Re: short-circuit control forms

[Jeffrey Carter <jeffrey.carter@boeing.com>]

Mark Lundquist wrote:
> 
> "Jeffrey Carter" <jeffrey.carter@boeing.com> wrote in message
> news:3B312495.15258B18@boeing.com...
> >
> > IANAL, nor do I play one on TV. However, "and" and "or" are normal
> > subprograms (you can overload them), and the semantics for subprograms
> > state that all actual parameters are evaluated, and the order of
> > evaluation is not specified by the language (ARM 6.4). Thus, it is
> > illegal for a compiler to convert them to short-circuit forms.
> 
> I don't think that's correct...
> 
> The compiler can do whatever it wants, as long as it's equivalent [RM
> 1.1.3].

Since preserving side effects is part of "equivalent", and deciding if
functions have side effects is "hard" (I'm not aware of any compilers
that do this), this is true in theory but irrelevant in practice. If a
function with a side effect is (part of) an operand of "and" or "or", a
compiler must generate code that calls the function. In real compilers,
that seems to mean that both operands are always evaluated. Is anyone
aware of a compiler that optimizes away evaluation of an operand to
"and" or "or" when neither operand has a function call? In cases where
one of the operands has a function call? Both operands?

As I said, IANAL. It would be nice to hear from Robert Duff, Tucker
Taft, or Robert Dewar about this.

-- 
Jeffrey Carter