On Mon, Oct 24, 2016 at 06:14:48PM +0200, Eric Botcazou wrote:
> > Maybe the best you can do is generate the double-width result, and then
> > check if the upper halve is the sign extension of the lower halve. Maybe
> > some trickery can help (for add/sub/neg at least).
>
> That's inefficient, even for additive operations.
It's better than the generic branch sequence below, or yours. It still
sucks, obviously.
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>
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>
> > You can also just FAIL the expander if !TARGET_MCRXR. I wonder just how
> > bad the generic code is.
>
> It is branchy. Here's a 32-bit overflow addition at -O2:
>
> cmpwi 7,4,0
> add 4,3,4
> blt- 7,.L4
> cmpw 7,4,3
> blt- 7,.L3
> .L5:
> mr 3,4
> blr
> .L4:
> cmpw 7,4,3
> ble+ 7,.L5
> .L3:
> <overflow>
>
> You can do it manually with just one branch:
>
> add 10,4,3
> srwi 4,4,31
> cmpw 7,10,3
> mfcr 9
> rlwinm 9,9,29,1
> cmpw 7,9,4
> bne- 7,.L5
> mr 3,10
> blr
> .L5:
> <overflow>
>
> and of course with -mmcrxr:
>
> addo 3,3,4
> mcrxr 7
> bgt- 7,.L10
> blr
> L10:
> <overflow>
Or using mcrxr (or mtxer) and SO:
mcrxr 0 # clear XER[SO], can use mtxer instead
...
addo. 3,3,4
bso .L10
blr
.L10:
etc.
(but keeping track of when your SO flag is clear is a pain, and if you
have to reset it all the time there is no big win).
Segher
