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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/50037] Unroll factor exceeds max trip count
Date: Wed, 10 Aug 2011 15:42:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50037

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rakdver at gcc dot gnu.org

--- Comment #3 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-08-10 15:42:19 UTC ---
Confirmed with

int foo (int n, int *addr)
{
  int count, sum;
  for ( count = n & 0x3; count > 0; count--, addr++ )
    sum += *addr;
  return sum;
}

and -O2 -funroll-loops.

Using

int foo (int n, int *addr)
{
  int count, sum;
  for ( count = n & 0x3; count >= 0; count--, addr++ )
    sum += *addr;
  return sum;
}

it works.  It looks like RTL number of iteration analysis is confused
by the adjustment

(insn 28 27 29 3 (parallel [
            (set (reg:SI 92)
                (plus:SI (reg/v:SI 77 [ count ])
                    (const_int -1 [0xffffffffffffffff])))
            (clobber (reg:CC 17 flags))
        ]) t.c:1 251 {*addsi_1}
     (expr_list:REG_DEAD (reg/v:SI 77 [ count ])
        (expr_list:REG_UNUSED (reg:CC 17 flags)
            (nil))))

neither works with

int foo (int n, int *addr)
{
  int count, sum;
  for ( count = 0; count < n & 0x3; count++, addr++ )
    sum += *addr;
  return sum;
}

or

int foo (int n, int *addr)
{
  int count, sum;
  for ( count = 0; count <= n & 0x3; count++, addr++ )
    sum += *addr;
  return sum;
}