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From: "glisse at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/58483] missing optimization opportunity for const std::vector compared to std::array
Date: Wed, 28 May 2014 12:24:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58483

Marc Glisse <glisse at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2014-05-28
     Ever confirmed|0                           |1

--- Comment #6 from Marc Glisse <glisse at gcc dot gnu.org> ---
With current trunk, for the vector version, the most obvious optimization
failure is the following:

  _57 = (unsigned long) &MEM[(void *)&._79 + 12B];
  _36 = (unsigned long) &MEM[(void *)&._79 + 4B];
  _51 = _57 - _36;
  _3 = _51 /[ex] 4;
  _26 = _3;
  _27 = _26 + 1;
  _31 = _27 * 4;
etc.

_51 should be folded to 8 (then we have the usual useless /4*4, but with a
constant it would be ok). This part is a 4.9/4.10 regression, gcc-4.8 managed
to get the constant 12:
  __builtin_memcpy (_33, &._80, 12);