<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 May 15, 2013, at 10:32 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><div class="im"><blockquote type="cite"><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 style="word-wrap:break-word"><div><div><blockquote type="cite"><div class="gmail_quote"><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><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></blockquote><div><br></div><div>No, because Clang cannot statically detect which indirect calls might call ::operator new. Instead, my proposal is to add a 'builtin' attribute to LLVM, and then for clang to add that attribute to the calls which can be optimized.</div>
</div></blockquote><div><br></div></div><div>Ugh. Having two different ways to represent "the same" thing is deeply unfortunate. I don't understand the full issue here though, how can you get an indirect call to ::operator new? Can you take its address in C++?</div>
</div></div></blockquote><div><br></div><div>Yes. operator new is an ordinary function that happens to have a funny name, and can have its address taken.</div></div></blockquote><br></div><div>To be clear, I'm only objecting because I don't want to add complexity to the IR for such a weird corner case.</div><div><br></div><div>Just brainstorming here, and yes, this is somewhat horrible, but would it be possible to handle this by having IRGen introduce a "thunk" function in the case when ::operator new has its address taken?</div><div><br></div><div>For example, you could have this pseudo code:</div><div><br></div><div><br></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>auto FP = & ::operator new; // I have no idea how to actually spell this in C++</div><div><br></div><div>IRGen into the equivalent of:</div><div><br></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>static void *thunk(size_t NumBytes) {</div><div><span class="Apple-tab-span" style="white-space:pre"> </span> return ::operator new(NumBytes); // Direct call, with nobuiltin attribute set.</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>}</div><div><br></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>auto FP = thunk;</div><div><br></div><div>That way the pain of this corner case is hoisted into clang, instead of subjecting all consumers of LLVM IR to a complexity increase.</div><div><br></div><div>-Chris</div></body></html>