Re: * Re: What is acceptable for -ffast-math? (Was: associative lawin combine)

[Linus Torvalds <torvalds at transmeta dot com>]

On Sun, 29 Jul 2001, Stephen L Moshier wrote:
>
> > Why not make this easier for the user:
> > - default to non-IEEE, but "reasonably safe".
>
> The default GCC behavior is not to allow any optimization
> that could change the value of an expression.  That rule is easy
> for users to understand and surely it is a good rule to keep.
>
> Floating point arithmetic, IEEE or otherwise, does not obey the
> associative or distributive laws.

This is YOUR opinion.

But most people who write floating point code do not think that way. They
consider FP to be "mathematically exact" (which _does_ obey associative
and distributive laws), and they write their programs that way.

And that is exactly my point: to the people who do not care about the
exact details of what it means from a mathematical standpoint, using
associative and distributive rules is _fine_. Their algorithms do not
care, simply because the person writing the code doesn't even _know_ about
the issues of limited precision and range of FP code.

See?

What I'm saying is that the compiler cannot fix "thinking errors" of the
programmer. Trying to fix them is self-defeating and ultimately stupid.
I'm claiming that 99%+ of all programmers do not know or care about the
fact that

	(a+b)*c

is not necessarily the same as

	a*c+b*c

when we're talking about floating point.

My _second_ point is that the ones that DO KNOW and DO CARE are also aware
enough that _they_ can be counted on to add a "-mieee" or other flags.

They are also knowledgeable enough that they know what "associative law"
means, which is already a step up from most programmers, I suspect. So
they can understand why the compiler has done an optimization, and can (if
it is documented) be instructed to use the appropriate switch for not
doing that optimization.

So: put the onus of understanding different FP optimizations on the people
who CAN understand them. Don't make the average person who wants to do his
own small project have to worry about it. By the time the person
understands about the range of FP arithmetic enough to care, he will also
have the capability to do something about it.

		Linus