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

Andy Shevchenko
2021-04-01T08:49:26Z

On Thu, Apr 1, 2021 at 3:41 AM Yury Norov <yury.norov@gmail.com> 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@gmail.com>

> Signed-off-by: Yury Norov <yury.norov@gmail.com>
> Acked-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> ---
> 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