Re: auto const ints and pointer issue

[Joe Buck <Joe dot Buck at synopsys dot COM>]

On Tue, Jun 17, 2008 at 08:51:24PM -0700, Karen Shaeffer wrote:
> I see your point. My sticking point is that the process is actually
> running on a physical machine.

And that's the problem.  You, like many C programmers, have in your head a
physical machine model where pointer variables are physical machine
addresses and other variables are physical memory locations.  But C
doesn't necessarily work that way (except at -O0); if it did, programs
would run substantially slower.  The compiler can make optimizations (like
reusing values in registers) in certain circumstances.  Anywhere in the
standard where you see that a behavior is undefined, read that as meaning,
the compiler is allowed to produce an arbitrary code sequence, so it will
produce the fastest, or smallest, or simplest-to-implement sequence.

> And the addresses, although virtual,
> do translate to a unique physical memory location. And, the value
> stored in that location cannot be 0 and 5 at the same time.

Right, but since it's const, the compiler is allowed to assume that it
is, well, const!  That means that if it has already read the value into
a register, it can assume that the register is still valid for the
remainder of the program execution, even if you decide to violate the
rules of C and change the "constant".  That's why it appears to be
0 and 5 at the same time.

To get rid of that behavior, you'd have to force the compiler to read
every variable from memory every time, and not use registers at all.