On Tue, Jul 23, 2019 at 12:15:39AM +0300, Alexey Dobriyan wrote: > On Mon, Jul 22, 2019 at 11:12:10PM +0200, Peter Zijlstra wrote: > > On Mon, Jul 22, 2019 at 11:27:20PM +0300, Alexey Dobriyan wrote: > > > Detect POPCNT instruction support and inline hweigth*() functions > > > if it is supported by CPU. > > > > > > Detect POPCNT at boot time and conditionally refuse to boot. > > > > > > Signed-off-by: Alexey Dobriyan <adobriyan@xxxxxxxxx> > > > --- > > > arch/x86/include/asm/arch_hweight.h | 24 +++++++++++++++++++ > > > arch/x86/include/asm/segment.h | 1 + > > > arch/x86/kernel/verify_cpu.S | 8 +++++++ > > > arch/x86/lib/Makefile | 5 +++- > > > .../drm/i915/display/intel_display_power.c | 2 +- > > > drivers/misc/sgi-gru/grumain.c | 2 +- > > > fs/btrfs/tree-checker.c | 4 ++-- > > > include/linux/bitops.h | 2 ++ > > > lib/Makefile | 2 ++ > > > scripts/kconfig/cpuid.c | 7 ++++++ > > > scripts/march-native.sh | 2 ++ > > > 11 files changed, 54 insertions(+), 5 deletions(-) > > > > *WHY* ? > > > > AFAICT this just adds lines and complexity and wins aboslutely nothing. > > If CPU is know to have POPCNT, it doesn't make sense to go through RDI. > Additionally some CPUs (still?) have fake dependency on the destination, > so "popcnt rax, rdi" is suboptimal. More general argument is that if -march=native is accepted, compiler will generate new instructions which will throw #UD on CPUs which aren't capable, so it doesn't make sense to _not_ go deeper and use all the knowledge about current CPU.
