<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Sep 8, 2017, at 12:53 PM, Martin J. O'Riordan via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org" class="">cfe-dev@lists.llvm.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="WordSection1" style="page: WordSection1; font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;"><div style="margin: 0cm 0cm 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Book Antiqua', serif;" class="">I was looking at a bug that a customer reported to me, and have reduced the code to the following fragment:<o:p class=""></o:p></span></div><div style="margin: 0cm 0cm 0.0001pt 36pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Courier New', serif;" class="">void func(int *z) {<o:p class=""></o:p></span></div><div style="margin: 0cm 0cm 0.0001pt 36pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Courier New', serif;" class=""> int *b = 0; // This is the customer bug<o:p class=""></o:p></span></div><div style="margin: 0cm 0cm 0.0001pt 36pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Courier New', serif;" class=""><o:p class=""> </o:p></span></div><div style="margin: 0cm 0cm 0.0001pt 36pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Courier New', serif;" class=""> for (int j = 0; j < 16; j++) {<o:p class=""></o:p></span></div><div style="margin: 0cm 0cm 0.0001pt 36pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Courier New', serif;" class=""> int c = *b;<o:p class=""></o:p></span></div><div style="margin: 0cm 0cm 0.0001pt 36pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Courier New', serif;" class=""> c -= *z++;<o:p class=""></o:p></span></div><div style="margin: 0cm 0cm 0.0001pt 36pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Courier New', serif;" class=""> *b++ = c;<o:p class=""></o:p></span></div><div style="margin: 0cm 0cm 0.0001pt 36pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Courier New', serif;" class=""> }<o:p class=""></o:p></span></div><div style="margin: 0cm 0cm 0.0001pt 36pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Courier New', serif;" class="">}</span><span style="font-size: 12pt; font-family: 'Book Antiqua', serif;" class=""><o:p class=""></o:p></span></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Book Antiqua', serif;" class="">It is clearly undefined behaviour, and this produces the following IR:<o:p class=""></o:p></span></div><div style="margin: 0cm 0cm 0.0001pt 36pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Courier New', serif;" class="">; Function Attrs: norecurse nounwind<o:p class=""></o:p></span></div><div style="margin: 0cm 0cm 0.0001pt 36pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Courier New', serif;" class="">define void @func(i32* nocapture readonly) local_unnamed_addr #0 {<o:p class=""></o:p></span></div><div style="margin: 0cm 0cm 0.0001pt 36pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Courier New', serif;" class=""> unreachable<o:p class=""></o:p></span></div><div style="margin: 0cm 0cm 0.0001pt 36pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Courier New', serif;" class="">}<o:p class=""></o:p></span></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Book Antiqua', serif;" class="">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 class=""></o:p></span></div><div style="margin: 0cm 0cm 0.0001pt 36pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Courier New', serif;" class="">; Function Attrs: norecurse nounwind readnone<o:p class=""></o:p></span></div><div style="margin: 0cm 0cm 0.0001pt 36pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Courier New', serif;" class="">define void @func(i32* nocapture) local_unnamed_addr #0 {<o:p class=""></o:p></span></div><div style="margin: 0cm 0cm 0.0001pt 36pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Courier New', serif;" class=""> ret void<o:p class=""></o:p></span></div><div style="margin: 0cm 0cm 0.0001pt 36pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Courier New', serif;" class="">}<o:p class=""></o:p></span></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Book Antiqua', serif;" class="">then it emits an function which returns immediately.<o:p class=""></o:p></span></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Book Antiqua', serif;" class=""><o:p class=""> </o:p></span></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Book Antiqua', serif;" class="">I was surprised though, that even with ‘</span><span style="font-size: 12pt; font-family: 'Courier New', serif;" class="">-Wall -Wextra</span><span style="font-size: 12pt; font-family: 'Book Antiqua', serif;" class="">’ the de-reference of the null pointer is not diagnosed.</span></div></div></div></blockquote><div><br class=""></div>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.</div><div><br class=""></div><div>I don't really see a bug here.</div><div><br class=""></div><div>John.</div><div><br class=""></div><div><blockquote type="cite" class=""><div class=""><div class="WordSection1" style="page: WordSection1; font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;"><div style="margin: 0cm 0cm 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Book Antiqua', serif;" class=""><o:p class=""></o:p></span></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Book Antiqua', serif;" class=""><o:p class=""> </o:p></span></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Book Antiqua', serif;" class="">I guess this is a bug, but I haven’t yet gone through the process of reporting bugs.<o:p class=""></o:p></span></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Book Antiqua', serif;" class=""><o:p class=""> </o:p></span></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Book Antiqua', serif;" class=""> MartinO<o:p class=""></o:p></span></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><span style="font-size: 12pt; font-family: 'Book Antiqua', serif;" class=""><o:p class=""> </o:p></span></div></div><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">_______________________________________________</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">cfe-dev mailing list</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><a href="mailto:cfe-dev@lists.llvm.org" style="color: rgb(149, 79, 114); text-decoration: underline; font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px;" class="">cfe-dev@lists.llvm.org</a><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><a href="http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev" style="color: rgb(149, 79, 114); text-decoration: underline; font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px;" class="">http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev</a></div></blockquote></div><br class=""></body></html>