Re: Computer architects leaving Intel...

George Neuner <gneuner2@comcast.net> writes:

On Thu, 12 Sep 2024 04:04:06 -0700, Tim Rentsch
<tr.17687@z991.linuxsc.com> wrote:
>

George Neuner <gneuner2@comcast.net> writes:

[...]

I definitely am NOT an expert on the C standard, but thinking
about it, it occurred to me that if an array is explicitly defined
that *might* cover all memory (or at least all heap), then the
compiler would have to honor any apparent pointers into it.
>
E.g., char (*all_memory)[] = 0;

>
This declaration introduces a pointer, not an array.  Similarly
the declaration
>
   char (*great_white_array)[ 999999999999999999 ] = 0;
>
does not introduce an array but just a pointer (and initializes
the pointer to be a null pointer).  There is no humongous array.

>
Of course there is no actual array ... the point was to (try to)
define *something* such that the compiler would think there was an
array and consider any char* as possibly pointing to an element of
that array.
[And yes!  it might end up pessimizing character manipulating code.]
>
The C standard guarantees that pointers to 2 elements of the same
array are comparable, and current (and past) compilers do allow
comparing arbitrary pointers when cast to char* without needing an
actual char array that covers the addresses.
>
But a guarantee wrt the standard requires the compiler to at least
*think* there is such an array.  The question is how to do that.

What the compiler thinks is irrelevant.  It's only what the C
standard thinks that matters.

If someone fools the C compiler today they might (or might not)
get what they want or expect.  But fooling the compiler is a
risky strategy, and it's almost never needed;  people try to
trick compilers a lot more often than circumstances actually
warrant, and that is even not counting non-guarantees about
future behavior.

Incidentally, I am in the middle of porting some code from one
platform to another.  Code works fine on the original platform,
millions of tests are picture perfect.  No undefined behavior in
sight.  The new platform is a nightmare, thanks to a certain
well-known company headquartered in the state of Washington.  Any
concerns about what happens with undefined behavior are so far
down the list we'd need a telescope to see them.  Given this
recent experience, it's hard for me to get too worked up about
defining these over-the-edge cases.