https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100137
Bug ID: 100137
Summary: -Werror=array-bounds false positive:"subscript -1 is
outside array bounds"
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: spamandnoise at gmail dot com
Target Milestone: ---
- exact version of GCC: 11.0.1 20210417 (bug first appeared in GCC 10.3)
- system type: x86_64-linux-gnu
- options given when GCC was configured/built: see below
- complete command line that triggers the bug: `g++ -O2 -Werror -Wall
-std=c++17 -c test.cpp`
compiler output:
-----
test.cpp: In function 'int main()':
test.cpp:38:11: error: array subscript -1 is outside array bounds of 'char [6]'
[-Werror=array-bounds]
38 | s.back() = '2';
| ~~~~~~^~
test.cpp:36:10: note: while referencing 'hello'
36 | char hello[] = "hello";
| ^~~~~
cc1plus: all warnings being treated as errors
Compiler returned: 1
-----
- preprocessed file that triggers the bug:
-----
typedef long unsigned int size_t; // expanded from <stddef.h>
struct span
{
span( char* _data, size_t _size )
: first_( _data ),
last_( _data != nullptr ? _data + _size : nullptr )
{
if ( _size != 0 && _data == nullptr ) throw 42;
}
char& back() const
{
//return *( first_ + ( last_ - first_ - 1 ) ); // this works
return *( last_ - 1 );
}
char* first_;
char* last_;
};
size_t string_length( char const * ptr, size_t max = (size_t) - 1 )
{
size_t len = 0;
while ( len < max && ptr[len] )
{
++len;
}
return len;
}
int
main()
{
char hello[] = "hello";
span s{ hello, string_length( hello ) };
s.back() = '2';
}
-----
`g++ -v` output:
-----
Using built-in specs.
COLLECT_GCC=/opt/compiler-explorer/gcc-snapshot/bin/g++
Target: x86_64-linux-gnu
Configured with: ../gcc-trunk-20210418/configure
--prefix=/opt/compiler-explorer/gcc-build/staging --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu --disable-bootstrap
--enable-multiarch --with-abi=m64 --with-multilib-list=m32,m64,mx32
--enable-multilib --enable-clocale=gnu --enable-languages=c,c++,fortran,ada,d
--enable-ld=yes --enable-gold=yes --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --enable-linker-build-id --enable-lto
--enable-plugins --enable-threads=posix
--with-pkgversion=Compiler-Explorer-Build
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 11.0.1 20210417 (experimental) (Compiler-Explorer-Build)
COLLECT_GCC_OPTIONS='-fdiagnostics-color=always' '-g' '-o' '/app/output.s'
'-masm=intel' '-S' '-v' '-O2' '-Werror' '-Wall' '-std=c++17' '-shared-libgcc'
'-mtune=generic' '-march=x86-64' '-dumpdir' '/app/'
/opt/compiler-explorer/gcc-trunk-20210418/bin/../libexec/gcc/x86_64-linux-gnu/11.0.1/cc1plus
-quiet -v -imultiarch x86_64-linux-gnu -iprefix
/opt/compiler-explorer/gcc-trunk-20210418/bin/../lib/gcc/x86_64-linux-gnu/11.0.1/
-D_GNU_SOURCE <source> -quiet -dumpdir /app/ -dumpbase output.cpp -dumpbase-ext
.cpp -masm=intel -mtune=generic -march=x86-64 -g -O2 -Werror -Wall -std=c++17
-version -fdiagnostics-color=always -o /app/output.s
GNU C++17 (Compiler-Explorer-Build) version 11.0.1 20210417 (experimental)
(x86_64-linux-gnu)
compiled by GNU C version 7.5.0, GMP version 6.1.0, MPFR version 3.1.4,
MPC version 1.0.3, isl version isl-0.18-GMP
GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096
[...]
Compiler executable checksum: c26ce8a3d2d070f1dc9f9a165aea3eaf
-----
