[Bug rtl-optimization/45434] x86 missed optimization: use high register (ah, bh, ch, dh) when available to make comparisons

[ubizjak at gmail dot com]

------- Comment #2 from ubizjak at gmail dot com  2010-08-29 16:40 -------
A bit simpler testcase:

int test(int i)
{
  return (i & 0xFF) == ((i & 0xFF00) >> 8);
}

fails two combine attempts:

Trying 8 -> 9:
Failed to match this instruction:
(set (reg:CCZ 17 flags)
    (compare:CCZ (zero_extract:SI (subreg:DI (reg/v:SI 63 [ i ]) 0)
            (const_int 8 [0x8])
            (const_int 8 [0x8]))
        (reg:SI 65)))

Trying 8, 6 -> 9:
Failed to match this instruction:
(set (reg:CCZ 17 flags)
    (compare:CCZ (subreg:QI (lshiftrt:SI (reg/v:SI 63 [ i ])
                (const_int 8 [0x8])) 0)
        (subreg:QI (reg/v:SI 63 [ i ]) 0)))

However, contrary to the first attempt, 8 -> 9 substitution in the second
attempt is over-optimized from ZERO_EXTRACT to LSHIFTRT. i386.md expects
ZERO_EXTRACT form.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45434