fabs(x) vs. (x) < 0 ? -(x) : (x)

[Rick Genter]

On machines which do not support floating-point instructions,
libraries which implement the DEC G-format floating point (I think
that's right) format can implement fabs() much more efficiently as a
subroutine than as a macro.  If you implement it as a macro you'll end
up generating something like:

	push	address of x
	push	address of constant 0
	call	fp-compare
	jumpge	L1
	push	address of x
	push	address of temp
	call	fp-negate-copy
	jump	L2

L1:
	push	address of x
	push	address of temp
	call	fp-copy

L2:

whereas the routine fabs() could be:

fabs:
	bitclr	x,#BIT_HIGH
	return

Obviously optimizations can be made depending on the intelligence of
your compiler, whether registers (or register-pairs) can hold floating
point values, etc.  Still, it would be hard to beat a single bit-clear
(or AND) operation.

					- Rick
--------
Rick Genter 				BBN Laboratories Inc.
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