</a><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">But I still don't quite understand Floating point ordered and unordered<o:p></o:p></p>
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<p class="MsoNormal">comparisons. For example, the following code<o:p></o:p></p>
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<p class="MsoNormal">```<o:p></o:p></p>
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<p class="MsoNormal">bool cmp1(float a) {<o:p></o:p></p>
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<p class="MsoNormal"> if (a > 0.0)<o:p></o:p></p>
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<p class="MsoNormal"> return true;<o:p></o:p></p>
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<p class="MsoNormal"> else<o:p></o:p></p>
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<p class="MsoNormal"> return false;<o:p></o:p></p>
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<p class="MsoNormal">}<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">bool cmp2(float a) {<o:p></o:p></p>
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<p class="MsoNormal"> if (!(a <= 0.0))<o:p></o:p></p>
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<p class="MsoNormal"> return true;<o:p></o:p></p>
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<p class="MsoNormal"> else<o:p></o:p></p>
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<p class="MsoNormal"> return false;<o:p></o:p></p>
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<p class="MsoNormal">}<o:p></o:p></p>
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<p class="MsoNormal">```<o:p></o:p></p>
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<p class="MsoNormal">The generated ir code is similar to the following:<o:p></o:p></p>
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<div>
<p class="MsoNormal">```<o:p></o:p></p>
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<div>
<p class="MsoNormal">define zeroext i1 @_Z4cmp1f(float %a) {<o:p></o:p></p>
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<p class="MsoNormal">entry:<o:p></o:p></p>
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<p class="MsoNormal"> %cmp = fcmp ogt float %a, 0.000000e+00<o:p></o:p></p>
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<p class="MsoNormal"> ret i1 %cmp<o:p></o:p></p>
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<p class="MsoNormal">}<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">define zeroext i1 @_Z4cmp2f(float %a) {<o:p></o:p></p>
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<p class="MsoNormal">entry:<o:p></o:p></p>
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<p class="MsoNormal"> %cmp = fcmp ugt float %a, 0.000000e+00<o:p></o:p></p>
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<p class="MsoNormal"> ret i1 %cmp<o:p></o:p></p>
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<p class="MsoNormal">}<o:p></o:p></p>
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<p class="MsoNormal">```<o:p></o:p></p>
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<p class="MsoNormal">If the parameter passed is NaN in this example, the result of cmp1 is false and<o:p></o:p></p>
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<p class="MsoNormal">the result of cmp2 is true. In this case, the result is completely opposite.
<o:p></o:p></p>
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<p class="MsoNormal">When will we get 'fcmp ugt' and when will we get 'fcmp ugt' ? When comparing<o:p></o:p></p>
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<p class="MsoNormal">ordered and unorder can be converted ? Such as converting an ordered less than<o:p></o:p></p>
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<p class="MsoNormal">"setolt" into unordered greater than "setuge" operation. <o:p>
</o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<div>
<p class="MsoNormal">Thanks in advance,<o:p></o:p></p>
</div>
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<p class="MsoNormal">Lijia He<o:p></o:p></p>
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