<div dir="ltr"><div>The language requires both sides of the | need to be evaluated. The compiler can't remove the function call if it has side effects.</div><div><br></div><br clear="all"><div><div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature">~Craig</div></div><br></div><br><div class="gmail_quote"><div dir="ltr">On Sun, Aug 19, 2018 at 10:15 PM Shoaib Meenai via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<p class="MsoNormal">This is better suited to llvm-dev, so I'm directing it there.<u></u><u></u></p>
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<p class="MsoNormal" style="margin-left:.5in"><b><span style="font-size:12.0pt;color:black">From:
</span></b><span style="font-size:12.0pt;color:black">llvm-commits <<a href="mailto:llvm-commits-bounces@lists.llvm.org" target="_blank">llvm-commits-bounces@lists.llvm.org</a>> on behalf of Qing Shan Zhang via llvm-commits <<a href="mailto:llvm-commits@lists.llvm.org" target="_blank">llvm-commits@lists.llvm.org</a>><br>
<b>Reply-To: </b>Qing Shan Zhang <<a href="mailto:qshanz@cn.ibm.com" target="_blank">qshanz@cn.ibm.com</a>><br>
<b>Date: </b>Sunday, August 19, 2018 at 10:10 PM<br>
<b>To: </b>"<a href="mailto:llvm-commits@lists.llvm.org" target="_blank">llvm-commits@lists.llvm.org</a>" <<a href="mailto:llvm-commits@lists.llvm.org" target="_blank">llvm-commits@lists.llvm.org</a>><br>
<b>Subject: </b>A question about the widely used "xxx | true" inside llvm<u></u><u></u></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",serif">Hi, guys,<u></u><u></u></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",serif">I notice that, we are widely using the statement "xxx | true" in llvm, such as "return SimplifyCFG(BB) | true;". I have no idea if there is any downside
to use the logic operator "||" here. Instead, I indeed see the potential problem to use bitwise operator "|". Because the bitwise operator do not short-circuit and the language std didn't specify the evaluation order for its lhs and rhs expression. That means,
compiler is free to optimize this statement to "return true". <u></u><u></u></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",serif">In shorts, IMO, it is legal to do the following optimization:<br>
bool bar() {<br>
return foo() | true;<br>
}<br>
--><br>
bool bar() {<br>
return true; // the call to foo() is missing.<u></u><u></u></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",serif">}<u></u><u></u></span></p>
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<p class="MsoNormal" style="margin-left:.5in"><span style="font-size:10.0pt;font-family:"Verdana",serif"> <br>
Best regards<br>
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