C++ standards question

[John Fine <johnsfine at verizon dot net>]

I have a construct that the optimizer in VS2010 64-bit changes to 
something that doesn't work.

I want to know whether this is a bug in the compiler or in the code 
being compiled.

I would have no idea where to look in the standards document for this 
answer.  I know it is off-topic (a C++ not gcc question) but this is 
where the experts are.

My function takes a parameter char* p
One of the things the function does is test and conditionally modifies a 
char pointed to through P.  Something like:

if ( expression of *p ) *p = '-';

The optimizer changes that to the equivalent of

*p = (expression of *p ) ? '-' : *p;

Is that a valid optimization?

The function is called sometimes with a pointer to read-only memory and 
sometimes with a pointer to read/write memory.
When the pointer points to read only memory, that expression of *p will 
always be false (that part works), so the write does not occur in 
unoptimized code.
When the pointer points to read/write memory, that expression of *p is 
usually false but might be true, so the write occasionally occurs.

But in the optimized code that byte is always written.  It is usually 
written with a copy of the value read from in during the evaluation of 
the test expression.

BTW, I have seen code like the following get optimized in a similar way, 
and that seems to me to be a reliably correct optimization:
*p = 'x'; if ( some expression) *p='y';
that gets correctly optimized to
*p = ( some expression ) ? 'y' : 'x';
That is safe because the original code always wrote to that location and 
the optimized code also always writes.