[Bug target/25111] New: [m68k] bset is not used for A = 1 << (B & 31) on ColdFire

[kazu at gcc dot gnu dot org]

Consider:

int bar (void);

int
foo (int b)
{
  int a = bar ();
  return a | (1 << (b & 31));
}

./cc1 -quiet -O2 -m5200 -fomit-frame-pointer generates

foo:
        move.l %d2,-(%sp)
        move.l 8(%sp),%d2
        jbsr bar
        moveq #31,%d1
        and.l %d1,%d2
        move.b #1,%d1
        lsl.l %d2,%d1
        or.l %d1,%d0
        move.l (%sp)+,%d2
        rts

Note that bset could be used in place of the five instructions between
moveq and or.l, inclusive.

Of course, the five instructions are too many for the combiner, but
the combiner already suggests an intermediate insn like so

(set (reg:SI 36)
     (ashift:SI (reg:SI 37)
                (and:SI (reg/v:SI 32 [ b ])
                        (const_int 31 [0x1f]))))

which we can implement using clr and bset.
Once this combination is established, going to

(set (reg:SI)
     (ior:SI (reg:SI)
             (ashift:SI (const_int 1)
                        (and:SI (reg:SI)
                                (const_int 31)))))

shouldn't be difficult.

Interestingly, if we feed A as an argument instead of a return value,
the combiner does not produce the ashift insn mentioned above.


-- 
           Summary: [m68k] bset is not used for A = 1 << (B & 31) on
                    ColdFire
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: kazu at gcc dot gnu dot org
GCC target triplet: m68k-elf


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25111