Re: [PATCH v2] pgo: add clang's Profile Guided Optimization infrastructure
From: Nick Desaulniers <ndesaulniers@google.com>
Date: 2021-01-12 17:38:11
Also in:
linux-kbuild, lkml
On Mon, Jan 11, 2021 at 9:14 PM Bill Wendling [off-list ref] wrote:
From: Sami Tolvanen <samitolvanen@google.com> Enable the use of clang's Profile-Guided Optimization[1]. To generate a profile, the kernel is instrumented with PGO counters, a representative workload is run, and the raw profile data is collected from /sys/kernel/debug/pgo/profraw. The raw profile data must be processed by clang's "llvm-profdata" tool before it can be used during recompilation: $ cp /sys/kernel/debug/pgo/profraw vmlinux.profraw $ llvm-profdata merge --output=vmlinux.profdata vmlinux.profraw Multiple raw profiles may be merged during this step. The data can now be used by the compiler: $ make LLVM=1 KCFLAGS=-fprofile-use=vmlinux.profdata ... This initial submission is restricted to x86, as that's the platform we
Please drop all changes to arch/* that are not to arch/x86/ then; we can cross that bridge when we get to each arch. For example, there's no point disabling PGO for architectures LLVM doesn't even have a backend for.
know works. This restriction can be lifted once other platforms have been verified to work with PGO. Note that this method of profiling the kernel is clang-native and isn't compatible with clang's gcov support in kernel/gcov.
Then the Kconfig option should depend on !GCOV so that they are mutually exclusive and can't be selected together accidentally; such as by bots doing randconfig tests. <large snip>
quoted hunk ↗ jump to hunk
+static inline int inst_prof_popcount(unsigned long long value) +{ + value = value - ((value >> 1) & 0x5555555555555555ULL); + value = (value & 0x3333333333333333ULL) + + ((value >> 2) & 0x3333333333333333ULL); + value = (value + (value >> 4)) & 0x0F0F0F0F0F0F0F0FULL; + + return (int)((unsigned long long)(value * 0x0101010101010101ULL) >> 56); +}
The kernel has a portable popcnt implementation called hweight64 if you #include <asm-generic/bitops/hweight.h>; does that work here? https://en.wikipedia.org/wiki/Hamming_weight -- Thanks, ~Nick Desaulniers