<html><head><meta http-equiv="Content-Type" content="text/html charset=iso-8859-1"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><div>On Jun 5, 2013, at 8:34 PM, Richard Smith <<a href="mailto:richard@metafoo.co.uk">richard@metafoo.co.uk</a>> wrote:</div><blockquote type="cite">On Wed, Jun 5, 2013 at 8:06 PM, Eli Friedman <span dir="ltr"><<a href="mailto:eli.friedman@gmail.com" target="_blank">eli.friedman@gmail.com</a>></span> wrote:<br><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div dir="ltr">Patch attached; fixes PR16144.<div><br></div><div>Currently, the data in TypeLocs consists of a bunch of tightly packed structures, and the structures can become misaligned because there isn't any padding.</div>
<div><br></div><div>There are basically three possible approaches to fixing the alignment issues in TypeLocs:</div><div><br></div><div><div>1) Force every piece of the TypeLoc's data to have alignment 8.</div><div>2) Perform dynamic alignment adjustments.</div>
<div>3) Use #pragma pack to let the compiler know the data is intentionally misaligned.</div></div><div><br></div><div>(1) has a substantial impact on memory usage (something like 1% on Cocoa.h), so I'd like to avoid it if possible. (2) is the attached patch; it avoids both misaligned loads and unnecessary memory usage. The primary downside is that TypeLocBuilder becomes a lot more complicated, because it doesn't know in advance where it needs to insert padding. (3) keeps around to misaligned data: there's a potential performance penalty, it requires being careful not to introduce incorrect accesses to the data, and it's just plain ugly.<br>
</div><div><br></div><div><br></div><div>Two questions to focus on for review: would (3) be a better approach?</div></div></blockquote><div><br></div><div>I've tried this. We expose pointers into the type source info block in a couple of places (for instance, the array of ParmVarDecl*s on a FunctionTypeLoc) and the misalignment is then exposed to quite a large body of code. Maybe a MisalignedArrayRef<...> would help, but the damage is still not very contained.</div></div></blockquote><div><br></div>Hmm. We could have an API that potentially copies out to a temporary buffer if the data is misaligned. Dynamic alignment is probably cleaner, though; ugh.</div><div><br></div><div><blockquote type="cite"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div>And is there any way to make the TypeLocBuilder implementation a bit less ugly?</div>
</div></blockquote></div><br><div>Perhaps we could remove the guarantee that the child locations of a TypeLoc produced by push<T> are valid, or require some explicit action to fix them?</div></blockquote><div><br></div></div>I *think* that should be fine.<div><br></div><div>John.</div></body></html>