https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114397
--- Comment #2 from GCC Commits ---
The master branch has been updated by Jakub Jelinek :
https://gcc.gnu.org/g:59b6cece54f33ac4994834d01e18269856576556
commit r14-9592-g59b6cece54f33ac4994834d01e18269856576556
Author: Jakub Jelinek
Date: Thu Mar 21 13:07:50 2024 +0100
libgcc: Fix up bitint division [PR114397]
The Knuth's division algorithm relies on the number of dividend limbs
to be greater ore equal to number of divisor limbs, which is why
I've added a special case for un < vn at the start of __divmodbitint4.
Unfortunately, my assumption that it then implies abs(v) > abs(u) and
so quotient must be 0 and remainder same as dividend is incorrect.
This is because this check is done before negation of the operands.
While bitint_reduce_prec reduces precision from clearly useless limbs,
the problematic case is when the dividend is unsigned or non-negative
and divisor is negative. We can have limbs (from MS to LS):
dividend: 0 M ?...
divisor:-1 -N ?...
where M has most significant bit set and M >= N (if M == N then it
also the following limbs matter) and the most significant limbs can
be even partial. In this case, the quotient should be -1 rather than
0. bitint_reduce_prec will reduce the precision of the dividend so
that M is the most significant limb, but can't reduce precision of the
divisor to more than having the -1 as most significant limb, because
-N doesn't have the most significant bit set.
The following patch fixes it by detecting this problematic case in the
un < vn handling, and instead of assuming q is 0 and r is u will
decrease vn by 1 because it knows the later code will negate the divisor
and it can be then expressed after negation in one fewer limbs.
2024-03-21 Jakub Jelinek
PR libgcc/114397
* libgcc2.c (__divmodbitint4): Don't assume un < vn always means
abs(v) > abs(u), check for a special case of un + 1 == vn where
u is non-negative and v negative and after v's negation vn could
be reduced by 1.
* gcc.dg/torture/bitint-65.c: New test.
