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</o:shapelayout></xml><![endif]--></head><body lang=EN-IE link="#0563C1" vlink="#954F72"><div class=WordSection1><p class=MsoNormal><span style='font-size:12.0pt;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:<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>void func(int *z) {<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'> int *b = 0; // This is the customer bug<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'><o:p> </o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'> for (int j = 0; j < 16; j++) {<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'> int c = *b;<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'> c -= *z++;<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'> *b++ = c;<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'> }<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>}</span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>It is clearly undefined behaviour, and this produces the following IR:<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>; Function Attrs: norecurse nounwind<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>define void @func(i32* nocapture readonly) local_unnamed_addr #0 {<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'> unreachable<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>}<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;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:<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>; Function Attrs: norecurse nounwind readnone<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>define void @func(i32* nocapture) local_unnamed_addr #0 {<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'> ret void<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>}<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>then it emits an function which returns immediately.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>I was surprised though, that even with ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'>-Wall -Wextra</span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’ the de-reference of the null pointer is not diagnosed.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>I guess this is a bug, but I haven’t yet gone through the process of reporting bugs.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'> MartinO<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p></div></body></html>