[Bug middle-end/56888] memcpy implementation optimized as a call to memcpy

["david at westcontrol dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56888

--- Comment #47 from David Brown <david at westcontrol dot com> ---
(In reply to M Welinder from comment #46)
> Should "-std=c99" imply turning off these optimizations?
> 
> Creating calls to, say, strlen is incompatible with the C99 standard and
> perhaps better limited to "-std=gnu-something" or an opt-in f-flag.

How is it incompatible with C99 to create calls to library functions?  I can
think of a two possibilities:

1. If the function implementation plays with errno (allowed in 7.5p3), in a way
that is visible to the code.

2. If the function is called with parameters that may invoke undefined
behaviour (such as calling "strlen" without being sure that the parameter
points to a null-terminated string), where such undefined behaviour is not
already present.

If the user writes code that acts like a call to strlen (let's assume the
implementation knows strlen does not change errno), then the compiler can
replace it with a library call.  Similarly, if the user writes a call to
strlen, then the compiler can replace it with inline code.

As long as there is no difference in the observable behaviour, the
transformation is allowed.

Or am I missing something here?