This patch explains the encoding used for the precision field in
TYPE_VECTOR_SUBPARTS when NUM_POLY_INT_COEFFS == 2. OK to install?
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
