Re: std::string add nullptr attribute

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

On Sun, 12 Mar 2023 at 22:10, Jonny Grant 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.
>
> How about adding a method that is called by these operators that has the __attribute__ ((nonnull)) ?
>
> example:
> https://godbolt.org/z/bqW86PP34
>
> >> 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.
> >
> > 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.
>
> There is a way to generate a build error for even string((const char*)nullptr)
>
> I made another example that detects nullptr being passed around (should such stupid code occur) at build time providing optimizer is on. With -O0 it just gives the error always; so I put in an __OPTIMIZE__ check. This example doesn't need -fanalyzer.
>
> https://godbolt.org/z/TdGnno4K5
>
> #if __OPTIMIZE__
> void nullptr_compile_abort() __attribute__((error("nullptr compile error")));
> #endif
>
> static void f2(const char * str)
> {
> #if __OPTIMIZE__
>     if (str == nullptr) nullptr_compile_abort();
> #endif
> }
>
> int main()
> {
>     f2((const char *)nullptr);
> }

This causes compilation to fail for code which is never executed at
run-time, which is not permitted by the standard.

You can use __attribute__((warning(""))) instead, but that is broken^W
inconvenient for inline functions. You need a non-inline definition of
the function, which means exporting a new function from the shared
library just for this diagnostic.

All these techniques you're rediscovering have been tried before :-)