Allocating scratch register

Allocating scratch register

[Boris Boesler]

Hi!

  I'm trying to allocate a scratch register: write immediate constant  
into scratch register r, write register r into memory

;; write imm into memory
(define_insn_and_split "mov<mode>_imm_by_store"
   [(set (match_operand:I8I16 0 "memory_operand"    "=m")
	(match_operand:I8I16 1 "immediate_operand" " i"))
   (clobber (match_scratch:I8I16 2 "=r"))]
   ""
   "#"
   ""
   [(parallel
     [(set (match_dup 2) (match_dup 1))
      (set (match_dup 0) (match_dup 2))])]
   ""
)

  I found that in a mips back-end. But this pattern is not recognized  
during code-generation [char c1; c1 = 1;]:
simple-memory.c:19: error: unrecognizable insn:
(insn 12 11 14 3 (set (mem/c/i:QI (reg/f:SI 105) [0 c1+0 S8])
         (const_int 1 [0x1])) -1 (nil)
     (nil))

  If I remove the clobber command and replace (match_dup 2) by  
(reg:I8I16 A15_REGNUM) code will be generated (but not as wanted).

  What is wrong with the code above?

Thanks,
Boris

Re: Allocating scratch register

[Ian Lance Taylor]

Boris Boesler <baembel@gmx.de> writes:

>   I'm trying to allocate a scratch register: write immediate constant  
> into scratch register r, write register r into memory
> 
> ;; write imm into memory
> (define_insn_and_split "mov<mode>_imm_by_store"
>    [(set (match_operand:I8I16 0 "memory_operand"    "=m")
> 	(match_operand:I8I16 1 "immediate_operand" " i"))
>    (clobber (match_scratch:I8I16 2 "=r"))]
>    ""
>    "#"
>    ""
>    [(parallel
>      [(set (match_dup 2) (match_dup 1))
>       (set (match_dup 0) (match_dup 2))])]
>    ""
> )
> 
>   I found that in a mips back-end. But this pattern is not recognized  
> during code-generation [char c1; c1 = 1;]:
> simple-memory.c:19: error: unrecognizable insn:
> (insn 12 11 14 3 (set (mem/c/i:QI (reg/f:SI 105) [0 c1+0 S8])
>          (const_int 1 [0x1])) -1 (nil)
>      (nil))
> 
>   If I remove the clobber command and replace (match_dup 2) by  
> (reg:I8I16 A15_REGNUM) code will be generated (but not as wanted).
> 
>   What is wrong with the code above?

There is nothing wrong with that code, but nothing is going to make
the compiler use it.  Moves are special.  If you need a scratch
register to do a move, then you need to look at the
TARGET_SECONDARY_RELOAD hook.

But if the problem is only that you need a register to store a
constant into memory, then you should be able to do that using
register constraints on your mov<mode> insn.

Ian

Re: Allocating scratch register

[Boris Boesler]

Hi!

Am 09.01.2008 um 23:54 schrieb Ian Lance Taylor:

> Boris Boesler <baembel@gmx.de> writes:
>
>>   I'm trying to allocate a scratch register: write immediate constant
>> into scratch register r, write register r into memory
...
>>   What is wrong with the code above?
>
> There is nothing wrong with that code, but nothing is going to make
> the compiler use it.  Moves are special.

  Yes, I can remember that constraints in a mov-insn can not be  
resolved by other/additional mov-insns.

  Ok, I rewrote my insns; there was a "mov<mode>"-insn with mode \in  
{QI, HI, SI}:

(define_insn_and_split "movqi"
   [(set (match_operand:QI 0 "reg_mem_operand" "=mr")
	(match_operand:QI 1 "rim_operand"     " mir"))
   (clobber (match_scratch:QI 2 "=r"))
  ]
   ""
   "#"
   "&& reload_completed"
   [(set (match_dup 2) (match_dup 1))
    (set (match_dup 0) (match_dup 2))]
   "")

;; help insns
(define_insn "mov<mode>_reg_by_store"
   [(set (match_operand:I8I16 0 "memory_operand"   "=  m")
	(match_operand:I8I16 1 "register_operand" " rAx"))]
   ""
   "bla bla bla")

(define_insn "mov<mode>_reg_imm"
   [(set (match_operand:I8I16 0 "register_operand"   "=rAx, rAx, rAx")
	(match_operand:I8I16 1 "immediate_operand"  "   I,   i, rAx"))]
   ""
   "some other bla bla bla")

  These insns handle byte-accesses as expected. But now a previous  
tests fail for something like ashift:SI(reg:SI, const_int:QI 5):
p.c:36: error: unrecognizable insn:
(insn 114 18 115 (set (scratch:QI)
         (const_int 5 [0x5])) -1 (nil)
     (nil))

  I do not understand why this happens does not happen in my byte- 
access example. So I added the two insns (I8I16 = {QI, HI}):

(define_insn "mov<mode>_to_scratch"
   [(set (match_scratch:I8I16 0                   "=r,r")
	(match_operand:I8I16 1 "reg_imm_operand" " r,i"))]
   ""
   "again bla bla bla")

(define_insn "mov<mode>_from_scratch"
   [(set (match_operand:I8I16 0 "register_operand" "=r")
	(match_scratch:I8I16 1                    " r"))]
   ""
   "even more bla bla bla")

  But now the compiler says:
p.c:36: error: insn does not satisfy its constraints:
(insn 114 18 115 (set (scratch:QI)
         (const_int 5 [0x5])) 79 {movqi_to_scratch} (nil)
     (nil))
p.c:36: internal compiler error: in final_scan_insn, at final.c:2382

  The two insns in the dump file .shorten are:

(insn 114 18 115 (set (scratch:QI)
         (const_int 5 [0x5])) 79 {movqi_to_scratch} (nil)
     (nil))

(insn 115 114 19 (set (reg:QI 31 R31)
         (scratch:QI)) 81 {movqi_from_scratch} (nil)
     (nil))

  I can't see what's wrong. I guess I don't understand that scratch- 
stuff in gcc?

Thanks,
Boris

Re: Allocating scratch register

[Paul Brook]

>   Yes, I can remember that constraints in a mov-insn can not be
> resolved by other/additional mov-insns.

I think you're doing this the wrong way. You don't have a i->m mov 
instruction, so why are you pretending you do?

Why aren't you doing this the same way as pretty much every other target? 
i.e.:

(define_insn "*movqi_insn"
   [(set (match_operand:QI 0 "reg_mem_operand" "=r,m")
        (match_operand:QI 1 "rim_operand"     " mi,r"))
 ""
 "mov %0,%1"
)

(define_expand "movqi"
  [(set (match...) (match...)]
 ""
 "
 if (GET_CODE (operands[0]) == MEM)
   operands[1] = force_reg (QImode, operands[1]);
  "
)

Plus the appropriate *_RELOAD_CLASS macros to keep reload happy.

Paul