[Bug target/49421] New: [arm] suboptimal choice of working regs

[philb at gnu dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49421

           Summary: [arm] suboptimal choice of working regs
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: philb@gnu.org


If a leaf function requires one more working register than can be accomodated
in the call-clobbered set, gcc currently tends to push r4 and use that next. 
However, in the specific case of a leaf function, it would be better to push lr
and use that as the working register, since then the return can be done with a
single pop.  Consider the made-up example:

int f(int *a, int *b, int *c, int *d)
{
  int i;
  for (i = 0; i < 4; i++)
    if (a[i] || b[i] || c[i] || d[i])
      return 1;

  return 0;
}

which compiles (-march=armv6 -mtune=arm1136jf-s -O2) to:

f:
    @ args = 0, pretend = 0, frame = 0
    @ frame_needed = 0, uses_anonymous_args = 0
    @ link register save eliminated.
    mov    ip, #0
    str    r4, [sp, #-4]!
.L3:
    ldr    r4, [r0, ip]
    cmp    r4, #0
    bne    .L7
    ldr    r4, [r1, ip]
    cmp    r4, #0
    bne    .L7
    ldr    r4, [r2, ip]
    cmp    r4, #0
    bne    .L7
    ldr    r4, [r3, ip]
    add    ip, ip, #4
    cmp    r4, #0
    bne    .L7
    cmp    ip, #16
    bne    .L3
    mov    r0, r4
.L2:
    ldmfd    sp!, {r4}
    bx    lr
.L7:
    mov    r0, #1
    b    .L2

If lr had been pushed instead of r4 then the return could have simply been "pop
{lr}".

Also, since this is arm11, it is no more expensive to push two words than one. 
If the compiler had stacked both r4 and lr, it would have freed up an extra
register for the loop which would probably have allowed the loads to be
scheduled better.