<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; "><br><div><div>On May 15, 2013, at 8:50 PM, Richard Smith <<a href="mailto:richard@metafoo.co.uk">richard@metafoo.co.uk</a>> wrote:</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 style="word-wrap:break-word"><div class="im"><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 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 class="gmail_extra"><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; position: static; z-index: auto; "><div>1) The 'nobuiltin' attribute doesn't actually prevent the optimization (see recent patch on llvmcommits)</div><div>2) We can't block the optimization if the call happens through a function pointer, unless we also annotate all calls through function pointers as 'nobuiltin'</div>
<div><br></div><div>How feasible would it be to make the 'builtin-ness' of _Znwm etc be opt-in rather than opt-out? Is there some other option we could pursue?</div></blockquote></div></div></div></blockquote>
</div></blockquote><div><br></div><div>Wow, this was spectacularly unclear, sorry about that. To avoid confusion, I'm suggesting that we add a 'builtin' attribute, and do not treat a call to _Znwm as a builtin call unless it has the attribute.</div>
<div> </div></div></blockquote><br></div><div>It's not clear to me that "builtin" is the right way to model this, but it definitely sounds like this should be an attribute on a call site (as opposed to on the function itself). What specific kinds of optimizations are we interested in doing to _Znwm calls?</div>
</div></blockquote><div><br></div><div>Initially, I'm just concerned about keeping the optimizations we already perform, such as globalopt lowering a new/delete pair into a global, while disabling the non-conforming variations of those optimizations. But we're also permitted to merge multiple allocations into one if they have sufficiently similar lifetimes.</div>
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</blockquote></div><br><div>So your proposal is for Clang to slap the attribute on explicit calls to ::operator new, but any other use of the symbol (e.g. from C code or something else weird) can be optimized?</div><div><br></div><div>If so, using nobuiltin seems perfectly fine to me. It seems like there is no auto-upgrade required.</div><div><br></div><div>-Chris</div></body></html>