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* [Bug libgcc/114397] New: wrong code with _BitInt() division at -O0
@ 2024-03-19 17:39 zsojka at seznam dot cz
2024-03-20 17:53 ` [Bug libgcc/114397] " jakub at gcc dot gnu.org
` (2 more replies)
0 siblings, 3 replies; 4+ messages in thread
From: zsojka at seznam dot cz @ 2024-03-19 17:39 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114397
Bug ID: 114397
Summary: wrong code with _BitInt() division at -O0
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Keywords: wrong-code
Severity: normal
Priority: P3
Component: libgcc
Assignee: unassigned at gcc dot gnu.org
Reporter: zsojka at seznam dot cz
CC: jakub at gcc dot gnu.org
Target Milestone: ---
Host: x86_64-pc-linux-gnu
Target: x86_64-pc-linux-gnu
Created attachment 57740
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=57740&action=edit
reduced testcase
Output:
$ x86_64-pc-linux-gnu-gcc testcase.c
$ ./a.out
Aborted
The result is 0 instead of -1.
$ x86_64-pc-linux-gnu-gcc -v
Using built-in specs.
COLLECT_GCC=/repo/gcc-trunk/binary-latest-amd64/bin/x86_64-pc-linux-gnu-gcc
COLLECT_LTO_WRAPPER=/repo/gcc-trunk/binary-trunk-r14-9532-20240318213256-g9eeca775367-checking-yes-rtl-df-extra-nobootstrap-amd64/bin/../libexec/gcc/x86_64-pc-linux-gnu/14.0.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /repo/gcc-trunk//configure --enable-languages=c,c++
--enable-valgrind-annotations --disable-nls --enable-checking=yes,rtl,df,extra
--disable-bootstrap --with-cloog --with-ppl --with-isl
--build=x86_64-pc-linux-gnu --host=x86_64-pc-linux-gnu
--target=x86_64-pc-linux-gnu --with-ld=/usr/bin/x86_64-pc-linux-gnu-ld
--with-as=/usr/bin/x86_64-pc-linux-gnu-as --enable-libsanitizer
--disable-libstdcxx-pch
--prefix=/repo/gcc-trunk//binary-trunk-r14-9532-20240318213256-g9eeca775367-checking-yes-rtl-df-extra-nobootstrap-amd64
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 14.0.1 20240319 (experimental) (GCC)
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug libgcc/114397] wrong code with _BitInt() division at -O0
2024-03-19 17:39 [Bug libgcc/114397] New: wrong code with _BitInt() division at -O0 zsojka at seznam dot cz
@ 2024-03-20 17:53 ` jakub at gcc dot gnu.org
2024-03-21 12:08 ` cvs-commit at gcc dot gnu.org
2024-03-21 12:13 ` jakub at gcc dot gnu.org
2 siblings, 0 replies; 4+ messages in thread
From: jakub at gcc dot gnu.org @ 2024-03-20 17:53 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114397
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Status|UNCONFIRMED |ASSIGNED
Last reconfirmed| |2024-03-20
Assignee|unassigned at gcc dot gnu.org |jakub at gcc dot gnu.org
--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 57749
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=57749&action=edit
gcc14-pr114397.patch
Untested fix.
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug libgcc/114397] wrong code with _BitInt() division at -O0
2024-03-19 17:39 [Bug libgcc/114397] New: wrong code with _BitInt() division at -O0 zsojka at seznam dot cz
2024-03-20 17:53 ` [Bug libgcc/114397] " jakub at gcc dot gnu.org
@ 2024-03-21 12:08 ` cvs-commit at gcc dot gnu.org
2024-03-21 12:13 ` jakub at gcc dot gnu.org
2 siblings, 0 replies; 4+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2024-03-21 12:08 UTC (permalink / raw)
To: gcc-bugs
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 <jakub@gcc.gnu.org>:
https://gcc.gnu.org/g:59b6cece54f33ac4994834d01e18269856576556
commit r14-9592-g59b6cece54f33ac4994834d01e18269856576556
Author: Jakub Jelinek <jakub@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 <jakub@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.
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug libgcc/114397] wrong code with _BitInt() division at -O0
2024-03-19 17:39 [Bug libgcc/114397] New: wrong code with _BitInt() division at -O0 zsojka at seznam dot cz
2024-03-20 17:53 ` [Bug libgcc/114397] " jakub at gcc dot gnu.org
2024-03-21 12:08 ` cvs-commit at gcc dot gnu.org
@ 2024-03-21 12:13 ` jakub at gcc dot gnu.org
2 siblings, 0 replies; 4+ messages in thread
From: jakub at gcc dot gnu.org @ 2024-03-21 12:13 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114397
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|ASSIGNED |RESOLVED
Resolution|--- |FIXED
--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Fixed.
^ permalink raw reply [flat|nested] 4+ messages in thread
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2024-03-19 17:39 [Bug libgcc/114397] New: wrong code with _BitInt() division at -O0 zsojka at seznam dot cz
2024-03-20 17:53 ` [Bug libgcc/114397] " jakub at gcc dot gnu.org
2024-03-21 12:08 ` cvs-commit at gcc dot gnu.org
2024-03-21 12:13 ` jakub at gcc dot gnu.org
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