<div dir="ltr"><div class="gmail_extra"><div class="gmail_quote">On Thu, Jul 23, 2015 at 11:42 AM, Piotr Padlewski <span dir="ltr"><<a href="mailto:prazek@google.com" target="_blank">prazek@google.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">HI,<div>Yep, our proposal doesn't cover it, because this load ; icmp ; assume; will land global initilizer function, and main will not see it. </div><div>At least if foo would be called multiple times, then we would only have one load from vtable, but unfortunatelly we will not be able to inline, or make direct call to it with this approach.</div><div>I think that this case is rare enough to solve it right now.</div><div><br></div><div>Piotr</div></div><div class="gmail_extra"><br><div class="gmail_quote"><span class="">On Thu, Jul 23, 2015 at 10:30 AM, Geoff Berry <span dir="ltr"><<a href="mailto:gberry@codeaurora.org" target="_blank">gberry@codeaurora.org</a>></span> wrote:<br></span><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div lang="EN-US" link="blue" vlink="purple"><div><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">Hi Piotr,<u></u><u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"><u></u> <u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">You may be interested in a recent patch I posted: <a href="https://urldefense.proofpoint.com/v2/url?u=http-3A__reviews.llvm.org_D11043&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=CnzuN65ENJ1H9py9XLiRvC_UQz6u3oG6GUNn7_wosSM&m=JLBFMHsMRUHs511X66Fr1fopHJ5WsTkKXLZ9GtX-koY&s=5EY4grsmJL7GyufHaG_GcSgPmas2OC8dUU-0BD8tM5g&e=" target="_blank">http://reviews.llvm.org/D11043</a><u></u><u></u></span></p><div><div class="h5"><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">This patch addresses a de-virtualization case that I’m not sure would be handled by your current proposal, namely that of a virtual call where the ‘this’ object is a global variable.<u></u><u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">For example:<u></u><u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"><u></u> <u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">struct A {<u></u><u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">  A();<u></u><u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">  virtual void foo();<u></u><u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">};<u></u><u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">void g(A * a) {<u></u><u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">  a->foo();<u></u><u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">}<u></u><u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"><u></u> <u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">A a;<u></u><u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">int main() {<u></u><u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">  g(&a);</span></p></div></div></div></div></blockquote></div></div></blockquote><div><br></div><div>We could in principle emit the load/icmp/assume for a's vptr here, but I'm concerned that this is not actually sound in general. Consider:</div><div><br></div><div>struct A;</div><div>extern A a;</div><div>void *p = &a; // emit load/icmp/assume here?</div><div>struct A { virtual void f(); };</div><div>A a;</div><div><br></div><div>If we emit an assume on the vptr at the point where the address of a is taken, we get:</div><div><br></div><div>@a = global %A zeroinitializer</div><div>void @global_init() {</div><div>  load @a</div><div>  %x = icmp @a, @A.vtable</div><div>  call @llvm.assume(%x)</div><div>  store @a, @p</div><div>  call @A::A(@a)</div><div>}</div><div><br></div><div>... where the assume call can be folded to "call @llvm.assume(i1 false)", which (presumably) can be reduced further to "unreachable" (oops). (In Geoff's example, this is sound only because we're guaranteed that 'a' is initialized before we enter 'main', but an optimization that only fires for code within 'main' doesn't sound very useful.)</div><div><br></div><div>We could support this with something weaker than the current load/icmp/assume pattern: we could imagine an instruction/intrinsic that says "any later load of pointer %p with !invariant.group !g will load the value %q", but that does not imply that the value %q is currently stored in %p. It's not clear to me whether that has sufficient value to be worthwhile.