https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114397
--- Comment #2 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jakub Jelinek <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:59b6cece54f33ac4994834d01e18269856576556 commit r14-9592-g59b6cece54f33ac4994834d01e18269856576556 Author: Jakub Jelinek <ja...@redhat.com> 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 <ja...@redhat.com> 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.
