Mark Dickinson <[email protected]> added the comment:
As to _why_ it's a false positive: at that point in the code, assuming 30-bit
limbs and an IEEE 754 binary64 "double", we have (using Python notation for
floor division)
a_size == 1 + (a_bits - 1) // 30
and
shift_digits == (a_bits - 55) // 30
from which it's clear that
shift_digits <= (a_bits - 1) // 30 < a_size
so a_size - shift_digits is always strictly positive.
The above doesn't depend on the precise values 55 and 30 - any other positive
values would have worked, so even with 15-bit digits and some other double
format with fewer bits, we still have "shift_digits < a_size".
And now since the v_rshift call writes "a_size - shift_digits" digits to x,
we're guaranteed that at least one digit is written, so `x[0]` is not
uninitialised.
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<https://bugs.python.org/issue40455>
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