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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">https://bugs.llvm.org/show_bug.cgi?id=37113<o:p></o:p></span></p>
<p class="MsoNormal"><a name="_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"><o:p> </o:p></span></a></p>
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<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> Riyaz Puthiyapurayil [mailto:riyaz@synopsys.com]
<br>
<b>Sent:</b> Thursday, April 12, 2018 12:21 PM<br>
<b>To:</b> Matthew Del Buono <mpdelbuono@gmail.com>; Maurizio Vitale <mav@uber.com><br>
<b>Cc:</b> Riyaz Puthiyapurayil <Riyaz.Puthiyapurayil@synopsys.com>; cfe-dev@lists.llvm.org<br>
<b>Subject:</b> RE: [cfe-dev] Clang tautological-compare fails to warn in cases where gcc doesn't<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoPlainText">I agree. Clang should warn in this case. If 'const' is changed to 'volatile', then gcc doesn't warn either (as expected).<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">I will file a bug.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">/Riyaz<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> Matthew Del Buono [<a href="mailto:mpdelbuono@gmail.com">mailto:mpdelbuono@gmail.com</a>]
<br>
<b>Sent:</b> Thursday, April 12, 2018 9:29 AM<br>
<b>To:</b> Maurizio Vitale <<a href="mailto:mav@uber.com">mav@uber.com</a>><br>
<b>Cc:</b> Riyaz Puthiyapurayil <<a href="mailto:Riyaz.Puthiyapurayil@synopsys.com">Riyaz.Puthiyapurayil@synopsys.com</a>>;
<a href="mailto:cfe-dev@lists.llvm.org">cfe-dev@lists.llvm.org</a><br>
<b>Subject:</b> Re: [cfe-dev] Clang tautological-compare fails to warn in cases where gcc doesn't<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<p class="MsoNormal">It's not volatile, so it seems inappropriate for clang to think that it could be modified in between. In fact, at -O1, the comparisons are optimized out anyway, so it's clear that the warning should be in place to tell developers that this
comparison is not going to be performed:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="background:#FFFFFE"><span style="color:teal">cmp(Foo const*, Foo const*):</span><span style="color:black">
</span><span style="color:green"># @cmp(Foo const*, Foo const*)</span><span style="color:black"><o:p></o:p></span></p>
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<div>
<p class="MsoNormal" style="background:#FFFFFE"><span style="color:blue">xor</span><span style="color:black">
</span><span style="color:#4864AA">eax</span><span style="color:black">, </span><span style="color:#4864AA">eax</span><span style="color:black"><o:p></o:p></span></p>
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<p class="MsoNormal" style="background:#FFFFFE"><span style="color:blue">ret</span><span style="color:black"><o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">(clang 6.0.0)<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Thu, Apr 12, 2018 at 8:35 AM, Maurizio Vitale via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-top:5.0pt;margin-right:0in;margin-bottom:5.0pt">
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<p class="MsoNormal">I suspect that clang cannot satisfy itself that the x->a cannot be changed by somebody else between the evaluation of the lhs and the rhs of the comparison.<o:p></o:p></p>
<div>
<p class="MsoNormal">All it knows is that the function cmp cannot modify it.<br>
I'd say clang is correct, but I haven't studied the problem much.<o:p></o:p></p>
</div>
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<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">Maurizio<o:p></o:p></p>
</div>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<div>
<div>
<p class="MsoNormal">On Thu, Apr 12, 2018 at 10:39 AM, Riyaz Puthiyapurayil via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p>
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<div>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Here is a simple test where clang fails to issue a warning in the first function but warns in the second function. gcc (6.x and above) gives a warning in both cases. Is the pointer
dereference throwing clang off? Why should the pointer dereference matter in this case?
<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<div style="border:solid #F2F2F2 1.0pt;padding:1.0pt 4.0pt 1.0pt 4.0pt">
<p class="m-2865762710056028528m3575411032359691794code">struct Foo {<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> int a;<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code">};<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> <o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code">// Case 1<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code">int cmp(const Foo* x, const Foo* y);<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code">int cmp(const Foo* x, const Foo* y)<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code">{<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> if (x->a < x->a) {<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> return -1;<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> } else if (y->a > y->a) {<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> return 1;<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> }<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> return 0;<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code">}<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> <o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code">// Case 2<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code">int cmp2(int x, int y);<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code">int cmp2(int x, int y)<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code">{<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> if (x < x) {<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> return -1;<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> } else if (y > y) {<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> return 1;<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> }<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> return 0;<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code">}<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> <o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code">// gcc 6.2 with -Wall<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code">x.cc: In function ?int cmp(const Foo*, const Foo*)?:<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code">x.cc:7:14: warning: self-comparison always evaluates to false [-Wtautological-compare]<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> if (x->a < x->a) {<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> ~~~~~^~~~~~<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code">x.cc:9:21: warning: self-comparison always evaluates to false [-Wtautological-compare]<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> } else if (y->a > y->a) {<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> ~~~~~^~~~~~<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code">x.cc: In function ?int cmp2(int, int)?:<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code">x.cc:17:11: warning: self-comparison always evaluates to false [-Wtautological-compare]<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> if (x < x) {<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> ~~^~~<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code">x.cc:19:18: warning: self-comparison always evaluates to false [-Wtautological-compare]<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> } else if (y > y) {<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> ~~^~~<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> <o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code">// clang 6.0.1 -Wall<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code">x.cc:17:11: warning: self-comparison always evaluates to false [-Wtautological-compare]<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> if (x < x) {<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> ^<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code">x.cc:19:18: warning: self-comparison always evaluates to false [-Wtautological-compare]<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> } else if (y > y) {<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code"> ^<o:p></o:p></p>
<p class="m-2865762710056028528m3575411032359691794code">2 warnings generated.<o:p></o:p></p>
</div>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
</div>
<p class="MsoNormal" style="margin-bottom:12.0pt">_______________________________________________<br>
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<p class="MsoNormal"><o:p> </o:p></p>
</div>
<p class="MsoNormal" style="margin-bottom:12.0pt"><br>
_______________________________________________<br>
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