On Mon, Nov 28, 2022 at 6:58 PM Segher Boessenkool <seg...@kernel.crashing.org> wrote: > > On Mon, Nov 28, 2022 at 09:42:05AM +0100, Richard Biener wrote: > > Since the function seems to be allowed to fail the patch looks > > reasonable - still I wonder > > what the "fallback" for a MODE_CC style compare-and-branch is? There > > are callers > > of this function that do not seem to expect failure at least, some > > suspiciously looking > > like MODE_CC candiates. > > Hi! > > cbranchcc4 is *not* a compare-and-branch, like ccbranch<mode>4 for other > modes are. Instead, it is a conditional branch. I still think it is a > bad idea to use this same pattern name for a completely different (and > much more basic) concept, it just confuses many things, makes us need > exceptions in most users of cbranch<mode>4 :-( > > cbranchcc4 does not do a comparison. Instead, it uses the result of > some previous comparison in some CC register (or anything else that set > such a register). We want to use a cbranchcc4 to reuse some earlier > comparison here. Which is great of course! But, redoing the > (potentially expensive) computation to prepare the CC for a more > complicated condition is not a good idea. Also, Power's conditional > branch insns just branch on one of the 32 condition bits (either set or > unset), not on a logical combination of multiple of those bits, as we > need with LTGT, UNLT, UNGT, UNEQ, and LE and GE without fastmath. So it > is much cleaner (and causes fewer problems later on) if we only allow > those codes we do support. > > Example of LTGT: > fcmpu 0,0,1 # compare f0 <=> f1 to cr0 (exactly one of > # cr0.lt, cr0.gt, cr0.eq, cr0.un will be set) > cror 2,0,1 # cr0.eq = cr0.lt | cr0.gt > beq 0 # branch if cr0.eq is set > > So, we want the cbranchcc4 here to just do that last insn, not the last > two insns (or all three as any other cbranch<mode>4 is!)
Anyhow - my question still stands - what's the fallback for the callers that do not check for failure? How are we sure we're not running into these when relaxing the requirement that a MODE_CC prepare_cmp_insn must not fail? Richard. > > Segher
