"Nicholas Piggin" <npig...@gmail.com> writes: > On Fri Oct 7, 2022 at 9:23 AM AEST, Segher Boessenkool wrote: >> On Fri, Oct 07, 2022 at 07:56:09AM +1000, Nicholas Piggin wrote: >> > On Fri Oct 7, 2022 at 5:54 AM AEST, Segher Boessenkool wrote: >> > > On Fri, Sep 23, 2022 at 01:30:04PM +1000, Nicholas Piggin wrote: ... >> > > > +# No AltiVec or VSX or MMA instructions when building kernel >> > > > KBUILD_CFLAGS += $(call cc-option,-mno-altivec) >> > > > KBUILD_CFLAGS += $(call cc-option,-mno-vsx) >> > > > +KBUILD_CFLAGS += $(call cc-option,-mno-mma) >> > > >> > > MMA code is never generated unless the code asks for it explicitly. >> > > This is fundamental, not just an implementations side effect. >> > >> > Well, now it double won't be generated :) >> >> Yeah, but there are many other things you can unnecessarily disable as >> well! :-) >> >> VMX and VSX are disabled here because the compiler *will* use those >> registers if it feels like it (that is, if it thinks that will be >> faster). MMA is a very different beast: the compiler can never know if >> it will be faster, to start with. > > True, but now I don't have to find the exact clause and have my lawyer > confirm that it definitely probably won't change in future and break > things.
Right. If someone asks "does the kernel ever use MMA instructions?" we can just point at that line and we have a definite answer. No need to audit the behaviour of all GCC and Clang versions ever released. cheers
