[rs6000] Add support for signed overflow arithmetic

[Eric Botcazou]

> It's better than the generic branch sequence below, or yours.  It still
> sucks, obviously.

OK, I thought you were talking about the double-width result.  The non-branch 
sequence I posted is the generic non-branch sequence (that Ada was using).

> Let's see.  Completely untested.  Inputs in regs 3 and 4, output in reg 3.
> 32-bit code all the way.
> 
> add:
> 	eqv 9,3,4
> 	add 3,3,4
> 	xor 4,3,4
> 	and. 4,9,4
> 	blt <overflow>
> 
> sub:
> 	xor 9,3,4
> 	sub 3,3,4
> 	eqv 4,3,4
> 	and. 4,9,4
> 	blt <overflow>

These ones (if correct) are quite better than the generic code!

> neg:
> 	neg 3,3
> 	xoris. 9,3,0x8000
> 	beq <overflow>
> 
> mul:
> 	mulhw 9,3,4
> 	mullw 3,3,4
> 	srawi 4,9,31
> 	cmpw 4,9
> 	bne <overflow>

These ones are essentially equivalent to the generic code.

-- 
Eric Botcazou