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Re: New optimization for reload_combine


On 07/16/2010 12:34 PM, Bernd Schmidt wrote:
This fixes one subproblem of PR42235.  In reload_combine, we recognize a
new pattern:

  REG1 = REG2 + CONST
  ...
  something MEM (REG1)

and try to use the addition in the MEM directly, moving the add insn
downwards if REG1 and REG2 are identical.  This has two benefits: it may
be possible to eliminate the add, and the load no longer depends on the
add instruction and may schedule sooner.

This can make the generated code quite a lot nicer:

-       adds    r3, r2, #2
         strb    r5, [r2, #0]
-       strb    r5, [r3, #0]
-       adds    r3, r2, #3
+       strb    r5, [r2, #2]
         strb    r5, [r2, #1]
-       strb    r5, [r3, #0]
-       adds    r3, r2, #4
+       strb    r5, [r2, #3]
         lsrs    r1, r1, #11
-       strb    r5, [r3, #0]
-       adds    r3, r2, #5
+       strb    r5, [r2, #4]
         add     r1, r1, r1, lsl #1
-       strb    r5, [r3, #0]
+       strb    r5, [r2, #5]

Nice. :) I suppose fwprop doesn't do it because the memory accesses are not present before reload?


Can you make the change dependent on

bool speed = optimize_bb_for_speed_p (BLOCK_FOR_INSN (insn));

  /* Prefer the new address if it is less expensive.  */
  gain = (address_cost (old_rtx, mode, as, speed)
          - address_cost (new_rtx, mode, as, speed));

  /* If the addresses have equivalent cost, prefer the new address
     if it has the highest `rtx_cost'.  That has the potential of
     eliminating the most insns without additional costs, and it
     is the same that cse.c used to do.  */
  if (gain == 0)
    gain = rtx_cost (new_rtx, SET, speed) - rtx_cost (old_rtx, SET, speed);

return (gain > 0);

as in fwprop (in turn taken from CSE)?

Does postreload.c fall under reload? If not, would someone approve this?

Well, it was part of reload1.c until a while ago...


Paolo


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