Jason Ekstrand
bd428162b6
The current align_mul calculation in the unknown-array-index
calculation is
align_mul = MIN3(parent_mul,
min_pow2_divisor(parent_offset),
min_pow2_divisor(stride))
which is certainly correct if parent_offset > 0. However, when
parent_offset = 0, min_pow2_divisor(parent_offset) isn't well-defined
and our calculation for it is 1 << -1 which isn't well-defined. That
said.... it's not actually needed.
The offset to the base of the array is
array_base = parent_mul * k + parent_offset
for some integer k. When we throw in an unknown array index i, we get
elem = parent_mul * k + parent_offset + stride * i.
If we set new_align = MIN2(parent_mul, min_pow2_divisor(stride)), then
both parent_mul and stride are divisible by new_align and
elem = (parent_mul / new_alig) * new_align * k +
(stride / new_align) * new_align * i + parent_offset
= new_align * ((parent_mul / new_alig) * k +
(stride / new_align) * i) + parent_offset
so elem = new_align * j + parent_offset where
j = (parent_mul / new_alig) * k + (stride / new_align) * i.
That's a very long-winded way of saying that we can delete one parameter
from the align_mul calculation and it's still fine. :-)
Fixes: 480329cf8b "nir: Add a helper for getting the alignment of a deref"
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Tested-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6628>
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