Control register spilling: Is it possible ?

["Petar Penchev" <ptr at melexis dot com>]

I am sorry if somebody gets this e-mail for second time, but I did not get  
a copy of it from GCC.

Here is the mail's body:
--------------------------------------



Hello all,
I have a problem with "movsi" pattern. The GCC target, I am workin on,  
does not support SImode directly. It is being splited to two "movhi"  
patterns using
scratch register, but I experiance problems when I try to compile newlib's  
qsort.c. When I try to do that gcc says :
../../../../../newlib_src/newlib/libc/search/qsort.c:222: error: unable to  
finda register to spill in class `CL_NDX_REGS'

the "movsi" pattern is:

(define_insn_and_split "movsi_internal"
  [(set (match_operand:SI 0 "nonimmediate_nonstack_operand" "=m,m, m,b, b")
(match_operand:SI 1 "general_operand"        "m,i, b,im,b"))
  (clobber (match_scratch:HI 2                   "=&a,&b,X, X,X"))]
  ""
  "#"
  "reload_completed"
  [(const_int 0)]
  "split_move (operands[0], operands[1], operands[2]);
   DONE;"
)

and the error message is related to the first alternative which defines  
move from memory to memory.
I tried to write RTL for alternative 0 , which pushes and pops the  
registers I need but the problem remained.

changed pattern looks like :

(define_insn_and_split "movsi_internal"
  [(set (match_operand:SI 0 "nonimmediate_nonstack_operand" "=m,m, m,b, b")
(match_operand:SI 1 "general_operand"        "m,i, b,im,b"))
  (clobber (match_scratch:HI 2                   "=&a,&b,X, X,X"))]
  ""
  "#"
  "reload_completed"
  [
; push X and Y registers
    (set (mem:HI (post_inc:HI (reg:HI S_HREG)))
          (reg:HI X_HREG))

    (set (mem:HI (post_inc:HI (reg:HI S_HREG)))
          (reg:HI Y_HREG))
; save scratch register just in case
   (set (mem:HI (post_inc:HI (reg:HI S_HREG)))
          (reg:HI A_HREG))
; move values
   (set (reg:HI Y_HREG) (match_dup 0))
   (set (reg:HI X_HREG) (match_dup 1))

   (set (reg:HI A_HREG) (mem:HI (reg:HI X_HREG)))
   (set (mem:HI (reg:HI Y_HREG)) (reg:HI A_HREG))

   (set (reg:HI A_HREG) (mem:HI (plus:HI (reg:HI X_HREG) (const_int 2) )))
  (set (mem:HI (plus:HI (reg:HI Y_HREG) (const_int 2)))  (reg:HI A_HREG))
; restore registers from stack

   (set (reg:HI A_HREG)
       (mem:HI (pre_dec (reg:HI S_HREG))))
   (set (reg:HI Y_HREG)
       (mem:HI (pre_dec (reg:HI S_HREG))))
   (set (reg:HI X_HREG)
       (mem:HI (pre_dec (reg:HI S_HREG))))
  ]
  "
    if (which_alternative!=0)
    {
      split_move (operands[0], operands[1], operands[2]);
      DONE;
    }"
)

X_HREG and Y_HREG are index registers A_HREG is used as scretch register.  
The error I get is related to X and Y regs.
Is it possible to control register splilling in RTL, i.e. when I push X  
reg to mark it as available for further usage ?
I am not so sure that this is the best way to handle this problem, so if  
anybody has any idea about it please let me know.

Regards Petar