Messages in this thread | | | Date | Thu, 25 Jul 2019 08:22:32 -0500 | From | Segher Boessenkool <> | Subject | Re: [PATCH v2] powerpc: slightly improve cache helpers |
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On Tue, Jul 23, 2019 at 09:21:53AM +1000, Michael Ellerman wrote: > Segher Boessenkool <segher@kernel.crashing.org> writes: > >> can use both RA and RB to compute the address, rather than us forcing RA > >> to 0. > >> > >> But at least with my compiler here (GCC 8 vintage) I don't actually see > >> GCC ever using both GPRs even with the patch. Or at least, there's no > >> difference before/after the patch as far as I can see. > > > > The benefit is small, certainly. > > Zero is small, but I guess some things are smaller? :P
Heh. 0 out of 12 is small.
It actually is quite easy to do trigger the macros to generate two-reg dcb* instructions; but all the places where that is especially useful, in loops for example, already use hand-written assembler code (and yes, using two-reg forms).
You probably will not want to write those routines as plain C ever given how important those are for performance (memset, clear-a-page), so the dcb* macros won't ever be very hot, oh well.
> >> So my inclination is to revert the original patch. We can try again in a > >> few years :D > >> > >> Thoughts? > > > > I think you should give the clang people time to figure out what is > > going on. > > Yeah fair enough, will wait and see what their diagnosis is.
Thanks!
Segher
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