On Tue, Apr 23, 2019 at 10:25 AM Richard Sandiford
<richard.sandif...@arm.com> wrote:
>
> This patch explains the encoding used for the precision field in
> TYPE_VECTOR_SUBPARTS when NUM_POLY_INT_COEFFS == 2. OK to install?
OK.
Richard.
> Richard
>
>
> 2019-04-19 Richard Sandiford <richard.sandif...@arm.com>
>
> gcc/
> * tree.h (TYPE_VECTOR_SUBPARTS, SET_TYPE_VECTOR_SUBPARTS): Add
> commentary about the encoding of precision.
>
> Index: gcc/tree.h
> ===================================================================
> --- gcc/tree.h 2019-04-23 09:21:46.206208219 +0100
> +++ gcc/tree.h 2019-04-23 09:21:58.898166354 +0100
> @@ -3734,6 +3734,8 @@ TYPE_VECTOR_SUBPARTS (const_tree node)
> unsigned int precision = VECTOR_TYPE_CHECK (node)->type_common.precision;
> if (NUM_POLY_INT_COEFFS == 2)
> {
> + /* See the corresponding code in SET_TYPE_VECTOR_SUBPARTS for a
> + description of the encoding. */
> poly_uint64 res = 0;
> res.coeffs[0] = HOST_WIDE_INT_1U << (precision & 0xff);
> if (precision & 0x100)
> @@ -3756,6 +3758,21 @@ SET_TYPE_VECTOR_SUBPARTS (tree node, pol
> gcc_assert (index >= 0);
> if (NUM_POLY_INT_COEFFS == 2)
> {
> + /* We have two coefficients that are each in the range 1 << [0, 63],
> + so supporting all combinations would require 6 bits per coefficient
> + and 12 bits in total. Since the precision field is only 10 bits
> + in size, we need to be more restrictive than that.
> +
> + At present, coeff[1] is always either 0 (meaning that the number
> + of units is constant) or equal to coeff[0] (meaning that the number
> + of units is N + X * N for some target-dependent zero-based runtime
> + parameter X). We can therefore encode coeff[1] in a single bit.
> +
> + The most compact encoding would be to use mask 0x3f for coeff[0]
> + and 0x40 for coeff[1], leaving 0x380 unused. It's possible to
> + get slightly more efficient code on some hosts if we instead
> + treat the shift amount as an independent byte, so here we use
> + 0xff for coeff[0] and 0x100 for coeff[1]. */
> unsigned HOST_WIDE_INT coeff1 = subparts.coeffs[1];
> gcc_assert (coeff1 == 0 || coeff1 == coeff0);
> VECTOR_TYPE_CHECK (node)->type_common.precision
