RE: [PATCH] lib/string.c: implement stpcpy

[David Laight <David.Laight@ACULAB.COM>]

From: Nick Desaulniers
> Sent: 15 August 2020 01:24
> 
> LLVM implemented a recent "libcall optimization" that lowers calls to
> `sprintf(dest, "%s", str)` where the return value is used to
> `stpcpy(dest, str) - dest`. This generally avoids the machinery involved
> in parsing format strings.
> 
> `stpcpy` is just like `strcpy` except:
> 1. it returns the pointer to the new tail of `dest`. This allows you to
>    chain multiple calls to `stpcpy` in one statement.
> 2. it requires the parameters not to overlap.  Calling `sprintf` with
>    overlapping arguments was clarified in ISO C99 and POSIX.1-2001 to be
>    undefined behavior.
> 
> `stpcpy` was first standardized in POSIX.1-2008.
> 
> Implement this so that we don't observe linkage failures due to missing
> symbol definitions for `stpcpy`.
> 
..
...
> diff --git a/include/linux/string.h b/include/linux/string.h
> index b1f3894a0a3e..e570b9b10f50 100644
> --- a/include/linux/string.h
> +++ b/include/linux/string.h
> @@ -31,6 +31,9 @@ size_t strlcpy(char *, const char *, size_t);
>  #ifndef __HAVE_ARCH_STRSCPY
>  ssize_t strscpy(char *, const char *, size_t);
>  #endif
> +#ifndef __HAVE_ARCH_STPCPY
> +extern char *stpcpy(char *__restrict, const char *__restrict__);
> +#endif
> 
>  /* Wraps calls to strscpy()/memset(), no arch specific code required */
>  ssize_t strscpy_pad(char *dest, const char *src, size_t count);
> diff --git a/lib/string.c b/lib/string.c
> index 6012c385fb31..81bc4d62c256 100644
> --- a/lib/string.c
> +++ b/lib/string.c
> @@ -272,6 +272,29 @@ ssize_t strscpy_pad(char *dest, const char *src, size_t count)
>  }
>  EXPORT_SYMBOL(strscpy_pad);
> 
> +#ifndef __HAVE_ARCH_STPCPY
...
> +#undef stpcpy
> +char *stpcpy(char *__restrict__ dest, const char *__restrict__ src)
> +{
> +	while ((*dest++ = *src++) != '\0')
> +		/* nothing */;
> +	return dest;
> +}
> +#endif
> +

Hmmmm....
Maybe the compiler should just inline the above?

OTOH there are faster copies on 64bit systems
(for moderate length strings).

It would also be nicer if the compiler actually used/required
a symbol in the 'reserved for the implementation' namespace.
Then the linker should be able to do a fixup to a differently
name symbol - if that is required.

But compiler writers enjoy making embedded coders life hell.

	David

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