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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/64121] [5 Regression] ICE: SSA corruption with -O -fsanitize=undefined
Date: Mon, 01 Dec 2014 10:00:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64121

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
  # xp_1(ab) = PHI <xp_3(2), xp_7(ab)(4)>
base:
  if (x_5(D) != 0)
    goto <bb 5>;
  else
    goto <bb 4>;

  <bb 4>:
  xp.0_6 = xp_1(ab);
  xp_7(ab) = xp.0_6 + 4;
  _15 = __builtin_object_size (ip_8(D), 0);
  _16 = ip_8(D);
  UBSAN_OBJECT_SIZE (_16, 4, _15, 0);
  _9 = *ip_8(D);
  _17 = __builtin_object_size (xp_1(ab), 0);
...
 _13 = &base + _12;
  gotovar.1_14 = _13;
  goto gotovar.1_14;


xp_1 and xp_7 are life at the same time - the above is from 017t.ubsan.

You may not refer to xp_1(ab) but should use xp.0_6 here.