Re: std::string add nullptr attribute

[Jonathan Wakely <jwakely.gcc@gmail.com>]

On Fri, 10 Feb 2023 at 21:30, Jonny Grant <jg@jguk.org> wrote:
>
>
>
> On 09/02/2023 17:52, Jonathan Wakely wrote:
> > On Thu, 9 Feb 2023 at 16:30, Xi Ruoyao wrote:
> >>
> >> On Thu, 2023-02-09 at 14:56 +0000, Jonathan Wakely via Gcc-help wrote:
> >>>> Note, my code isn't like this, it is just an example to suggest
> >>>> adding the nullptr attribute, as its clearly already rejected at
> >>>> runtime.
> >>>
> >>> I assume you mean the nonnull attribute. That was added in 2020 and
> >>> then reverted because it broke some things:
> >>
> >> I remember I'd once made the same mistake when I suggested to add
> >> nonnull for ostream::operator<<(const string &) and I was lectured:
> >> nonnull is not only a diagnostic attribute, it also allows the compiler
> >> to assume the parameter is never null and rendering std::string(nullptr)
> >> an undefined behavior.
> >
> > Yes, I think that's what might have happened with the std::string change.
>
> My apologies, Jonathan, Xi, yes it is the __attribute__((nonnull)); I was mistaken to type as nullptr.
>
> I re-read, and it does seem nonnull is really an optimization that as a side effect may give some warnings. So I'm going to stop using it.
> https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes
>
> (there is a typo in that manual section saying "nonnul" - I don't know if you have a moment to make a change in git? I didn't get replies on gcc-patches to my patches...)
>
> I searched and see like someone investigated this problem and saw it removed NULL checks http://www.rkoucha.fr/tech_corner/nonnull_gcc_attribute.html
>
> I saw wget2 removed the nonnull attribute due to the optimizer removing checks against NULL too
> https://gitlab.com/gnuwget/wget2/-/issues/200
>
> >> Then the example may just silently continue to run, instead of throwing
> >> an exception.  It would be an ironic example: an attempt to improve
> >> diagnostic finally made diagnostic more difficult.
> >
> > Indeed.
> >
> > Maybe we can add __attribute__((access(read, 1))) instead, which says
> > that we will read from the pointer, which also implies it must be
> > non-null.
>
> I tried this with gcc 12, as read_only, but it didn't stop when compiling. Maybe you have an example that demonstrates please?
>
> void f(const char * p) __attribute__((access(read_only, 1)));
>
> >
> > N.B. in C++23 string(nullptr) produces an error, although
> > string((const char*)nullptr) doesn't, so in practice it only prevents
> > the dumbest calls with a literal 'nullptr' token, and not the more
> > realistic problems where you have a pointer that happens to be null.
>
> That's good it stops compiling, the error is not that clear "use of deleted function" for me though.
>
> string.cpp: In function ‘int main()’:
> string.cpp:13:26: error: use of deleted function ‘std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(std::nullptr_t) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; std::nullptr_t = std::nullptr_t]’
>    13 |     std::string c(nullptr);
>
>
>
>
> I made my own test class str_string which stops the build a different way. It only works if the dumbest calls with 'nullptr' as you found in your test.
>
> void nullptr_compile_abort() __attribute__((error("nullptr compile error")));
>
> str_string(nullptr_t) { nullptr_compile_abort(); }

This doesn't work because std::is_constructible_v<std::string,
std::nullptr_t> would be true, and we want it to be false.

>
>
>  g++ -std=c++23 -Wall -O1 -o string2 string2.cpp
> In constructor ‘str_string::str_string(nullptr_t)’,
>     inlined from ‘int main()’ at string2.cpp:48:25:
> string2.cpp:20:50: error: call to ‘nullptr_compile_abort’ declared with attribute error: nullptr compile error
>    20 |     str_string(nullptr_t) { nullptr_compile_abort(); }
>
> Jonny