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From: "dl.soluz at gmx dot net" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug tree-optimization/58483] New: missing optimization opportunity for const std::vector compared to std::array Date: Fri, 20 Sep 2013 11:03:00 -0000 [thread overview] Message-ID: <bug-58483-4@http.gcc.gnu.org/bugzilla/> (raw) http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58483 Bug ID: 58483 Summary: missing optimization opportunity for const std::vector compared to std::array Product: gcc Version: 4.8.1 Status: UNCONFIRMED Severity: enhancement Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: dl.soluz at gmx dot net this small testprogram shows a missing optimization opportunity for const std::vector when using initialization_list - in compared to std::array #include <vector> #include <numeric> #include <array> static int calc(const std::array<int,3> p_ints, const int& p_init) //static int calc(const std::vector<int> p_ints, const int& p_init) { return std::accumulate(p_ints.begin(), p_ints.end(), p_init); } int main() { const int result = calc({10,20,30},100); return result; } optimizer-result using std::array int main() () { <bb 2>: return 160; } the result using std::vector int main() () { int __init; int _2; int _11; int _32; int * _33; <bb 2>: _33 = operator new (12); __builtin_memcpy (_33, &._79, 12); _32 = MEM[(const int &)_33]; __init_28 = _32 + 100; _2 = MEM[(const int &)_33 + 4]; __init_18 = _2 + __init_28; _11 = MEM[(const int &)_33 + 8]; __init_13 = _11 + __init_18; if (_33 != 0B) goto <bb 3>; else goto <bb 4>; <bb 3>: operator delete (_33); <bb 4>: return __init_13; } according to Marc Gliss's answer in (http://gcc.gnu.org/ml/gcc/2013-09/msg00179.html) "... We don't perform such high-level optimizations. But if you expand, inline and simplify this program, the optimizers sees something like: p=operator new(12); memcpy(p,M,12); // M contains {10, 20, 30} res=100+p[0]+p[1]+p[2]; if(p!=0) operator delete(p); A few things that go wrong: * because p is filled with memcpy and not with regular assignments, the compiler doesn't realize that p[0] is known. * the test p != 0 is unnecessary (a patch that should help is pending review) * we would then be left with: p=new(12); delete p; return 160; gcc knows how to remove free(malloc(12)) but not the C++ variant (I don't even know if it is legal, or what conditions and flags are required to make it so). ..." the pending patch is http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19476 but the question is still can new(delete(12)) also removed like free(malloc(12)) in this scenario?
next reply other threads:[~2013-09-20 11:03 UTC|newest] Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top 2013-09-20 11:03 dl.soluz at gmx dot net [this message] 2013-09-20 11:14 ` [Bug tree-optimization/58483] " glisse at gcc dot gnu.org 2013-09-20 11:17 ` dl.soluz at gmx dot net 2013-09-20 14:46 ` dl.soluz at gmx dot net 2013-09-28 18:52 ` dl.soluz at gmx dot net 2013-09-29 8:06 ` dl.soluz at gmx dot net 2013-10-03 23:57 ` glisse at gcc dot gnu.org 2013-10-08 10:43 ` glisse at gcc dot gnu.org 2014-05-28 12:24 ` glisse at gcc dot gnu.org 2014-06-04 5:48 ` dl.soluz at gmx dot net 2014-06-04 6:21 ` glisse at gcc dot gnu.org 2021-11-14 14:19 ` dl.soluz at gmx dot net 2023-05-30 18:43 ` pinskia at gcc dot gnu.org
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