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</o:shapelayout></xml><![endif]--></head><body bgcolor=white lang=EN-US link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='color:#1F497D'>Hi John,<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'>It seems the dereferencing a NULL pointer is undefined behavior but<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Calling a function through a null pointer seems o.k.<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'>If so , for this place, we need comment out the check.<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'><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#232">http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#232</a><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'>look at Notes from the October 2003 meeting.<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'>Yin <o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><div><div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif";color:windowtext'>From:</span></b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif";color:windowtext'> John Criswell [mailto:criswell@illinois.edu] <br><b>Sent:</b> Wednesday, November 06, 2013 6:28 PM<br><b>To:</b> Yin Ma; 'llvmdev Dev'<br><b>Subject:</b> Re: [LLVMdev] Should remove calling NULL pointer or not<o:p></o:p></span></p></div></div><p class=MsoNormal><o:p> </o:p></p><div><p class=MsoNormal>On 11/6/13 6:36 PM, Yin Ma wrote:<o:p></o:p></p></div><blockquote style='margin-top:5.0pt;margin-bottom:5.0pt'><p class=MsoNormal>Hi,<o:p></o:p></p><p class=MsoNormal> <o:p></o:p></p><p class=MsoNormal>For a small case, that calls NULL pointer function. LLVM explicitly converts<o:p></o:p></p><p class=MsoNormal>It to a store because it thinks it is not reachable like calling undefvalue.<o:p></o:p></p><p class=MsoNormal>In InstCombineCalls.cpp:930<o:p></o:p></p><p class=MsoNormal> <o:p></o:p></p><p class=MsoNormal>I think it is not a right approach because calling null pointer function <o:p></o:p></p><p class=MsoNormal>Will segfault the program. Converting to a store will make program pass<o:p></o:p></p><p class=MsoNormal>Silently. This changes the behavior of a program. <o:p></o:p></p><p class=MsoNormal> <o:p></o:p></p><p class=MsoNormal>So we need remove the case if (isa<ConstantPointerNull>(Callee) at <o:p></o:p></p><p class=MsoNormal>InstCombineCalls.cpp:918 and treat calling Null pointer reachable.<o:p></o:p></p><p class=MsoNormal> <o:p></o:p></p><p class=MsoNormal>How do you think? Is there any reason that we should convert <o:p></o:p></p><p class=MsoNormal>a calling null pointer to a store?<o:p></o:p></p></blockquote><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Times New Roman","serif"'><br>If calling a NULL function pointer yields undefined behavior (as defined by the C/C++ standards), then the optimization is correct: since the behavior is undefined, the compiler can change it as it sees fits. In other words, the compiler is not required to maintain "incorrect" behavior.<br><br>The remaining question, then, is whether the C/C++ standards consider calling a NULL function pointer undefined behavior. I suspect that it is undefined behavior, but to be honest, I do not know for certain.<br><br>-- John T.<br><br><br><br><br><o:p></o:p></span></p><p class=MsoNormal> <o:p></o:p></p><p class=MsoNormal>Thanks,<o:p></o:p></p><p class=MsoNormal> <o:p></o:p></p><p class=MsoNormal>Yin <o:p></o:p></p><p class=MsoNormal> <o:p></o:p></p><p class=MsoNormal> <o:p></o:p></p><p class=MsoNormal> <o:p></o:p></p><p class=MsoNormal> <o:p></o:p></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Times New Roman","serif"'><br><br><br><o:p></o:p></span></p><pre>_______________________________________________<o:p></o:p></pre><pre>LLVM Developers mailing list<o:p></o:p></pre><pre><a href="mailto:LLVMdev@cs.uiuc.edu">LLVMdev@cs.uiuc.edu</a> <a href="http://llvm.cs.uiuc.edu">http://llvm.cs.uiuc.edu</a><o:p></o:p></pre><pre><a href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev">http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev</a><o:p></o:p></pre><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Times New Roman","serif"'><o:p> </o:p></span></p></div></body></html>