On 4/9/20 2:52 PM, Segher Boessenkool wrote: > Hi! > > On Thu, Apr 02, 2020 at 11:53:47AM -0700, Richard Henderson wrote: >> The rtl description of signed/unsigned overflow from subtract >> was fine, as far as it goes -- we have CC_Cmode and CC_Vmode >> that indicate that only those particular bits are valid. >> >> However, it's not clear how to extend that description to >> handle signed comparison, where N == V (GE) N != V (LT) are >> the only valid bits. >> >> Using an UNSPEC means that we can unify all 3 usages without >> fear that combine will try to infer anything from the rtl. >> It also means we need far fewer variants when various inputs >> have constants propagated in, and the rtl folds. >> >> Accept -1 for the second input by using ADCS. > > If you use an unspec anyway, why do you need a separate UNSPEC_SBCS? > It is just the same as UNSPEC_ADCS, with one of the inputs inverted? > > Is there any reason to pretend borrows are different from carries?
Good point. If we go this way, I'll make sure and merge them. But I've also just sent v4 that does away with the unspecs and uses the forms that Earnshaw used for config/arm. r~
