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From: "matt at godbolt dot org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c++/68148] New: Devirtualization only applies to last of multiple successive calls Date: Thu, 29 Oct 2015 15:49:00 -0000 [thread overview] Message-ID: <bug-68148-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68148 Bug ID: 68148 Summary: Devirtualization only applies to last of multiple successive calls Product: gcc Version: 5.2.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: matt at godbolt dot org Target Milestone: --- Given the code: ---- struct Interface { virtual ~Interface() {} virtual void virtualFunc() = 0; virtual void virtualFunc2() = 0; }; struct Concrete : Interface { int counter_; Concrete() : counter_(0) {} void virtualFunc() { counter_++; } void virtualFunc2() { counter_++; } }; void test(Interface &c) { c.virtualFunc(); c.virtualFunc2(); } ---- (Compiled at -O3 -fdevirtualize-speculatively) Speculative devirtualization is applied to the call to virtualFunc2, but not to virtualFunc. (See https://goo.gl/Vtx5Fe). If one comments out the call to virtualFunc2, then the virtualFunc() call *is* speculatively devirtualized (https://goo.gl/G8f505). It seems to me that either both should be spec devirtualized, or none. Or perhaps even more generally, if the vtable pointer is that of "Concrete" then both calls can be inlined in one and converted to counter+=2 (provided inspection proved that Concrete's virtualFunc() does not modify the vtable, which I believe is otherwise a barrier to this kind of optimization). Am I missing something here, or is this a missed opportunity? Thanks, Matt
next reply other threads:[~2015-10-29 15:49 UTC|newest] Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top 2015-10-29 15:49 matt at godbolt dot org [this message] 2015-10-30 9:27 ` [Bug ipa/68148] " rguenth at gcc dot gnu.org 2015-10-30 13:52 ` matt at godbolt dot org
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