Re: [PATCH 09/12] lib: add fast path for find_next_*_bit()

[Andy Shevchenko]

On Thu, Apr 1, 2021 at 3:41 AM Yury Norov <yury.norov@xxxxxxxxx> wrote:
>
> Similarly to bitmap functions, find_next_*_bit() users will benefit
> if we'll handle a case of bitmaps that fit into a single word inline.
> In the very best case, the compiler may replace a function call with
> a few instructions.
>
> This is the quite typical find_next_bit() user:
>
>         unsigned int cpumask_next(int n, const struct cpumask *srcp)
>         {
>                 /* -1 is a legal arg here. */
>                 if (n != -1)
>                         cpumask_check(n);
>                 return find_next_bit(cpumask_bits(srcp), nr_cpumask_bits, n + 1);
>         }
>         EXPORT_SYMBOL(cpumask_next);
>
> Currently, on ARM64 the generated code looks like this:
>         0000000000000000 <cpumask_next>:
>            0:   a9bf7bfd        stp     x29, x30, [sp, #-16]!
>            4:   11000402        add     w2, w0, #0x1
>            8:   aa0103e0        mov     x0, x1
>            c:   d2800401        mov     x1, #0x40                       // #64
>           10:   910003fd        mov     x29, sp
>           14:   93407c42        sxtw    x2, w2
>           18:   94000000        bl      0 <find_next_bit>
>           1c:   a8c17bfd        ldp     x29, x30, [sp], #16
>           20:   d65f03c0        ret
>           24:   d503201f        nop
>
> After applying this patch:
>         0000000000000140 <cpumask_next>:
>          140:   11000400        add     w0, w0, #0x1
>          144:   93407c00        sxtw    x0, w0
>          148:   f100fc1f        cmp     x0, #0x3f
>          14c:   54000168        b.hi    178 <cpumask_next+0x38>  // b.pmore
>          150:   f9400023        ldr     x3, [x1]
>          154:   92800001        mov     x1, #0xffffffffffffffff         // #-1
>          158:   9ac02020        lsl     x0, x1, x0
>          15c:   52800802        mov     w2, #0x40                       // #64
>          160:   8a030001        and     x1, x0, x3
>          164:   dac00020        rbit    x0, x1
>          168:   f100003f        cmp     x1, #0x0
>          16c:   dac01000        clz     x0, x0
>          170:   1a800040        csel    w0, w2, w0, eq  // eq = none
>          174:   d65f03c0        ret
>          178:   52800800        mov     w0, #0x40                       // #64
>          17c:   d65f03c0        ret
>
> find_next_bit() call is replaced with 6 instructions. find_next_bit()
> itself is 41 instructions plus function call overhead.
>
> Despite inlining, the scripts/bloat-o-meter report smaller .text size
> after applying the series:
>         add/remove: 11/9 grow/shrink: 233/176 up/down: 5780/-6768 (-988)

Acked-by: Andy Shevchenko <andy.shevchenko@xxxxxxxxx>

> Signed-off-by: Yury Norov <yury.norov@xxxxxxxxx>
> Acked-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
> ---
>  include/asm-generic/bitops/find.h | 30 ++++++++++++++++++++++++++++++
>  include/asm-generic/bitops/le.h   | 21 +++++++++++++++++++++
>  2 files changed, 51 insertions(+)
>
> diff --git a/include/asm-generic/bitops/find.h b/include/asm-generic/bitops/find.h
> index 7ad70dab8e93..4148c74a1e4d 100644
> --- a/include/asm-generic/bitops/find.h
> +++ b/include/asm-generic/bitops/find.h
> @@ -20,6 +20,16 @@ static inline
>  unsigned long find_next_bit(const unsigned long *addr, unsigned long size,
>                             unsigned long offset)
>  {
> +       if (small_const_nbits(size)) {
> +               unsigned long val;
> +
> +               if (unlikely(offset >= size))
> +                       return size;
> +
> +               val = *addr & GENMASK(size - 1, offset);
> +               return val ? __ffs(val) : size;
> +       }
> +
>         return _find_next_bit(addr, NULL, size, offset, 0UL, 0);
>  }
>  #endif
> @@ -40,6 +50,16 @@ unsigned long find_next_and_bit(const unsigned long *addr1,
>                 const unsigned long *addr2, unsigned long size,
>                 unsigned long offset)
>  {
> +       if (small_const_nbits(size)) {
> +               unsigned long val;
> +
> +               if (unlikely(offset >= size))
> +                       return size;
> +
> +               val = *addr1 & *addr2 & GENMASK(size - 1, offset);
> +               return val ? __ffs(val) : size;
> +       }
> +
>         return _find_next_bit(addr1, addr2, size, offset, 0UL, 0);
>  }
>  #endif
> @@ -58,6 +78,16 @@ static inline
>  unsigned long find_next_zero_bit(const unsigned long *addr, unsigned long size,
>                                  unsigned long offset)
>  {
> +       if (small_const_nbits(size)) {
> +               unsigned long val;
> +
> +               if (unlikely(offset >= size))
> +                       return size;
> +
> +               val = *addr | ~GENMASK(size - 1, offset);
> +               return val == ~0UL ? size : ffz(val);
> +       }
> +
>         return _find_next_bit(addr, NULL, size, offset, ~0UL, 0);
>  }
>  #endif
> diff --git a/include/asm-generic/bitops/le.h b/include/asm-generic/bitops/le.h
> index 21305f6cea0b..5a28629cbf4d 100644
> --- a/include/asm-generic/bitops/le.h
> +++ b/include/asm-generic/bitops/le.h
> @@ -5,6 +5,7 @@
>  #include <asm-generic/bitops/find.h>
>  #include <asm/types.h>
>  #include <asm/byteorder.h>
> +#include <linux/swab.h>
>
>  #if defined(__LITTLE_ENDIAN)
>
> @@ -37,6 +38,16 @@ static inline
>  unsigned long find_next_zero_bit_le(const void *addr, unsigned
>                 long size, unsigned long offset)
>  {
> +       if (small_const_nbits(size)) {
> +               unsigned long val = *(const unsigned long *)addr;
> +
> +               if (unlikely(offset >= size))
> +                       return size;
> +
> +               val = swab(val) | ~GENMASK(size - 1, offset);
> +               return val == ~0UL ? size : ffz(val);
> +       }
> +
>         return _find_next_bit(addr, NULL, size, offset, ~0UL, 1);
>  }
>  #endif
> @@ -46,6 +57,16 @@ static inline
>  unsigned long find_next_bit_le(const void *addr, unsigned
>                 long size, unsigned long offset)
>  {
> +       if (small_const_nbits(size)) {
> +               unsigned long val = *(const unsigned long *)addr;
> +
> +               if (unlikely(offset >= size))
> +                       return size;
> +
> +               val = swab(val) & GENMASK(size - 1, offset);
> +               return val ? __ffs(val) : size;
> +       }
> +
>         return _find_next_bit(addr, NULL, size, offset, 0UL, 1);
>  }
>  #endif
> --
> 2.25.1
>


-- 
With Best Regards,
Andy Shevchenko