<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 16 Oct 2014, at 19:59, David Blaikie <<a href="mailto:dblaikie@gmail.com" class="">dblaikie@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><br class="Apple-interchange-newline"><br style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><div class="gmail_quote" style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;">On Thu, Oct 16, 2014 at 7:26 PM, Frédéric Riss<span class="Apple-converted-space"> </span><span dir="ltr" class=""><<a href="mailto:friss@apple.com" target="_blank" class="">friss@apple.com</a>></span><span class="Apple-converted-space"> </span>wrote:<br class=""><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex;"><div style="word-wrap: break-word;" class=""><div class=""><div class=""><br class=""><div class=""><blockquote type="cite" class=""><div class="">On 16 Oct 2014, at 18:58, David Blaikie <<a href="mailto:dblaikie@gmail.com" target="_blank" class="">dblaikie@gmail.com</a>> wrote:</div><br class=""><div class=""><div dir="ltr" class=""><br class=""><div class="gmail_extra"><br class=""><div class="gmail_quote">On Thu, Oct 16, 2014 at 6:20 PM, Duncan P. N. Exon Smith<span class="Apple-converted-space"> </span><span dir="ltr" class=""><<a href="mailto:dexonsmith@apple.com" target="_blank" class="">dexonsmith@apple.com</a>></span><span class="Apple-converted-space"> </span>wrote:<br class=""><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex;"><span class=""><br class="">> On Oct 16, 2014, at 6:08 PM, Eric Christopher <<a href="mailto:echristo@gmail.com" target="_blank" class="">echristo@gmail.com</a>> wrote:<br class="">><br class="">> On Thu, Oct 16, 2014 at 6:06 PM, Duncan P. N. Exon Smith<br class="">> <<a href="mailto:dexonsmith@apple.com" target="_blank" class="">dexonsmith@apple.com</a>> wrote:<br class="">>><br class="">>>> On Oct 16, 2014, at 3:54 PM, Frederic Riss <<a href="mailto:friss@apple.com" target="_blank" class="">friss@apple.com</a>> wrote:<br class="">>>><br class="">>>> If you look at the added testcase, it cast a void (*func)() to a int (*func)(). The int typed result is referenced in a debug info metadata node (an argument to the dbg.value intrinsic). Then instcombine would remove the bitcast and turn the int returning function into a void returning function that gets RAUWd over the value in the MDNode. And at this point in the current code you have invalid IR.<br class="">>>><br class="">>>> The code in the testcase is reduced from a huge LTO link that encountered this situation in a much more complicated setup.<br class="">>>><br class="">>>><span class="Apple-converted-space"> </span><a href="http://reviews.llvm.org/D5828" target="_blank" class="">http://reviews.llvm.org/D5828</a><br class="">>><br class="">>> Weird. This LGTM (after fixing a few super minor whitespace changes);<br class="">>> not sure if Eric is still looking.<br class="">><br class="">> Enh, I was trying to figure out if there was some way to avoid the<br class="">> void metadata node being create rather than avoiding RAUW'ing it. It's<br class="">> probably not a huge distinction at the moment.<br class=""><br class=""></span>I don't think it is a void metadata node; it's a void `CallInst`.<br class=""><br class="">We have:<br class=""><br class=""> <span class="Apple-converted-space"> </span>call void @llvm.dbg.declare(metadata !{i32* %deadvar}, ...)<br class=""> <span class="Apple-converted-space"> </span>%call = ...<br class=""><span class=""> <span class="Apple-converted-space"> </span>store i32 %call, i32* %deadvar, align 4, !dbg !21<br class=""><br class=""></span>Which converts to:<br class=""><br class=""> <span class="Apple-converted-space"> </span>call void @llvm.dbg.value(metadata !{i32* %call}, ...)<br class=""> <span class="Apple-converted-space"> </span>%call = ...<br class=""><br class="">Then, when %call gets RAUW'ed, `metadata !{i32* %call}` gets updated<br class="">to `void`.<br class=""></blockquote><div class=""><br class=""></div><div class="">My only thought is that this code has UB, right? I don't know whether we should do something else with it, which might involve killing the dbg.value metadata along with all the other instructions that are fruit of the UB tree. (the whole basic block, anything else reachable from it, etc)</div></div></div></div></div></blockquote><br class=""></div></div></div><div class="">Are you advocating against the patch, or merely pointing out that the compiler should handle the testcase code differently?</div></div></blockquote><div class=""><br class=""></div><div class="">A little of both.</div><div class=""> </div><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex;"><div style="word-wrap: break-word;" class=""><div class="">In the former case, here’s my answer :-) :</div><div class=""><br class=""></div><div class="">Yes, it involves UB (at least I think, the bogus return value is never used, but I suppose the call through the wrong pointer type in itself is UB). And thus we would be in our right to just drop the call and the associated dbg.value, but the fix in itself has nothing to do with UB. It prevents any call of RAUW with a void type replacement Value to generate invalid MDNodes. We could audit all RAUV callers to never do that, but handling it directly in MDNode seems safer (and what would the caller do anyway? the end-result would certainly be the same).</div><div class=""><br class=""></div><div class="">The counter argument could be that only input code with UB could trigger such RAUW calls, but I wouldn’t bet on it.</div></div></blockquote><div class=""><br class=""></div><div class="">Generally my interest is in not conceding conditionals like this until we know they're necessary. It's an easy example, but consider my recent commit <span style="font-family: monospace;" class="">219702</span> - a fairly innocuous conditional allowed a slew of bugs to be introduced. Where possible I'd like to understand the root cause of bugs and continue to fix those until we find cases where a certain thing actually needs to be supported. We can add assertions to make sure violations of those invariants fail early and are easier to debug, but silently accepting them because its expedient now may cost us not just the bugs we have today but many more we may introduce in the future by giving up that invariant.<br class=""><br class="">So it's not that I bet there wouldn't be other violations of this rule - I just wouldn't bet that the next one we find will be for unquestionably valid code (maybe the 5th one we find is, and we have to remove this invariant then - but if so, we've just found/fixed 5 bugs in the mean time - granted, we still lose the ability to enforce the invariant, but we have a solid example as to why its not a reasonable invariant).<br class=""><br class="">Not a "over my dead body" argument against this patch, just a thought/perspective.<br class=""></div></div></div></blockquote><div><br class=""></div><div>I can see your general point, but I have a hard time applying it to this case. Could you explicit what you think would be the right fix? From your first message I gather that you’d want to just drop the code in the name of UB, but I'm not sure this would always be the right thing to do (I must concede that I’ve never been a proponent of compilers using UB to prune the code. Especially so when there is no diagnostic of this behaviour.) In the trivial case of the testcase, the issue would be diagnosed to the user through warnings. But in an LTO scenario, this issue can be introduced without any diagnostic message.</div><div><br class=""></div><div>Coming back to the patch in itself, I also don’t see how it could be the wrong thing to do. The TrackingVH in the MDNode is meant to handle the case of the Value going away. RAUWing with a void value is just another case of a vanishing value.</div><div><br class=""></div><div>Fred</div><div><br class=""></div><div><br class=""></div><blockquote type="cite" class=""><div class=""><div class="gmail_quote" style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;"><div class="">- David </div><div class=""> </div><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex;"><div style="word-wrap: break-word;" class=""><div class=""><br class=""></div><div class="">Fred</div></div></blockquote></div></div></blockquote></div><br class=""></body></html>