Re: [PATCH 11/14] lib: add fast path for find_next_*_bit()

[Andy Shevchenko]

On Wed, Feb 17, 2021 at 08:05:09PM -0800, Yury Norov 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. 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);
> 
> On ARM64 if CONFIG_FAST_PATH is disabled it generates:

bloat-o-meter over specific module and/or vmlinux.o?

> 	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
> 
> If CONFIG_FAST_PATH is enabled:
> 	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. (And I suspect
> we can improve the GENMASK() for better code generation.) find_next_bit()
> itself is 41 instructions.

...

> +	if (SMALL_CONST(size - 1)) {
> +		unsigned long val;
> +
> +		if (unlikely(offset >= size))
> +			return size;

> +		val = *addr & GENMASK(size - 1, offset);

Yeah, GENMASK() basically for constant values or cases like (x,0). I think here
is something what has been done in BITMAP_FIRST/LAST_WORD_MASK will give better
results.

> +		return val ? __ffs(val) : size;
> +	}

-- 
With Best Regards,
Andy Shevchenko