[Bug libstdc++/89979] New: subtract_with_carry_engine incorrect carry flag

gcc@christoph-conrads.name gcc-bugzilla@gcc.gnu.org
Thu Apr 4 21:46:00 GMT 2019 

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89979

            Bug ID: 89979
           Summary: subtract_with_carry_engine incorrect carry flag
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc@christoph-conrads.name
  Target Milestone: ---

Created attachment 46090
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=46090&action=edit
The initial state in combination with the incorrect carry computation makes the
PRNG enter the unescapable all zero state.

The carry inside `std::subtract_with_carry_engine` is computed incorrectly if
word_size == std::numeric_limits<result_type>::digits. 

The bug can be triggered neither by std::ranlux24 nor by std::ranlux48.

The bug occurs with GCC 7.3.0, GCC 8.2.0, and Clang when using the GNU Standard
C++ Library shipping with GCC 7.3.0.

$ gcc --version
gcc (Gentoo 7.3.0-r3 p1.4) 7.3.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ g++-7.3.0 -Wextra -Wall -std=c++11 -pedantic -v -save-temps
~/libstdcxx-swc-bug.cpp
Using built-in specs.
COLLECT_GCC=g++-7.3.0
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-pc-linux-gnu/7.3.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with:
/var/tmp/portage/sys-devel/gcc-7.3.0-r3/work/gcc-7.3.0/configure
--host=x86_64-pc-linux-gnu --build=x86_64-pc-linux-gnu --prefix=/usr
--bindir=/usr/x86_64-pc-linux-gnu/gcc-bin/7.3.0
--includedir=/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/include
--datadir=/usr/share/gcc-data/x86_64-pc-linux-gnu/7.3.0
--mandir=/usr/share/gcc-data/x86_64-pc-linux-gnu/7.3.0/man
--infodir=/usr/share/gcc-data/x86_64-pc-linux-gnu/7.3.0/info
--with-gxx-include-dir=/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/include/g++-v7
--with-python-dir=/share/gcc-data/x86_64-pc-linux-gnu/7.3.0/python
--enable-languages=c,c++,fortran --enable-obsolete --enable-secureplt
--disable-werror --with-system-zlib --enable-nls --without-included-gettext
--enable-checking=release --with-bugurl=https://bugs.gentoo.org/
--with-pkgversion='Gentoo 7.3.0-r3 p1.4' --disable-esp --enable-libstdcxx-time
--enable-shared --enable-threads=posix --enable-__cxa_atexit
--enable-clocale=gnu --disable-multilib --with-multilib-list=m64
--disable-altivec --disable-fixed-point --enable-targets=all --enable-libgomp
--disable-libmudflap --disable-libssp --disable-libcilkrts --disable-libmpx
--enable-vtable-verify --enable-libvtv --enable-lto --without-isl
--enable-libsanitizer --enable-default-pie --enable-default-ssp
Thread model: posix
gcc version 7.3.0 (Gentoo 7.3.0-r3 p1.4) 
COLLECT_GCC_OPTIONS='-Wextra' '-Wall' '-std=c++11' '-Wpedantic' '-v'
'-save-temps' '-shared-libgcc' '-mtune=generic' '-march=x86-64'
 /usr/libexec/gcc/x86_64-pc-linux-gnu/7.3.0/cc1plus -E -quiet -v -D_GNU_SOURCE
/home/starfish/libstdcxx-swc-bug.cpp -mtune=generic -march=x86-64 -std=c++11
-Wextra -Wall -Wpedantic -fpch-preprocess -o libstdcxx-swc-bug.ii
ignoring nonexistent directory "/usr/local/include"
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../x86_64-pc-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/include/g++-v7
 /usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/include/g++-v7/x86_64-pc-linux-gnu
 /usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/include/g++-v7/backward
 /usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/include
 /usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/include-fixed
 /usr/include
End of search list.
COLLECT_GCC_OPTIONS='-Wextra' '-Wall' '-std=c++11' '-Wpedantic' '-v'
'-save-temps' '-shared-libgcc' '-mtune=generic' '-march=x86-64'
 /usr/libexec/gcc/x86_64-pc-linux-gnu/7.3.0/cc1plus -fpreprocessed
libstdcxx-swc-bug.ii -quiet -dumpbase libstdcxx-swc-bug.cpp -mtune=generic
-march=x86-64 -auxbase libstdcxx-swc-bug -Wextra -Wall -Wpedantic -std=c++11
-version -o libstdcxx-swc-bug.s
GNU C++11 (Gentoo 7.3.0-r3 p1.4) version 7.3.0 (x86_64-pc-linux-gnu)
        compiled by GNU C version 7.3.0, GMP version 6.1.2, MPFR version 3.1.6,
MPC version 1.0.3, isl version none
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C++11 (Gentoo 7.3.0-r3 p1.4) version 7.3.0 (x86_64-pc-linux-gnu)
        compiled by GNU C version 7.3.0, GMP version 6.1.2, MPFR version 3.1.6,
MPC version 1.0.3, isl version none
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 09ac773f85349e020b5b36ead38f9924
COLLECT_GCC_OPTIONS='-Wextra' '-Wall' '-std=c++11' '-Wpedantic' '-v'
'-save-temps' '-shared-libgcc' '-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../x86_64-pc-linux-gnu/bin/as
-v --64 -o libstdcxx-swc-bug.o libstdcxx-swc-bug.s
GNU assembler version 2.30.0 (x86_64-pc-linux-gnu) using BFD version (Gentoo
2.30 p5) 2.30.0
COMPILER_PATH=/usr/libexec/gcc/x86_64-pc-linux-gnu/7.3.0/:/usr/libexec/gcc/x86_64-pc-linux-gnu/7.3.0/:/usr/libexec/gcc/x86_64-pc-linux-gnu/:/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/:/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../x86_64-pc-linux-gnu/bin/
LIBRARY_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../lib64/:/lib/../lib64/:/usr/lib/../lib64/:/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../x86_64-pc-linux-gnu/lib/:/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-Wextra' '-Wall' '-std=c++11' '-Wpedantic' '-v'
'-save-temps' '-shared-libgcc' '-mtune=generic' '-march=x86-64'
 /usr/libexec/gcc/x86_64-pc-linux-gnu/7.3.0/collect2 -plugin
/usr/libexec/gcc/x86_64-pc-linux-gnu/7.3.0/liblto_plugin.so
-plugin-opt=/usr/libexec/gcc/x86_64-pc-linux-gnu/7.3.0/lto-wrapper
-plugin-opt=-fresolution=libstdcxx-swc-bug.res
-plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lgcc
-plugin-opt=-pass-through=-lc -plugin-opt=-pass-through=-lgcc_s
-plugin-opt=-pass-through=-lgcc --eh-frame-hdr -m elf_x86_64 -dynamic-linker
/lib64/ld-linux-x86-64.so.2 -pie
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../lib64/Scrt1.o
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../lib64/crti.o
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/crtbeginS.o
-L/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0
-L/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../lib64 -L/lib/../lib64
-L/usr/lib/../lib64
-L/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../x86_64-pc-linux-gnu/lib
-L/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../.. libstdcxx-swc-bug.o -lstdc++
-lm -lgcc_s -lgcc -lc -lgcc_s -lgcc
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/crtendS.o
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../lib64/crtn.o
COLLECT_GCC_OPTIONS='-Wextra' '-Wall' '-std=c++11' '-Wpedantic' '-v'
'-save-temps' '-shared-libgcc' '-mtune=generic' '-march=x86-64'

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