Re: GCC beaten by ICC in stupid trig test!

[Dale Johannesen <dalej at apple dot com>]

On Mar 24, 2004, at 11:12 AM, Joe Buck wrote:

> On Wed, Mar 24, 2004 at 02:10:13PM -0500, Paul Koning wrote:
> >  Joe> Consider a, b, and c as single precision floating point values,
> >  Joe> and a=1, b=-1, c=1.2345e-8.  (a+b)+c will compute as 1.2345e-8.
> >  Joe> a+(b+c) will return zero, as will (a+c)+b.
> >
> > I don't have a C standard, but my copy of Harbison & Steele says what
> > I expected about parentheses: "Parentheses do not necessarily force a
> > particular evaluation order".
>
> This was true of K&R C, but I seem to recall that the standards 
> committee
> changed that.  Any standards gurus out there care to comment?

This is correct.  My copy of H&S (1987) is explicit that reassociation 
is allowed
even when that would change the result.  It does have an appendix on
Draft Proposed ANSI C, as it was then, but that also does not mention 
the rule
that became standard; apparently this was introduced late in the 
standardization
process.  (It does list unary + as a means of forcing order of 
evaulation, thus:
  (+(a+b))+c
Unary minus did not have this effect.  I don't much like the rule 
that's in the standard,
but I have to admit it's better than that.)

The current standards define order of evaluation implicitly from the 
parsing order.
Rearrangements are permitted only if they can't change the result 
(generally true
for integer, not for FP).  Thus, it is more restrictive than Fortran, 
where K&R C was
less so.

But weren't we talking about -ffast-math?  Standards are irrelevant to 
that discussion.
-ffast-math is for things that aren't safe according to the standard, 
but are frequently
useful; that's what it's for.