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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Hello!<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span lang="EN-US">In my humble opinion that is a genuine false positive.<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">Unless we can see that printInt modifies zero, we should not write a warning. I would assume that external functions are “well behaved”.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><![if !supportLists]><span lang="EN-US" style="font-family:Wingdings"><span style="mso-list:Ignore">Ø<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span lang="EN-US">Why? Because even though it is static and printInt should not be able to modify it, printInt might be able to call into another function which can modify the value of zero.<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">Then the translation unit would have to for instance have a globally accessible function that modifies zero.<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">As I see it.. we know that zero is not modified neither directly/indirectly by printInt if the provided code is the entire TU.<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">Best regards,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Daniel Marjamäki<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"><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"><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">On 19 October 2015 at 13:13, Bhargava Shastry via cfe-dev <</span><a href="mailto:cfe-dev@lists.llvm.org" target="_blank"><span lang="EN-US">cfe-dev@lists.llvm.org</span></a><span lang="EN-US">> wrote:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Hi All,<br>
<br>
I came across a curious pattern of FPs while evaluating Clang SA:<br>
<br>
$ cat garbageread-fp.c<br>
1. extern void printInt(int x); // prints int<br>
2.<br>
3. static int zero = 0;<br>
4.<br>
5. void f() {<br>
6. int x;<br>
7. if (zero != 0)<br>
8. printInt(0); // dead code<br>
9. else<br>
10. x = 0;<br>
11. if (zero == 0) {<br>
12. if (!x)<br>
13. printInt(x);<br>
14. }<br>
15. }<br>
<br>
$ clang --analyze garbageread-fp.c<br>
garbageread-fp.c:12:8: warning: Branch condition evaluates to a garbage<br>
value<br>
if (!x)<br>
^~<br>
1 warning generated.<br>
<br>
I have uploaded an html report here: </span><a href="http://output.jsbin.com/jiqewevihi" target="_blank"><span lang="EN-US">http://output.jsbin.com/jiqewevihi</span></a><span lang="EN-US"><br>
<br>
Why does Clang SA forget the constraint down the execution path?<br>
<br>
Notes:<br>
1. Replacing static with extern retains the FP.<br>
2. The FP has something to do with function inlining. Replacing printInt<br>
with standard library function makes the FP go away!<br>
<br>
</span>Regards,<br>
Bhargava<br>
<br>
<span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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