Messages in this thread Patch in this message | | | Date | Mon, 31 Aug 2015 14:19:10 +0300 | From | "Michael S. Tsirkin" <> | Subject | Re: [PATCH 1/2] x86/bitops: implement __test_bit |
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On Mon, Aug 31, 2015 at 09:59:47AM +0200, Ingo Molnar wrote: > > * Michael S. Tsirkin <mst@redhat.com> wrote: > > > On Sun, Aug 30, 2015 at 11:13:20PM -0700, H. Peter Anvin wrote: > > > Presumably because gcc can't generate bt... whether or not it is worth it is another matter. > > > > > > On August 30, 2015 11:05:49 PM PDT, Ingo Molnar <mingo@kernel.org> wrote: > > > > > > > >* Michael S. Tsirkin <mst@redhat.com> wrote: > > > > > > > >> +static __always_inline int __constant_test_bit(long nr, const > > > >unsigned long *addr) > > > >> +{ > > > >> + return ((1UL << (nr & (BITS_PER_LONG-1))) & > > > >> + (addr[nr >> _BITOPS_LONG_SHIFT])) != 0; > > > >> +} > > > >> + > > > >> +static inline int __variable_test_bit(long nr, const unsigned long > > > >*addr) > > > >> +{ > > > >> + int oldbit; > > > >> + > > > >> + asm volatile("bt %2,%1\n\t" > > > >> + "sbb %0,%0" > > > >> + : "=r" (oldbit) > > > >> + : "m" (*addr), "Ir" (nr)); > > > >> + > > > >> + return oldbit; > > > >> +} > > > > > > > >Color me confused, why use assembly for this at all? > > > > > > > >Why not just use C for testing the bit (i.e. turn __constant_test_bit() > > > >into > > > >__test_bit()) - that would also allow the compiler to propagate the > > > >result, > > > >potentially more optimally than we can do it via SBB... > > > > > > > >Thanks, > > > > > > > > Ingo > > > > Exactly: > > > > > > Disassembly of section .text: > > > > 00000000 <__variable_test_bit>: > > __variable_test_bit(): > > 0: 8b 54 24 08 mov 0x8(%esp),%edx > > 4: 8b 44 24 04 mov 0x4(%esp),%eax > > 8: 0f a3 02 bt %eax,(%edx) > > b: 19 c0 sbb %eax,%eax > > d: c3 ret > > e: 66 90 xchg %ax,%ax > > > > 00000010 <__constant_test_bit>: > > __constant_test_bit(): > > 10: 8b 4c 24 04 mov 0x4(%esp),%ecx > > 14: 8b 44 24 08 mov 0x8(%esp),%eax > > 18: 89 ca mov %ecx,%edx > > 1a: c1 fa 04 sar $0x4,%edx > > 1d: 8b 04 90 mov (%eax,%edx,4),%eax > > 20: d3 e8 shr %cl,%eax > > 22: 83 e0 01 and $0x1,%eax > > 25: c3 ret > > But that's due to the forced interface of generating a return code. Please compare > it at an inlined usage site, where GCC is free to do the comparison directly and > use the result in flags. > > Thanks, > > Ingo
I applied this patch on top of mine:
diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h index 9229334..2aed985 100644 --- a/arch/x86/include/asm/bitops.h +++ b/arch/x86/include/asm/bitops.h @@ -323,24 +323,17 @@ static inline int variable_test_bit(long nr, volatile const unsigned long *addr) return oldbit; } -static __always_inline int __constant_test_bit(long nr, const unsigned long *addr) +/** + * __test_bit - Determine whether a bit is set + * @nr: bit number to test + * @addr: Address to start counting from + */ +static __always_inline int __test_bit(long nr, const unsigned long *addr) { return ((1UL << (nr & (BITS_PER_LONG-1))) & (addr[nr >> _BITOPS_LONG_SHIFT])) != 0; } -static inline int __variable_test_bit(long nr, const unsigned long *addr) -{ - int oldbit; - - asm volatile("bt %2,%1\n\t" - "sbb %0,%0" - : "=r" (oldbit) - : "m" (*addr), "Ir" (nr)); - - return oldbit; -} - #if 0 /* Fool kernel-doc since it doesn't do macros yet */ /** * test_bit - Determine whether a bit is set @@ -348,13 +341,6 @@ static inline int __variable_test_bit(long nr, const unsigned long *addr) * @addr: Address to start counting from */ static int test_bit(int nr, const volatile unsigned long *addr); - -/** - * __test_bit - Determine whether a bit is set - * @nr: bit number to test - * @addr: Address to start counting from - */ -static int __test_bit(int nr, const volatile unsigned long *addr); #endif #define test_bit(nr, addr) \ @@ -362,10 +348,6 @@ static int __test_bit(int nr, const volatile unsigned long *addr); ? constant_test_bit((nr), (addr)) \ : variable_test_bit((nr), (addr))) -#define __test_bit(nr, addr) \ - (__builtin_constant_p((nr)) \ - ? __constant_test_bit((nr), (addr)) \ - : __variable_test_bit((nr), (addr))) /** * __ffs - find first set bit in word
And the code size went up:
134836 2997 8372 146205 23b1d arch/x86/kvm/kvm-intel.ko -> 134846 2997 8372 146215 23b27 arch/x86/kvm/kvm-intel.ko
342690 47640 441 390771 5f673 arch/x86/kvm/kvm.ko -> 342738 47640 441 390819 5f6a3 arch/x86/kvm/kvm.ko
I tried removing __always_inline, this had no effect.
-- MST
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