Differential D20250
Build arm64 kernel with inline ffs, fls Authored by dougm on May 12 2019, 9:16 PM. Tags None Referenced Files
Details Implement the ffs and fls instructions, and their longer counterparts, in cpufunc, in terms of gcc extensions like __builtin_ffs, for arm64 architectures, and use those, rather than simple libkern implementations, in building arm64 kernels. Tested by: greg_unrelenting.technology I can complete an arm64 build, and I can examine an object file like pmap.o or subr_blist.o for signs that the inlining has happened, as instruction sequences like 244: dac001c9 rbit x9, x14 appear there to indicate a fast implementation of ffs. My reference on smart bit ops for arm64 and how to recognize them is https://fgiesen.wordpress.com/2013/10/18/bit-scanning-equivalencies/. Without the patch, the arm64 build does not have these sequences in these object files and has lines like this instead: af34: 94000000 bl 0 <ffsl> which do not appear with the patch in place. None of this suggests that this patch is correct, but only that it does something. I don't have the means to test an arm64 kernel at this time. If Peter Holm can help, that would be great. If not, I'll have to wait for the freebsd arm64 test boxes to come back online.
Diff Detail
Event TimelineComment Actions Can somebody explain to me why don't we use __builtin_ffs() and other functions everywhere ?
Comment Actions In case you are asking why I wrote code using builtin_clz and not builtin_ffs, it is because I believe that many call to ffs and fls always subtract one from the result, and always know that the argument is nonzero, and I think that the way I wrote things might let the compiler optimize out the zero-check and/or the plus-one sometimes, and maybe with __builtin_ffs that wouldn't happen. But that is speculation. I case you are asking about the whole freebsd codebase in general, I can't say. Comment Actions Kernel with this patch boots fine on Amazon EC2 (which btw you can use too, freebsd-snapshots posts have aarch64 AMIs) Comment Actions The only difference in the pmap.o code on arm64 between using builtin_ffsl and _builtin_ctzl (with a zero-test and an add) is that the version with builtin_ffsl has an 'orr' instruction where the other version has an 'add'. So this version uses builtin_ffs and builtin_ffsl.
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