> 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.
> 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>
--
Eric Botcazou
