RE: RFA: Make LRA temporarily eliminate addresses before testing constraints

[Robert Suchanek <Robert dot Suchanek at imgtec dot com>]

Pinging for approval.

This part of the patch will be needed for MIPS16. The second part to enable
LRA in MIPS has been already approved.

> Hi Richard, 
> 
> >> Robert: you also had an LRA change, but is it still needed after this one?
> >> If so, could you repost it and explain the case it handles? 
>
> For just turning the LRA for the MIPS backend is not needed but we have issues
> with the code size for MIPS16. LRA inserted a lot of reloads and the code size
> increased on average by about 10% IIRC. To fix this, a number of patterns 
> have to accept the stack pointer and a new class, M16_SP_REGS with 
> M16_REGS + $sp was added.
> 
> However, this triggered a reloading problem as the stack pointer was rejected
> by the back end and LRA tried to insert base+disp with the displacement not
> always present. It only affects $sp not directly accessible as in MIPS16 case.
>
> Regards,
> Robert
>         
> gcc/                                                      
> 	* lra-constraints.c (base_to_reg): New function.       
> 	(process_address): Use new function.                   
> 
> diff --git gcc/lra-constraints.c gcc/lra-constraints.c
> index 08716fe..d5ed37f 100644
> --- gcc/lra-constraints.c
> +++ gcc/lra-constraints.c
> @@ -2686,6 +2686,39 @@ process_alt_operands (int only_alternative)
>    return ok_p;
>  }
>  
> +/* Make reload base reg from address AD.  */
> +static rtx
> +base_to_reg (struct address_info *ad)
> +{
> +  enum reg_class cl;
> +  int code = -1;
> +  rtx new_inner = NULL_RTX;
> +  rtx new_reg = NULL_RTX;
> +  rtx insn;
> +  rtx last_insn = get_last_insn();
> +
> +  lra_assert (ad->base == ad->base_term && ad->disp == ad->disp_term);
> +  cl = base_reg_class (ad->mode, ad->as, ad->base_outer_code,
> +                       get_index_code (ad));
> +  new_reg = lra_create_new_reg (GET_MODE (*ad->base_term), NULL_RTX,
> +                                cl, "base");
> +  new_inner = simplify_gen_binary (PLUS, GET_MODE (new_reg), new_reg,
> +                                   ad->disp_term == NULL
> +                                   ? gen_int_mode (0, ad->mode)
> +                                   : *ad->disp_term);
> +  if (!valid_address_p (ad->mode, new_inner, ad->as))
> +    return NULL_RTX;
> +  insn = emit_insn (gen_rtx_SET (ad->mode, new_reg, *ad->base_term));
> +  code = recog_memoized (insn);
> +  if (code < 0)
> +    {
> +      delete_insns_since (last_insn);
> +      return NULL_RTX;
> +    }
> +
> +  return new_inner;
> +}
> +
>  /* Make reload base reg + disp from address AD.  Return the new pseudo.  */
>  static rtx
>  base_plus_disp_to_reg (struct address_info *ad)
> @@ -2908,6 +2941,8 @@ process_address_1 (int nop, rtx *before, rtx *after)
>  
>       3) the address is a frame address with an invalid offset.
>  
> +     4) the address is a frame address with an invalid base.
> +
>       All these cases involve a non-autoinc address, so there is no
>       point revalidating other types.  */
>    if (ad.autoinc_p || valid_address_p (&ad))
> @@ -2989,14 +3024,19 @@ process_address_1 (int nop, rtx *before, rtx *after)
>        int regno;
>        enum reg_class cl;
>        rtx set, insns, last_insn;
> +      /* Try to reload base into register only if the base is invalid
> +         for the address but with valid offset, case (4) above.  */
> +      start_sequence ();
> +      new_reg = base_to_reg (&ad);
> +
>        /* base + disp => new base, cases (1) and (3) above.  */
>        /* Another option would be to reload the displacement into an
>  	 index register.  However, postreload has code to optimize
>  	 address reloads that have the same base and different
>  	 displacements, so reloading into an index register would
>  	 not necessarily be a win.  */
> -      start_sequence ();
> -      new_reg = base_plus_disp_to_reg (&ad);
> +      if (new_reg == NULL_RTX)
> +        new_reg = base_plus_disp_to_reg (&ad);
>        insns = get_insns ();
>        last_insn = get_last_insn ();
>        /* If we generated at least two insns, try last insn source as