ANSII C, optimization, and "hardware registers"

[PAD Powell]

I have just run into a really fun thing with an optimizer.  The problem
was in the code for a hardware level driver, which wanted to:
1. Stuff a value into a register.
2. Look at the register until a flag (bit) went high.

The code written was
	struct regs{
		int r1;
	} *csr;
	...
	csr->r1 = ST_START;
	while( (csr->r1 & ST_DONE) == 0 );

Well, imagine my surprise when the code generated only did:
	1.  loaded the ST_START value into CPU register (byte value actually)
	2.  placed the CPU register value into memory (word value)
	3.  Did not generate a test instruction, cause the ST_START and
		ST_DONE value were identical.

Now here is the question:
1.  Was this legal code generation?
2.  Note that this compiler did "simple" optimizations as part of the code
	generation. Is this legal?

I know of several ways around this, but I thought that it should be addressed
by the ANSII standard.

Patrick Powell