[PATCH] Add new target-hook truncated_to_mode

[Paolo Bonzini]

> +mips_truncated_to_mode (enum machine_mode mode, rtx x)
> +{
> +  gcc_assert (!TRULY_NOOP_TRUNCATION (GET_MODE_BITSIZE (mode),
> +				      GET_MODE_BITSIZE (GET_MODE (x)))
> +	      && GET_MODE (x) == DImode);
> +
> +  return num_sign_bit_copies (x, GET_MODE (x)) >= 33;

I may be wrong as I still have to understand the magic of TRUNCATE, but 
I don't think a target hook is necessary, as this logic is completely 
generic as far as I can tell.  Why not this:

bool
truncated_to_mode (enum machine_mode mode, rtx x)
{
   if (REG_P (x) && rtl_hooks.reg_truncated_to_mode (mode, x))
     return true;

   gcc_assert (!TRULY_NOOP_TRUNCATION (GET_MODE_BITSIZE (mode),
                                       GET_MODE_BITSIZE (GET_MODE (x)));
   return num_sign_bit_copies (x, GET_MODE (x)) >
	 GET_MODE_BITSIZE (GET_MODE (x)) - GET_MODE_BITSIZE (mode);
}

In the MIPS case, you would have n_s_b_c (x, GET_MODE (x)) > 64 - 32.

Paolo