On Thu, Mar 07, 2024 at 11:27:28PM +0100, Uros Bizjak wrote: > On Thu, Mar 7, 2024 at 11:07 PM Uros Bizjak <ubiz...@gmail.com> wrote: > > > > (unspec:DI [ > > > > (reg:CC 17 flags) > > > > ] UNSPEC_PUSHFL) > > > > > > But that is invalid RTL? The only valid use of a CC is written as > > > cc-compared-to-0. It means "the result of something compared to 0, > > > stored in that cc reg". > > > > > > (And you can copy a CC reg around, but that is not a use ;-) ) > > Hm... under this premise, we can also say that any form that is not > cc-compared-to-0 is not a use.
Well, no. All uses of CC are written as comparisons to 0, or are pure dataflow. Anything else is not "not a use" but just invalid. > Consequently, if it is not a use, then > the CC reg should not be updated at its use location, so my v1 patch, > where we simply skip the update (but retain the combined insn), > actually makes sense. With more asserts, perhaps. > In this concrete situation, we don't care about CC register mode in > the PUSHFL insn. And we should not change CC reg mode of the use, > because any other mode than the generic CCmode won't be recognized by > the insn pattern. You always care about the CC mode, that is why you always write it as comparison, so the backend can choose a mode based on what the flag bits mean in this context. For an extreme example look at 390, but on pretty much any target both signed and unsigned comparisons use the same flag bits, and maybe fp comparisons as well. But pushfl does sound like pure dataflow. Why is this a builtin, is it ever a good idea if the user writes stuff the compiler can do a better job doing itself, not to mention it is way easier for the compiler anyway? :-) Segher
