Message367858
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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Date |
User |
Action |
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2020-05-01 16:08:41 | mark.dickinson | set | recipients:
+ mark.dickinson, vstinner, petdance |
2020-05-01 16:08:41 | mark.dickinson | set | messageid: <1588349321.11.0.468948336038.issue40455@roundup.psfhosted.org> |
2020-05-01 16:08:41 | mark.dickinson | link | issue40455 messages |
2020-05-01 16:08:40 | mark.dickinson | create | |
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