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</o:shapelayout></xml><![endif]--></head><body lang=EN-IE link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'>Thanks John,<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'>I decided to try this with GCC v4.8.3, v6.3.0 and VC2015, and none of these compiler detect it either. However, as you expected, the static analyser does identify this perfectly:<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-family:"Courier New";color:black;mso-fareast-language:EN-US'>ANALYZE (Syntax): bug.cpp func(int *)<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-family:"Courier New";color:black;mso-fareast-language:EN-US'>ANALYZE (Path, Inline_Regular): bug.cpp func(int *)<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-family:"Courier New";color:black;mso-fareast-language:EN-US'>bug.cpp:5:13: warning: Dereference of null pointer (loaded from variable 'b')<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-family:"Courier New";color:black;mso-fareast-language:EN-US'> int c = *b;<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-family:"Courier New";color:black;mso-fareast-language:EN-US'> ^~<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-family:"Courier New";color:black;mso-fareast-language:EN-US'>1 warning generated.<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'>All the best,<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'> MartinO<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Book Antiqua",serif;color:#943634;mso-fareast-language:EN-US'><o:p> </o:p></span></p><div><div style='border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0cm 0cm 0cm'><p class=MsoNormal><b><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri",sans-serif'>From:</span></b><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri",sans-serif'> rjmccall@apple.com [mailto:rjmccall@apple.com] <br><b>Sent:</b> 08 September 2017 20:55<br><b>To:</b> Martin J. O'Riordan <MartinO@theheart.ie><br><b>Cc:</b> clang developer list <cfe-dev@lists.llvm.org><br><b>Subject:</b> Re: [cfe-dev] Undiagnosed null pointer dereference<o:p></o:p></span></p></div></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal><o:p> </o:p></p><div><blockquote style='margin-top:5.0pt;margin-bottom:5.0pt'><div><p class=MsoNormal>On Sep 8, 2017, at 12:53 PM, Martin J. O'Riordan via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org">cfe-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p></div><p class=MsoNormal><o:p> </o:p></p><div><div><p class=MsoNormal><span style='font-family:"Book Antiqua",serif'>I was looking at a bug that a customer reported to me, and have reduced the code to the following fragment:</span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div style='margin-left:36.0pt'><p class=MsoNormal><span style='font-family:"Courier New"'>void func(int *z) {</span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div style='margin-left:36.0pt'><p class=MsoNormal><span style='font-family:"Courier New"'> int *b = 0; // This is the customer bug</span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div style='margin-left:36.0pt'><p class=MsoNormal><span style='font-family:"Courier New"'> </span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div style='margin-left:36.0pt'><p class=MsoNormal><span style='font-family:"Courier New"'> for (int j = 0; j < 16; j++) {</span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div style='margin-left:36.0pt'><p class=MsoNormal><span style='font-family:"Courier New"'> int c = *b;</span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div style='margin-left:36.0pt'><p class=MsoNormal><span style='font-family:"Courier New"'> c -= *z++;</span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div style='margin-left:36.0pt'><p class=MsoNormal><span style='font-family:"Courier New"'> *b++ = c;</span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div style='margin-left:36.0pt'><p class=MsoNormal><span style='font-family:"Courier New"'> }</span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div style='margin-left:36.0pt'><p class=MsoNormal><span style='font-family:"Courier New"'>}</span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-family:"Book Antiqua",serif'>It is clearly undefined behaviour, and this produces the following IR:</span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div style='margin-left:36.0pt'><p class=MsoNormal><span style='font-family:"Courier New"'>; Function Attrs: norecurse nounwind</span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div style='margin-left:36.0pt'><p class=MsoNormal><span style='font-family:"Courier New"'>define void @func(i32* nocapture readonly) local_unnamed_addr #0 {</span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div style='margin-left:36.0pt'><p class=MsoNormal><span style='font-family:"Courier New"'> unreachable</span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div style='margin-left:36.0pt'><p class=MsoNormal><span style='font-family:"Courier New"'>}</span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-family:"Book Antiqua",serif'>which emits a label for the function in assembly, but no instructions. But if I comment out any of the lines in the loop, I get:</span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div style='margin-left:36.0pt'><p class=MsoNormal><span style='font-family:"Courier New"'>; Function Attrs: norecurse nounwind readnone</span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div style='margin-left:36.0pt'><p class=MsoNormal><span style='font-family:"Courier New"'>define void @func(i32* nocapture) local_unnamed_addr #0 {</span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div style='margin-left:36.0pt'><p class=MsoNormal><span style='font-family:"Courier New"'> ret void</span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div style='margin-left:36.0pt'><p class=MsoNormal><span style='font-family:"Courier New"'>}</span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-family:"Book Antiqua",serif'>then it emits an function which returns immediately.</span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-family:"Book Antiqua",serif'> </span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-family:"Book Antiqua",serif'>I was surprised though, that even with ‘</span><span style='font-family:"Courier New"'>-Wall -Wextra</span><span style='font-family:"Book Antiqua",serif'>’ the de-reference of the null pointer is not diagnosed.</span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div></div></blockquote><div><p class=MsoNormal><o:p> </o:p></p></div><p class=MsoNormal>The compiler frontend doesn't normally do the kind of cross-statement data-flow analysis that would be necessary to report this. I'm sure the static analyzer would report it, though. It would also be exposed dynamically if you built with UBSan.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>I don't really see a bug here.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>John.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><blockquote style='margin-top:5.0pt;margin-bottom:5.0pt'><div><div><p class=MsoNormal><span style='font-family:"Book Antiqua",serif'> </span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-family:"Book Antiqua",serif'>I guess this is a bug, but I haven’t yet gone through the process of reporting bugs.</span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-family:"Book Antiqua",serif'> </span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-family:"Book Antiqua",serif'> MartinO</span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><div><p class=MsoNormal><span style='font-family:"Book Antiqua",serif'> </span><span style='font-size:11.0pt;font-family:"Calibri",sans-serif'><o:p></o:p></span></p></div><p class=MsoNormal><span style='font-size:9.0pt;font-family:"Helvetica",sans-serif'>_______________________________________________<br>cfe-dev mailing list<br></span><a href="mailto:cfe-dev@lists.llvm.org"><span style='font-size:9.0pt;font-family:"Helvetica",sans-serif;color:#954F72'>cfe-dev@lists.llvm.org</span></a><span style='font-size:9.0pt;font-family:"Helvetica",sans-serif'><br></span><a href="http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev"><span style='font-size:9.0pt;font-family:"Helvetica",sans-serif;color:#954F72'>http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev</span></a><o:p></o:p></p></div></blockquote></div><p class=MsoNormal><o:p> </o:p></p></div></body></html>