Re: [Patch, RTL] Eliminate redundant vec_select moves.

[Richard Sandiford]

Tejas Belagod <tbela...@arm.com> writes:
> +  /* This is big-endian-safe because the elements are kept in target
> +     memory order.  So, for eg. PARALLEL element value of 2 is the same in
> +     either endian-ness.  */
> +  if (GET_CODE (src) == VEC_SELECT
> +      && REG_P (XEXP (src, 0)) && REG_P (dst)
> +      && REGNO (XEXP (src, 0)) == REGNO (dst))
> +    {
> +      rtx par = XEXP (src, 1);
> +      int i;
> +
> +      for (i = 0; i < XVECLEN (par, 0); i++)
> +     {
> +       rtx tem = XVECEXP (par, 0, i);
> +       if (!CONST_INT_P (tem) || INTVAL (tem) != i)
> +         return 0;
> +     }
> +      return 1;
> +    }
> +

I think for big endian it needs to be:

    INTVAL (tem) != i + base

where base is something like:

    int base = GET_MODE_NUNITS (GET_MODE (XEXP (src, 0))) - XVECLEN (par, 0);

E.g. a big-endian V4HI looks like:

    msb          lsb
    0000111122223333

and shortening it to say V2HI only gives the low 32 bits:

            msb  lsb
            22223333

Thanks,
Richard