</div><div><br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div lang="EN-US" link="blue" vlink="purple"><div><div><div class="h5"><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"><u></u><u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">}<u></u><u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"><u></u> <u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">It might be worth considering if your approach could be extended to handle this case.<u></u><u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"><u></u> <u></u></span></p><div style="border:dashed #2f6fab 1.0pt;padding:12.0pt 12.0pt 12.0pt 12.0pt;background:#f9f9f9"><p class="MsoNormal" style="line-height:15.6pt;background:#f9f9f9;border:none;padding:0in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">--<u></u><u></u></span></p><p class="MsoNormal" style="line-height:15.6pt;background:#f9f9f9;border:none;padding:0in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">Geoff Berry<u></u><u></u></span></p><p class="MsoNormal" style="line-height:15.6pt;background:#f9f9f9;border:none;padding:0in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d">Employee of Qualcomm Innovation Center, Inc.<u></u><u></u></span></p><p class="MsoNormal" style="line-height:15.6pt;background:#f9f9f9;border:none;padding:0in"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project<u></u><u></u></span></p></div><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1f497d"><u></u> <u></u></span></p><p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> <a href="mailto:llvmdev-bounces@cs.uiuc.edu" target="_blank">llvmdev-bounces@cs.uiuc.edu</a> [mailto:<a href="mailto:llvmdev-bounces@cs.uiuc.edu" target="_blank">llvmdev-bounces@cs.uiuc.edu</a>] <b>On Behalf Of </b>Piotr Padlewski<br><b>Sent:</b> Wednesday, July 22, 2015 5:56 PM<br><b>To:</b> <a href="mailto:cfe-dev@cs.uiuc.edu" target="_blank">cfe-dev@cs.uiuc.edu</a> Developers; <a href="mailto:llvmdev@cs.uiuc.edu" target="_blank">llvmdev@cs.uiuc.edu</a><br><b>Subject:</b> [LLVMdev] Clang devirtualization proposal<u></u><u></u></span></p><p class="MsoNormal"><u></u> <u></u></p><div><span><div><p class="MsoNormal">Hi folks,<u></u><u></u></p></div><div><p class="MsoNormal">this summer I will work with Richard Smith on clang devirtualization. Check out our proposal:<u></u><u></u></p></div><div><p class="MsoNormal"><u></u> <u></u></p></div></span><p class="MsoNormal"><a href="https://urldefense.proofpoint.com/v2/url?u=https-3A__docs.google.com_document_d_1f2SGa4TIPuBGm6y6YO768GrQsA8awNfGEJSBFukLhYA_edit-3Fusp-3Dsharing&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4_bhmuhMHA&m=-sGvXxkjadRLtXcTi4kOVPumoH-0XOKmk_vgUTcYugY&s=hHoo6tgC-NooXdIwbBwT_D8sIw8fcYF4XvBRI8Lr9Eg&e=" target="_blank">https://docs.google.com/document/d/1f2SGa4TIPuBGm6y6YO768GrQsA8awNfGEJSBFukLhYA/edit?usp=sharing</a><u></u><u></u></p><div><p class="MsoNormal"><u></u> <u></u></p></div><div><p class="MsoNormal">And modified LangRef<u></u><u></u></p></div><div><p class="MsoNormal"><a href="https://urldefense.proofpoint.com/v2/url?u=http-3A__reviews.llvm.org_D11399&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4_bhmuhMHA&m=-sGvXxkjadRLtXcTi4kOVPumoH-0XOKmk_vgUTcYugY&s=L6_vdinD06uAwgm4OJGL5QxKw8Tzfa_4DxPwf3Zj704&e=" target="_blank">http://reviews.llvm.org/D11399</a><u></u><u></u></p></div><span><div><p class="MsoNormal"><u></u> <u></u></p></div><div><p class="MsoNormal">You can also check out previous disscussion that was started before our proposal was ready - <a href="http://lists.cs.uiuc.edu/pipermail/cfe-dev/2015-July/044052.html" target="_blank">http://lists.cs.uiuc.edu/pipermail/cfe-dev/2015-July/044052.html</a><u></u><u></u></p></div><div><p class="MsoNormal"><u></u> <u></u></p></div><div><p class="MsoNormal">Regards<u></u><u></u></p></div><div><p class="MsoNormal">Piotr Padlewski<u></u><u></u></p></div></span></div></div></div></div></div></blockquote></div><br></div>
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