diff options
Diffstat (limited to 'gcc')
| -rw-r--r-- | gcc/ChangeLog | 5 | ||||
| -rw-r--r-- | gcc/tree.h | 17 |
2 files changed, 22 insertions, 0 deletions
diff --git a/gcc/ChangeLog b/gcc/ChangeLog index 3a927ed..3330d1c 100644 --- a/gcc/ChangeLog +++ b/gcc/ChangeLog @@ -1,3 +1,8 @@ +2019-04-26 Richard Sandiford <richard.sandiford@arm.com> + + * tree.h (TYPE_VECTOR_SUBPARTS, SET_TYPE_VECTOR_SUBPARTS): Add + commentary about the encoding of precision. + 2019-04-25 Andreas Tobler <andreast@gcc.gnu.org> * config/i386/freebsd64.h: Add bits for 32-bit multilib support. @@ -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, poly_uint64 subparts) 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 |