Re: [f-cpu] signed cmpl

[Michael Riepe <michael@stud.uni-hannover.de>]

On Mon, Dec 16, 2002 at 10:41:00AM +0100, devik wrote:
> Hi,
> 
> when I was adding conditionals into gcc code I found
> that signed compare is often used by gcc backend
> (more than unsigned one for most of code).
> 
> IMHO the signed compare can be (even for SIMD) done as:
> 
> xor.N a0,a1,t0       // get sign difference
> cmpl[e].N a0,a1,t1   // unsigned compare
> shiftari.N N-1,t0,t0 // convert MSB bit to negation mask
> // free slot here
> xor.N t0,t1,t0       // negate result for different signs

> it is 6 cycle latency! Can some HW guy here tell whether can
> I count on signed cmp being computed by INC unit too ? It
> would be one xor gate in its critical path AFAIK (negate
> result if sign bits were different).

My version would be:

	[s]bchg.N N-1, a0, t0		// change sign(s)
	[s]bchg.N N-1, a1, t1		// change sign(s)
	// free slot
	[s]cmpl[e].N t0, t1, t0		// unsigned compare

but that's hardly faster. However, it's easy to realize in hardware:
The unsigned cmpl is equivalent to

	y = (first_bit(a ^ b) & b) ? -1 : 0

and its signed counterpart will be

	y = (first_bit(a ^ b) & change_sign(b)) ? -1 : 0

which also needs a single XOR per chunk - but that's located *outside* the
critical datapath, because first_bit() takes more time than a single XOR.

Of course there will be both signed and unsigned integer compare
instructions (and also min/max/sort). Projected names for the signed
counterparts are cmpls, cmples, mins, maxs and minmaxs (aka sorts).

-- 
 Michael "Tired" Riepe <Michael.Riepe@stud.uni-hannover.de>
 "All I wanna do is have a little fun before I die"
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