<html><head><meta http-equiv="Content-Type" content="text/html charset=windows-1252"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;">On Jan 3, 2014, at 1:55 PM, Chandler Carruth <<a href="mailto:chandlerc@google.com">chandlerc@google.com</a>> wrote:<div><blockquote type="cite"><div class="gmail_extra" style="font-family: Helvetica; font-size: 12px; 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="gmail_quote"><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 class="im">><br>> The key thing then is to make sure that it's safe to enable the<br>> assertions in the headers if an application is built with !<span class="il">NDEBUG</span><span class="Apple-converted-space"> </span>and<br>> linked against an<span class="Apple-converted-space"> </span><span class="il">NDEBUG</span><span class="Apple-converted-space"> </span>version of LLVM.<br><br></div>Sounds great. I'm pretty confident that there will be no problems - in practice - from any ODR violations that might arise from "assert" differing across library boundaries. We would want some pretty strong practical justification for breaking away from standard assert.</blockquote></div><br>Sorry to dig up this thread, but when re-reading it, I was surprised that everyone seems to think this will be easily done across all of LLVM.</div><div class="gmail_extra" style="font-family: Helvetica; font-size: 12px; 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;"><br></div><div class="gmail_extra" style="font-family: Helvetica; font-size: 12px; 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;">How can we support AssertingVH, which behaves as a POD-like struct around a pointer in NDEBUG, and as a class with significant (important) functionality to implement asserts on dangling value handles in !NDEBUG builds?</div><div class="gmail_extra" style="font-family: Helvetica; font-size: 12px; 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;"><br></div><div class="gmail_extra" style="font-family: Helvetica; font-size: 12px; 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;">While having different components of LLVM and consumers of LLVM able to intermix NDEBUG and !NDEBUG built code freely without ABI issues is nice-to-have in my book, the functionality provided by AssertingVH is significantly more nice-to-have, and I don't see any easy ways to contain or limit the exposure of this facility to library-level consumers.</div></blockquote><div><br></div><div>I hadn’t considered AssertVH, and I agree that losing it isn’t an option.</div><div><br></div><div>Would it be possible to redesign AssertVH to be non-ABI fragile across debug/release builds? I haven’t looked at it recently, but maybe it could be a pointer to a CallbackVH in the debug mode, or a PointerUnion<rawpointer, callbackvh> or something.</div><br><blockquote type="cite"><div class="gmail_extra" style="font-family: Helvetica; font-size: 12px; 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;">We also have quite a few places in LLVM today that conserve memory usage in non-assert builds by removing unnecessary members of classes. We can say that this makes the ABI more fragile, but it is a valuable space optimization. Chris, are you saying to strongly believe that these should only be allowed for classes that are not visible in the C++ API? I find that surprising, as LLVM has historically prioritized efficiency and developer tools over ABI stability in our C++ APIs. Build-level instability is certainly a different beast from version stability, but I wanted to make sure the ramifications of this shift were being considered by everyone. (They hadn't by me.)</div></blockquote></div><br><div>Do you have any specific examples in mind?</div><div><br></div><div>-Chris</div></body></html>