[preliminary patch] Optimize mov in h8300 port (revised!)

[Kazu Hirata <kazu at hxi dot com>]

Hi,

> Or one could write this in a generic way and have it controlled by
> the cost of the various ways to load up the constant.

I started to tweak const_costs() in config/h8300/h8300.c.  Is the
attached patch in the right direction?

As you suggested, I am looking at how well newlib gets compiled.

Under stdio, stdlib, and math, I counted the number of lines in the
assembly output (direct from gcc, not objdump).  I got:

         stdio stdlib  math
original  8059  11615 33319
patched   8034  11591 33319

So I see some marginal improvement.  I see both good and bad things
happening.  The math library did change; it is just a coincidence that
the number of lines stayed the same.  Here are some noteworthy
differences.

Good (1):

This is due to reload_cse_move2add.  (Thank you, Jeff, for letting me
know this!)  This is a kind of improvement that does not show up in
the number of assembly instructions.  mov.w takes 4 bytes, and adds 2
bytes.

-       mov.w   #1,r3
+       adds    #1,r3

Good (2):

Sometimes knowing a reg being 0 results in a fewer branches.  A common
scenario would be a function having a sanity check at its beginning
like "if (p == NULL) return 0;".  I see the same happen for -1 as
well.

        mov.w   r0,r0
        beq     .L3
        mov.w   #1,r0
-       bra     .L2
 .L3:
-       sub.w   r0,r0
-.L2:
        rts

Bad (1):

Sometimes a desire to reuse a reg to save constant-loading time
results in more register usage (and more longevity!?).  The following
function simply got worse with other differences in the function
having no effect on either speed or space.

@@ -10,8 +10,9 @@
 __init_signal_r:
        push    r4
+       push    r5
        mov.w   r0,r4

Bad (2):

I haven't figured out how this is caused.  Ugly.

-       btst    #1,r2l
+       mov.w   r2,r5
+       and     #2,r5l
+       and     #0,r5h
+       mov.w   r5,r5

Bad (3):

Uglier.  matherr() should just return 0.  Haven't chased this one,
either.

 _matherr:
+       mov.w   @(6,r0),r1
+       mov.w   @(4,r0),r0
+       mov.w   r1,@-r7
+       mov.w   r0,@-r7
+       jsr     @___nesf2
+       adds    #2,r7
+       adds    #2,r7
+       mov.w   r0,r0
+       beq     .L3
        sub.w   r0,r0
+.L3:
        rts

Here is the patch for config/h8300/h8300.c.  No change in h8300.md! :-)

Kazu Hirata

@@ -852,9 +872,10 @@ function_arg (cum, mode, type, named)
 /* Return the cost of the rtx R with code CODE.  */
 
 int
-const_costs (r, c)
+const_costs (r, c, outer_code)
      rtx r;
      enum rtx_code c;
+     enum rtx_code outer_code;
 {
   switch (c)
     {
@@ -862,19 +883,19 @@ const_costs (r, c)
       switch (INTVAL (r))
        {
        case 0:
+         return (outer_code == SET) ? 1 : 0;
        case 1:
        case 2:
        case -1:
        case -2:
-         return 0;
+         return (outer_code == SET) ? 2 : 1;
        case 4:
        case -4:
          if (TARGET_H8300H || TARGET_H8300S)
-           return 0;
-         else
-           return 1;
+           return (outer_code == SET) ? 2 : 1;
+         /* Fall through.  */
        default:
-         return 1;
+         return 2;
        }
 
     case CONST: