Re: std::string add nullptr attribute

[Jonny Grant <jg@jguk.org>]

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(); }


 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