[Algorithms] Fast Floating-Point compares?

[Stuart H. <stu...@ku...>]

This may be slightly off-topic, if so, I apologise in advance.

Floating point compares, that is, doing something like:
	float x, y;

	x =3D some_value;
	y =3D different_value;
	if (x > y)
		do_something();
	else
		do_something_else();

.seem to be particularly slow on most architectures.  Short of avoiding
the problem entirely (i.e. not using floats) - are there any tricks for
improving the performance of floating point compares?

I know of the trick of testing the "sign" bit (assuming IEEE compliance)
to do:
	if (x > 0.0f)
		...
.tests, and by rearranging terms slightly you can generalise this to
any constant:
	if (x > 12.3f)
		...
becomes:
	if ((x - 12.3f) > 0.0f)
		...
. meaning you can then test the sign of (x - 12.3f).

However, there don't appear to be similar tricks for two variables,
unless:
	if (x > y)
		...
. can be sped up by simply testing the sign of (x - y).  But that
sounds too simple to be true...

-Dino.


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