<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 Mar 28, 2016, at 3:12 PM, Chandler Carruth <<a href="mailto:chandlerc@google.com" class="">chandlerc@google.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class="">FWIW, I agree with Mehdi that we should just assert that our types don't get called with size zero.</div></div></blockquote>Yeah, I agree. I’ve tried running the tests with the assert in place and there’s about 1000 failures across llvm/clang. I’ll see what I can fix as I would like to get these to behave. There may be actual logic errors in some of these cases.<br class=""><blockquote type="cite" class=""><div class=""><div dir="ltr" class=""><div class=""><br class=""></div><div class="">That said, I don't think we can be terribly cavalier with what we expect from standard allocator types, operator new, or malloc. And so I would expect LLVM_ATTRIBUTE_RETURNS_NOALIAS to not imply NONNULL, and while it seems reasonable to put NONNULL on *our* allocate function because of the assert and the fact that we assume the underlying allocation routine never produces a null, it doesn't seem reasonable for any old function with NOALIAS to have NONNULL inferred.</div></div></div></blockquote>I agree. I don’t actually know if NOALIAS and NONNULL were put on this function independently, or if the committer assumed one implied the other. But I do agree that they should be independent in general, even if they happen to both apply here.</div><div><br class=""></div><div>Cheers,</div><div>Pete<br class=""><blockquote type="cite" class=""><div class=""><div dir="ltr" class=""><div class=""><br class=""></div><div class="">-Chandler</div></div><br class=""><div class="gmail_quote"><div dir="ltr" class="">On Tue, Mar 22, 2016 at 9:18 PM Mehdi Amini via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" class="">llvm-dev@lists.llvm.org</a>> wrote:<br class=""></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word" class=""><div class=""><blockquote type="cite" class=""><div class="">On Mar 22, 2016, at 5:39 PM, Pete Cooper via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank" class="">llvm-dev@lists.llvm.org</a>> wrote:</div><br class=""><div class=""><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class=""><blockquote type="cite" class=""><div class=""><br class="">On Mar 22, 2016, at 5:35 PM, David Blaikie <<a href="mailto:dblaikie@gmail.com" target="_blank" class="">dblaikie@gmail.com</a>> wrote:</div><br class=""><div class=""><br class=""><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class=""><div class="gmail_quote" style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px">On Tue, Mar 22, 2016 at 5:30 PM, Pete Cooper<span class=""> </span><span dir="ltr" class=""><<a href="mailto:peter_cooper@apple.com" target="_blank" class="">peter_cooper@apple.com</a>></span><span class=""> </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="">Hi all<div class=""><br class=""></div><div class="">(No idea if I have the correct audience. Please CC more people as needed).</div><div class=""><br class=""></div><div class="">I have an UBSan failure in BumpPtrAllocatorImpl.Allocate.</div><div class=""><br class=""></div><div class="">The problem is that lld requests that we StringRef::copy an empty string. This passes a length of 0 to a BumpPtrAllocator. The BumpPtrAllocator happened to not have anything allocated yet so the CurPtr is nullptr, but given that we need 0 space we think we have enough space and return an allocation of size 0 at address nullptr. This therefore returns nullptr from Allocate, but that method is marked with LLVM_ATTRIBUTE_RETURNS_NONNULL and LLVM_ATTRIBUTE_RETURNS_NOALIAS, both of which aren’t true in this case.</div><div class=""><br class=""></div><div class="">To put this in code, if I have</div><div class=""><br class=""></div><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px" class=""><div class="">BumpPtrAllocator allocator;</div><div class="">StringRef s;</div><div class="">s.copy(allocator);</div></blockquote><div class=""><br class=""></div><div class="">then i’m going to allocate 0 bytes in the allocator and get a StringRef(nullptr, 0). Its a valid StringRef, but an UBSan failures in the allocator.</div><div class=""><br class=""></div><div class="">Lang and I looked up malloc behaviour online as this is fairly analogous. The answer there is that you are allowed to return nullptr, or not, its implementation defined. So no help there.</div><div class=""><br class=""></div><div class="">So the question is, how do we want this to behave in our code?</div><div class=""><br class=""></div><div class="">Some options:</div><div class="">- Assert that Allocate never gets a size 0 allocation. So fix StringRef::copy to see this case</div><div class="">- Remove the attributes from Allocate but continue to return nullptr (or base of the allocator?) in this case</div><div class="">- Keep the attributes on Allocate and treat size 0 allocations as size 1</div></div></blockquote><div class=""><br class=""></div><div class="">I believe the last is closer to 'new's behavior - which I think returns a unique non-null address (well, unique amongst current allocations - can be recycled once deleted) if I recall correctly.<span class=""> </span></div></div></div></blockquote>That’s what I would have expected too. Its like sizeof(struct {}) which can be a 1 depending on factors we don’t need to get in to here.<br class=""></div></div></blockquote><div class=""><br class=""></div></div></div><div style="word-wrap:break-word" class=""><div class=""><div class="">Well except that if sizeof(struct{}) is 1, the allocator is never called with a 0.</div><div class=""><br class=""></div><div class="">I would consider forbidding zero sized allocation in the allocator (assert()) by design (hey we're controlling every possible uses!), unless there is a real use-case for that.</div><div class=""><br class=""></div><div class="">This would also be in line with the C++ standard requirement for allocator which specifies that the result of "a.allocate(0)" is unspecified (ref: C++14 Table 28 — Allocator requirements).</div></div></div><div style="word-wrap:break-word" class=""><div class=""><br class=""><div class="">-- </div><div class="">Mehdi</div></div></div><div style="word-wrap:break-word" class=""><div class=""><div class=""><br class=""></div><br class=""><blockquote type="cite" class=""><div class=""><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class=""><blockquote type="cite" class=""><div class=""><div class="gmail_quote" style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px"><div class="">Can check for wording if that's helpful/desired.</div></div></div></blockquote>No need for my benefit :) I’m in agreement that this is a good behavior to go for, but will leave it to others to say if they’d like the extra detail.</div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class=""><br class=""></div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="">One thing I did forget to say is that I’d like to fix StringRef::copy in all of the above cases. I think that this method should always avoid the allocator and return StringRef(nullptr, 0) when length is 0. I’ll get a patch up on llvm-commits if there’s no objections there.</div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class=""><br class=""></div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="">Thanks,</div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="">Pete<br class=""><blockquote type="cite" class=""><div class=""><div class="gmail_quote" style="font-family:Helvetica;font-size:12px;font-style:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px"><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="">Thanks,</div><div class="">Pete</div></div></blockquote></div></div></blockquote></div><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class=""><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;float:none;display:inline!important" class="">_______________________________________________</span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class=""><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;float:none;display:inline!important" class="">LLVM Developers mailing list</span><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class=""><a href="mailto:llvm-dev@lists.llvm.org" style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" target="_blank" class="">llvm-dev@lists.llvm.org</a><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class=""><a href="http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev" style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" target="_blank" class="">http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev</a></div></blockquote></div></div>_______________________________________________<br class="">
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