Messages in this thread | | | Date | Wed, 29 Jul 2020 17:44:09 -0300 | From | Arnaldo Carvalho de Melo <> | Subject | Re: [PATCH] perf bench: Add benchmark of find_next_bit |
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Em Wed, Jul 29, 2020 at 04:59:18PM -0300, Arnaldo Carvalho de Melo escreveu: > Em Tue, Jul 28, 2020 at 08:51:52AM -0300, Arnaldo Carvalho de Melo escreveu: > > Em Fri, Jul 24, 2020 at 12:19:59AM -0700, Ian Rogers escreveu: > > > for_each_set_bit, or similar functions like for_each_cpu, may be hot > > > within the kernel. If many bits were set then one could imagine on > > > Intel a "bt" instruction with every bit may be faster than the function > > > call and word length find_next_bit logic. Add a benchmark to measure > > > this. > > > > Thanks, applied.
> > > This benchmark on AMD rome and Intel skylakex shows "bt" is not a good > > > option except for very small bitmaps. > > > > +++ b/tools/perf/bench/find-bit-bench.c > > > > +#if defined(__i386__) || defined(__x86_64__) > > > +static bool asm_test_bit(long nr, const unsigned long *addr) > > > +{ > > > + bool oldbit; > > > + > > > + asm volatile("bt %2,%1" > > > + : "=@ccc" (oldbit) > > > + : "m" (*(unsigned long *)addr), "Ir" (nr) : "memory"); > > > + > > > + return oldbit; > > Some old clang versions are not liking this:
Failed with:
clang version 3.8.0 (tags/RELEASE_380/final) clang version 3.8.1 (tags/RELEASE_381/final) clang version 4.0.0 (tags/RELEASE_400/final) Alpine clang version 8.0.0 (tags/RELEASE_800/final) (based on LLVM 8.0.0) Alpine clang version 5.0.0 (tags/RELEASE_500/final) (based on LLVM 5.0.0) Alpine clang version 5.0.1 (tags/RELEASE_501/final) (based on LLVM 5.0.1) Alpine clang version 5.0.1 (tags/RELEASE_502/final) (based on LLVM 5.0.1)
Worked with:
Alpine clang version 9.0.0 (https://git.alpinelinux.org/aports f7f0d2c2b8bcd6a5843401a9a702029556492689) (based on LLVM 9.0.0) Alpine clang version 10.0.0 (https://gitlab.alpinelinux.org/alpine/aports.git 7445adce501f8473efdb93b17b5eaf2f1445ed4c) Alpine clang version 10.0.0 (git://git.alpinelinux.org/aports 7445adce501f8473efdb93b17b5eaf2f1445ed4c)
Also failed for;
# grep FAIL dm.log/summary | grep -v alpine alt:p8: FAIL clang version 3.8.0 (tags/RELEASE_380/final) alt:p9: FAIL clang version 7.0.1 amazonlinux:1: FAIL clang version 3.6.2 (tags/RELEASE_362/final) amazonlinux:2: FAIL clang version 7.0.1 (Amazon Linux 2 7.0.1-1.amzn2.0.2) #
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