Re: RFC -- update_equiv_regs and friends

[Joern Rennecke <amylaar at cambridge dot redhat dot com>]

> My time is very limited, so I can't take on any large scale development
> work to solve this problem.  Thus, I'm thinking seriously about A, C, D,
> E & f.

I vote for G: prune the equivalences while they are made in a
machine-dependent manner.  That can then also takew care of situations
where a constant is more expensive to rematerialize on a particular target
than it is in general.

Something like:

Index: local-alloc.c
===================================================================
RCS file: /cvs/cvsfiles/devo/gcc/local-alloc.c,v
retrieving revision 1.114
retrieving revision 1.114.6.2
diff -p -r1.114 -r1.114.6.2
*** local-alloc.c	2000/12/29 21:07:17	1.114
--- local-alloc.c	2001/02/03 04:31:03	1.114.6.2
*************** function_invariant_p (x)
*** 798,803 ****
--- 798,807 ----
     into the using insn.  If it succeeds, we can eliminate the register
     completely.  */
  
+ #ifndef LEGITIMATE_EQUIVALENCE
+ #define LEGITIMATE_EQUIVALENCE(X) (! rtx_varies_p (X))
+ #endif
+ 
  static void
  update_equiv_regs ()
  {
*************** update_equiv_regs ()
*** 944,950 ****
  
        if (REG_N_SETS (regno) != 1
  	  && (! note
! 	      || rtx_varies_p (XEXP (note, 0))
  	      || (reg_equiv[regno].replacement
  		  && ! rtx_equal_p (XEXP (note, 0),
  				    reg_equiv[regno].replacement))))
--- 948,954 ----
  
        if (REG_N_SETS (regno) != 1
  	  && (! note
! 	      || ! LEGITIMATE_EQUIVALENCE (XEXP (note, 0))
  	      || (reg_equiv[regno].replacement
  		  && ! rtx_equal_p (XEXP (note, 0),
  				    reg_equiv[regno].replacement))))
*************** update_equiv_regs ()
*** 958,964 ****
  
        /* If this register is known to be equal to a constant, record that
  	 it is always equivalent to the constant.  */
!       if (note && ! rtx_varies_p (XEXP (note, 0)))
  	PUT_MODE (note, (enum machine_mode) REG_EQUIV);
  
        /* If this insn introduces a "constant" register, decrease the priority
--- 962,968 ----
  
        /* If this register is known to be equal to a constant, record that
  	 it is always equivalent to the constant.  */
!       if (note && LEGITIMATE_EQUIVALENCE (XEXP (note, 0)))
  	PUT_MODE (note, (enum machine_mode) REG_EQUIV);
  
        /* If this insn introduces a "constant" register, decrease the priority

Only the first punk would probably go into defaults.h, and we'd document
the stuff in tm.texi.