Re: Computer architects leaving Intel...

On Sun, 08 Sep 2024 15:36:39 GMT, anton@mips.complang.tuwien.ac.at
(Anton Ertl) wrote:

Tim Rentsch <tr.17687@z991.linuxsc.com> writes:

anton@mips.complang.tuwien.ac.at (Anton Ertl) writes:
>

There was still no easy way to determine whether your software
that calls memcpy() actually works as expected on all hardware,

>
There may not be a way to tell if memcpy()-calling code will work
on platforms one doesn't have, but there is a relatively simple
and portable way to tell if some memcpy() call crosses over into
the realm of undefined behavior.

>
1) At first I thought that yes, one could just check whether there is
an overlap of the memory areas.  But then I remembered that you cannot
write such a check in standard C without (in the general case)
exercising undefined behaviour; and then the compiler could eliminate
the check or do something else that's unexpected.  Do you have such a
check in mind that does not exercise undefined behaviour in the
general case?

The result of comparing pointers to two elements of the same array is
defined.  Cast to (char*), both src and dst can be considered to point
to elements of the [address space sized] char array at address zero.

Adding size_t to a pointer yields another pointer of the same type.


All of gcc, clang and MSVC seem happy with this.

2) Even if there is such a check, you have to be aware that there is a
potential problem with memcpy().  In that case the way to go is to
just use memmove().  But that does not help you with the next "clever"
idea that some compiler or library maintainer has.
>
- anton