[PATCH] Avoid signed overflow in num_get::_M_extract_int (PR libstdc++/67214)

[Jonathan Wakely]

On 20/05/17 15:10 +0800, Xi Ruoyao wrote:
>On 2017-05-19 15:38 +0100, Jonathan Wakely wrote:
>> On 18/05/17 19:10 +0800, Xi Ruoyao wrote:
>> > This UB has been hiding so long...
>>
>> Indeed! Thanks for the patch.
>>
>> > 2017-03-11  Xi Ruoyao  <ryxi@stu.xidian.edu.cn>
>> >
>> > 	PR libstdc++/67214
>> > 	* include/bits/locale_facets.tcc (_M_extract_int):
>> > 	Â Â Add explicit conversion to avoid signed overflow.
>> > ---
>> > Â libstdc++-v3/include/bits/locale_facets.tcc | 3 ++-
>> > Â 1 file changed, 2 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/libstdc++-v3/include/bits/locale_facets.tcc b/libstdc++-v3/include/bits/locale_facets.tcc
>> > index 351190c..5f85d15 100644
>> > --- a/libstdc++-v3/include/bits/locale_facets.tcc
>> > +++ b/libstdc++-v3/include/bits/locale_facets.tcc
>> > @@ -470,7 +470,8 @@ _GLIBCXX_BEGIN_NAMESPACE_LDBL
>> > Â 	bool __testoverflow = false;
>> > Â 	const __unsigned_type __max =
>> > Â 	Â Â (__negative && __gnu_cxx::__numeric_traits<_ValueT>::__is_signed)
>> > -	Â Â ? -__gnu_cxx::__numeric_traits<_ValueT>::__min
>> > +	Â Â ? -static_cast<__unsigned_type>(__gnu_cxx::
>> > +	Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â __numeric_traits<_ValueT>::__min)
>>
>> Do we need to keep the negation, or can we just cast to
>> __unsigned_type?
>
>For 2's complement we can just cast to __unsigned_type. Â But for
>clarity and other strange architectures I think we should keep
>the negation.

https://gcc.gnu.org/onlinedocs/gcc/Integers-implementation.html
"GCC only supports two's complement integer types"

I doubt that's ever going to change, but keeping the negation also
doesn't do any harm. I'll test and commit this, thanks.