RTL sharing problem

[Jeffrey A Law <law at cygnus dot com>]

Boy another fun bug.  It's amazing how many interesting problems the PA64 port
is exposing.

copy_rtx_if_shared allowed a MEM of this form to be shared:

(mem (virtual_stack_vars_rtx)

Virtual register instantiation changed the shared copy (but did not unshare
the node) into

(mem (plus (fp) (const_int))


We had two references to this shared node in the insn chain.  During
instruction combination we ended up turning the mem into something like

(mem (ior (fp) (const_int)) inside a larger RTL expression.


Then we broke out the (ior (fp) (const_int) using the generic splitting
code in the combiner.  This resulted in a valid 3->2 combination with
the first insn looking like
(insn X Y Z (set (dest) (ior (fp) (const_int)))

Which was perfectly fine in the context of the 3->2 combination; however
it produced an unrecognizable insn in the other reference to the shared
nodes -- (mem (ior (...)) is not a valid memory reference...


This patch fixes the problem by unsharing nodes which reference the
special registers.  It may be possible to delete some code in
instantiate_virtual_registers_1 because of this change -- I'm still
evaluating the potential cleanup.



	* emit-rtl.c (copy_rtx_if_shared): A MEM which references
	virtual_stack_vars_rtx or virtual_incoming_args_rtx can not
	be shared.
	
Index: emit-rtl.c
===================================================================
RCS file: /cvs/cvsfiles/devo/gcc/emit-rtl.c,v
retrieving revision 1.136
diff -c -3 -p -r1.136 emit-rtl.c
*** emit-rtl.c	1999/08/24 05:11:25	1.136
--- emit-rtl.c	1999/08/31 06:19:01
*************** copy_rtx_if_shared (orig)
*** 1695,1710 ****
        return x;
  
      case MEM:
!       /* A MEM is allowed to be shared if its address is constant
! 	 or is a constant plus one of the special registers.  */
!       if (CONSTANT_ADDRESS_P (XEXP (x, 0))
! 	  || XEXP (x, 0) == virtual_stack_vars_rtx
! 	  || XEXP (x, 0) == virtual_incoming_args_rtx)
  	return x;
  
        if (GET_CODE (XEXP (x, 0)) == PLUS
- 	  && (XEXP (XEXP (x, 0), 0) == virtual_stack_vars_rtx
- 	      || XEXP (XEXP (x, 0), 0) == virtual_incoming_args_rtx)
  	  && CONSTANT_ADDRESS_P (XEXP (XEXP (x, 0), 1)))
  	{
  	  /* This MEM can appear in more than one place,
--- 1695,1712 ----
        return x;
  
      case MEM:
!       /* A MEM is allowed to be shared if its address is constant.
! 
! 	 We used to allow sharing of MEMs which referenced 
! 	 virtual_stack_vars_rtx or virtual_incoming_args_rtx, but
! 	 that can lose.  instantiate_virtual_regs will not unshare
! 	 the MEMs, and combine may change the structure of the address
! 	 because it looks safe and profitable in one context, but
! 	 in some other context it creates unrecognizable RTL.  */
!       if (CONSTANT_ADDRESS_P (XEXP (x, 0)))
  	return x;
  
        if (GET_CODE (XEXP (x, 0)) == PLUS
  	  && CONSTANT_ADDRESS_P (XEXP (XEXP (x, 0), 1)))
  	{
  	  /* This MEM can appear in more than one place,