[Bug target/54089] [SH] Refactor shift patterns

["olegendo at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54089

--- Comment #25 from Oleg Endo <olegendo at gcc dot gnu.org> 2012-11-07 23:31:20 UTC ---
Created attachment 28633
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28633
Arithmetic right shift rework 2

This could be an alternative approach for the arith right shifts.
Applied to rev 193240:

CSiBE -m4-single -O2 -ml -mpretend-cmove
sum:  3159747 -> 3160047    +300 / +0.009494 %

CSiBE -m2 -O2 -ml
sum:  3314907 -> 3313319    -1588 / -0.047905 %

The idea here is to let arith shifts initially go through a slightly more
complex pattern 'ashrsi3_bridge_const', which allows the lib function address
or shad constant to be CSE'ed early on.
As mentioned in comment #18, the arith -> logical shift conversion is done by
combine only under certain circumstances.  After some trial and error I arrived
at the 'ashrsi3_bridge_const' pattern, which makes combine convert most of the
arith shifts into logical shifts.  In some cases it also finds more
opportunities, but regressions like this one still remain:

void MC_put_xy_16_c
(uint8_t * dest, const uint8_t * ref, const int stride, int height)
{
  // must see a shlr instead of 2x shar.
  // somehow the loop prevents this...
  do
  {
    dest[0] = (((ref[0]+ref[0 +1]+(ref+stride)[0]+(ref+stride)[0 +1]+2)>>2));
    ref += stride; dest += stride;
  } while (--height);
}

In addition to that, some unlucky register allocations appear, which cause
unneeded reg-reg copies (it seems something has trouble utilizing commutative
insns such as 'add').

The other problem is that patterns such as:

(define_insn_and_split "*lshrsi3"
  [(set (match_operand:SI 0 "arith_reg_dest")
    (lshiftrt:SI (match_operand:SI 1 "arith_reg_operand")
             (match_operand:SI 2 "shift_count_operand")))
   (clobber (reg:SI T_REG))
   (use (match_operand:SI 3 "general_operand"))
   (use (match_operand:SI 4 "general_operand"))]
  "TARGET_SH1 && can_create_pseudo_p () && CONST_INT_P (operands[2])"
  "#"
  "&& 1"
  [(const_int 0)]
{
  emit_insn (gen_lshrsi3 (operands[0], operands[1], operands[2]));
  DONE;
})

have to be added for combine to convert more arith -> logical shifts.  In this
patch I've added only one such pattern.  However, if for example zero_extract
patterns are added, variations such as above would be required, too.  This
looks discomforting to me.  Maybe the better solution would indeed be to add a
arith -> logical shift conversion pass before combine, or try to convert arith
shifts in the split pass after combine by looking at the data flow of the
shifted values.