[Nickle]Sequencing of expressions with side effects

[Keith Packard]

Around 15 o'clock on Jul 23, Carl Worth wrote:

> 	a = a++;
> 
> the behavior of which is undefined.
> 
> Do nickle's semantics prevent such undefined behavior? I couldn't find
> any definition of ++/-- nor any discussion of the sequencing of the
> side effects of expressions.

The intent is for the semantics to have well defined behaviour in all 
cases.  The evaluation order within expressions is controlled by 
precedence.  Within operators, or within a list of operators of equal 
precedence, the evaluation is strictly left to right.

However, the compilation of assignment operators currently violates this
rule; the right hand side is evaluated before the address of the left hand
side:

	> int[2] a = { 3, 4 };
	> int j = 0;
	> a[j] = j++;
	> a
	[2] {3, 0}
	>

Fixing this will be a bit tricky, as the compiler must handle the multiple 
assignment case:

	> b[j] = a[j++] = j++;

It looks like nickle will need to stack the lvalues until reaching the 
rvalue and then unwind the stack with multiple stores.  In the case
of += style operators, the evaluation of the lvalue will be followed by 
fetching the referenced value before the evaluation of the right hand side.

I think this is easy.

Keith Packard        XFree86 Core Team        HP Cambridge Research Lab