On Wed, 2023-05-24 at 13:32 +0800, Kewen.Lin wrote: > on 2023/5/24 06:30, Peter Bergner wrote: > > On 5/23/23 12:24 AM, Kewen.Lin wrote: > > > on 2023/5/23 01:31, Carl Love wrote: > > > > The builtins were requested for use in GLibC. As of version > > > > 2.31 they > > > > were added as inline asm. They requested a builtin so the asm > > > > could be > > > > removed. > > > > > > So IMHO we also want the similar support for mffscrn, that is to > > > make > > > use of mffscrn and mffscrni on Power9 and later, but falls back > > > to > > > __builtin_set_fpscr_rn + mffs similar on older platforms. > > > > So __builtin_set_fpscr_rn everything we want (sets the RN bits) and > > uses mffscrn/mffscrni on P9 and later and uses older insns on pre- > > P9. > > The only problem is we don't return the current FPSCR bits, as the > > bif > > is defined to return void. > > Yes. > > > Crazy idea, but could we extend the built-in > > with an overload that returns the FPSCR bits? > > So you agree that we should make this proposed new bif handle pre-P9 > just > like some other existing bifs. :) I think extending it is good and > doable, > but the only concern here is the bif name "__builtin_set_fpscr_rn", > which > matches the existing behavior (only set rounding) but doesn't match > the > proposed extending behavior (set rounding and get some env bits > back). > Maybe it's not a big deal if the documentation clarify it well.
Extending the builtin to pre Power 9 is straight forward and I agree would make good sense to do. I am a bit concerned on how to extend __builtin_set_fpscr_rn to add the new functionality. Peter suggests overloading the builtin to either return void or returns FPSCR bits. It is my understanding that the return value for a given builtin had to be the same, i.e. you can't overload the return value. Maybe you can with Bill's new infrastructure? I recall having problems trying to overload the return value in the past and Bill said you couldn't do it. I play with this and see if I can overload the return value. > > > > To be honest, I like > > the __builtin_set_fpscr_rn name better than __builtin_mffscrn[i]. > > +1 > > BR, > Kewen > > > The built-in machinery can see that the usage is expecting a return > > value > > or not and for the pre-P9 code, can skip generating the ending mffs > > if > > we don't want the return value. > > > > Peter > > > >
