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Date: Mon, 07 Feb 2005 19:36:00 -0000
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From: "kazu at cs dot umass dot edu" <gcc-bugzilla@gcc.gnu.org>
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Reply-To: gcc-bugzilla@gcc.gnu.org
Subject: [Bug tree-optimization/19804] Missed jump threading opportunity on "else" arm of COND_EXPR
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------- Additional Comments From kazu at cs dot umass dot edu  2005-02-07 06:45 -------
Here is a test case where a jump threading opportunity would be missed
if the change suggested in the original post were made:

void
foo (int *p)
{
  if (*p != 0)
    bar ();

  if (*p != 0)
    bar ();
}

Here is a tree dump right before the first DOM.

foo (p)
{
  int D.1120;

<bb 0>:
  D.1120_2 = *p_1;
  if (D.1120_2 != 0) goto <L0>; else goto <L1>;

<L0>:;
  bar ();

<L1>:;
  D.1120_3 = *p_1;
  if (D.1120_3 != 0) goto <L2>; else goto <L3>;

<L2>:;
  bar ();

<L3>:;
  return;

}

The change I suggested would record "D.1120_2 == 2" while following
edge from <L0> to <L2>.

Then th jump threading selection code figures out that

  D.1120_3 = *p_1;

is the same as

  D.1120_3 = 0;

via equality "D.1120_3 == D.1120_2".

For whatever reason, the jump threading selection code does not want to
see anything but SSA_NAME on the rhs.

At that point, DOM throws away this jump threading opportunity.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19804