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    SubjectRe: [PATCH 1/2] x86/bitops: implement __test_bit

    * Ingo Molnar <mingo@kernel.org> wrote:

    > > 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.

    So I was thinking about the patch below on top of yours.

    But turns out GCC indeed generates worse code even under the best of
    circumstances. For example the nested_vmx_disable_intercept_for_msr() change:

    @@ -4275,24 +4275,24 @@ static void nested_vmx_disable_intercept
    */
    if (msr <= 0x1fff) {
    if (type & MSR_TYPE_R &&
    - !test_bit(msr, msr_bitmap_l1 + 0x000 / f))
    + !__test_bit(msr, msr_bitmap_l1 + 0x000 / f))
    /* read-low */
    __clear_bit(msr, msr_bitmap_nested + 0x000 / f);

    before (i.e. your series):

    ffffffff818b1082: 89 d0 mov %edx,%eax
    ffffffff818b1084: 48 0f a3 07 bt %rax,(%rdi)
    ffffffff818b1088: 45 19 c0 sbb %r8d,%r8d
    ffffffff818b108b: 45 85 c0 test %r8d,%r8d
    ffffffff818b108e: 75 04 jne ffffffff818b1094 <nested_vmx_disable_intercept_for_msr+0x43>

    after (with my 'optimization' patch applied):

    ffffffff818b1091: 89 d0 mov %edx,%eax
    ffffffff818b1093: 49 89 c0 mov %rax,%r8
    ffffffff818b1096: 49 c1 f8 06 sar $0x6,%r8
    ffffffff818b109a: 4e 8b 04 c7 mov (%rdi,%r8,8),%r8
    ffffffff818b109e: 49 0f a3 d0 bt %rdx,%r8
    ffffffff818b10a2: 72 04 jb ffffffff818b10a8 <nested_vmx_disable_intercept_for_msr+0x48>

    So GCC when left to its own devices, generates one more instruction and 4 more
    bytes. Why does GCC do that? Why doesn't it use BT directly and use the flag, i.e.
    something like [pseudocode]:

    ffffffff818b1082: 89 d0 mov %edx,%eax
    ffffffff818b1084: 48 0f a3 07 bt %rax,(%rdi)
    ffffffff818b108e: 75 04 jne ffffffff818b1094 <nested_vmx_disable_intercept_for_msr+0x43>

    ?

    In any case I take back my objection:

    Acked-by: Ingo Molnar <mingo@kernel.org>

    Thanks,

    Ingo


    ---
    arch/x86/include/asm/bitops.h | 19 +------------------
    1 file changed, 1 insertion(+), 18 deletions(-)

    Index: tip/arch/x86/include/asm/bitops.h
    ===================================================================
    --- tip.orig/arch/x86/include/asm/bitops.h
    +++ tip/arch/x86/include/asm/bitops.h
    @@ -323,24 +323,12 @@ static inline int variable_test_bit(long
    return oldbit;
    }

    -static __always_inline int __constant_test_bit(long nr, const unsigned long *addr)
    +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
    @@ -362,11 +350,6 @@ static int __test_bit(int nr, const vola
    ? 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
    * @word: The word to search


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