Re: [edk2-devel] Using ekd2-libc with C++20

["Pedro Falcato" <pedro.falcato@gmail.com>]

On Fri, Feb 16, 2024 at 8:06 PM Pedro Falcato <pedro.falcato@gmail.com> wrote:
>
> On Fri, Feb 16, 2024 at 2:46 PM pawel.karczewski via groups.io
> <pawel.karczewski=solidigm.com@groups.io> wrote:
> >
> > Hi,
> >
> > I'm successfully building C++20 project with edk2 under the GCC compiler.
> > Currently I'm trying to add edk2-libc to this project, but encountered issue with <string.h> header.
> >
> > ```
> > edk2/StdLib/Include/string.h:487:19: error: ISO C++17 does not allow ‘register’ storage class specifier [-Werror=register] register char **stringp
> > ```
> >
> > Why `strsep(register char **stringp, register const char *delim)` parameters has register storage class-class specifier?
> > C standard states the `register` keyword is only hint for the compiler and the extent to which such
> > suggestions are effective is implementation-defined. Do you see any real performance improvements thanks to this?
>
> Considering the file is Copyright(C) University of California (and the
> SCCS tags suggest 1993, which places it around 4.4BSD), this very much
> looks like a historical artifact (of old UNIX and bad compilers). I
> certainly doubt any compiler capable of building EDK2 changes its
> codegen because of it (and certainly not the calling convention, at
> least in all ABIs I know of).

I was bored and had to check :)

Old 4.4BSD string.h:
https://www.tuhs.org/cgi-bin/utree.pl?file=4.4BSD/usr/src/include/string.h

So we can see that it didn't even have the register specifier in the
declaration.

However, in the definition:
https://www.tuhs.org/cgi-bin/utree.pl?file=4.4BSD/usr/src/lib/libc/string/strsep.c
We can see that 'register' was present there, for what I assume was
handholding the compiler into keeping the arg variables in registers
on old UNIX (they did that *a lot*, even back in the original UNIX C
versions).

-- 
Pedro


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