[Bug target/34789] [avr] sometimes the compiler keeps addresses in registers unnecessarily

["hutchinsonandy at aim dot com" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #1 from hutchinsonandy at aim dot com  2008-02-22 01:22 -------
This appears to be due to avr_rtx_costs not assigning same cost to SYMBOL_REF
and CONST_INT. So SYMBOL_REF looks expensive - so is held in register to avoid
"recalculating" it.

Quick change to make SYMBOL_REF same cost as CONST_INT in both avr_rtx_cost and
avr_operand_rtx_cost gave the desired result 

  22                    /* prologue: function */
  23                    /* frame size = 0 */
  24                    .LM2:
  25 0000 E091 0000             lds r30,a
  26 0004 F091 0000             lds r31,(a)+1
  27 0008 EE0F                  lsl r30
  28 000a FF1F                  rol r31
  29 000c E050                  subi r30,lo8(-(data))
  30 000e F040                  sbci r31,hi8(-(data))
  31 0010 8081                  ld r24,Z
  32 0012 9181                  ldd r25,Z+1
  33 0014 0E94 0000             call foo
  34                    .LM3:
  35 0018 FC01                  movw r30,r24
  36 001a EE0F                  lsl r30
  37 001c FF1F                  rol r31
  38 001e E050                  subi r30,lo8(-(data))
  39 0020 F040                  sbci r31,hi8(-(data))
  40 0022 8081                  ld r24,Z
  41 0024 9181                  ldd r25,Z+1
  42 0026 0E94 0000             call foo
  43                    /* epilogue start */
  44                    .LM4:
  45 002a 0895                  ret

CONST and LABEL_REF might also have same problem as they cost same as MEM.

But note, if SYMBOL_REF were part of memory address, then it might be better
held in register (like Y or Z) - this should be done by checking outer code.
With outer code of "MEM" SYMBOL_REF would be more expensive than register.
(which might same a few LDS/STS that appear in code)

Avr_rtx_cost needs some serious work done to correct these are other anomalies
in cost assumptions and the recursion on operand costs.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34789