Re: [PATCH 03/15] print_integer: new and improved way of printing integers

[Andy Shevchenko]

On Mon, Apr 20, 2020 at 11:57:31PM +0300, Alexey Dobriyan wrote:
> Time honored way to print integers via vsnprintf() or equivalent has
> unavoidable slowdown of parsing format string. This can't be fixed in C,
> without introducing external preprocessor.
> 
> seq_put_decimal_ull() partially saves the day, but there are a lot of
> branches inside and overcopying still.
> 
> _print_integer_*() family of functions is meant to make printing
> integers as fast as possible by deleting format string parsing and doing
> as little work as possible.
> 
> It is based on the following observations:
> 
> 1) memcpy is done in forward direction
> 	it can be done backwards but nobody does that,
> 
> 2) digits can be extracted in a very simple loop which costs only
> 	1 multiplication and shift (division by constant is not division)
> 
> All the above asks for the following signature, semantics and pattern of
> printing out beloved /proc files:
> 
> 	/* seq_printf(seq, "%u %llu\n", A, b); */
> 
> 	char buf[10 + 1 + 20 + 1];
> 	char *p = buf + sizeof(buf);
> 
> 	*--p = '\n';
> 	p = _print_integer_u64(p, B);
> 	*--p = ' ';
> 	p = _print_integer_u32(p, A);
> 
> 	seq_write(seq, p, buf + sizeof(buf) - p);
> 
> 1) stack buffer capable of holding the biggest string is allocated.
> 
> 2) "p" is pointer to start of the string. Initially it points past
> 	the end of the buffer WHICH IS NOT NUL-TERMINATED!
> 
> 3) _print_integer_*() actually prints an integer from right to left
> 	and returns new start of the string.
> 
> 			     <--------|
> 				123
> 				^
> 				|
> 				+-- p
> 
> 4) 1 character is printed with
> 
> 	*--p = 'x';
> 
> 	It generates very efficient code as multiple writes can be
> 	merged.
> 
> 5) fixed string is printed with
> 
> 	p = memcpy(p - 3, "foo", 3);
> 
> 	Complers know what memcpy() does and write-combine it.
> 	4/8-byte writes become 1 instruction and are very efficient.
> 
> 6) Once everything is printed, the result is written to seq_file buffer.
> 	It does only one overflow check and 1 copy.
> 
> This generates very efficient code (and small!).
> 
> In regular seq_printf() calls, first argument and format string are
> constantly reloaded. Format string will most likely with [rip+...] which
> is quite verbose.
> 
> seq_put_decimal_ull() will do branches (and even more branches
> with "width" argument)
> 

> 	TODO
> 	benchmark with mainline because nouveau is broken for me -(
> 	vsnprintf() changes make the code slower

Exactly main point of this exercise. I don't believe that algos in vsprintf.c
are too dumb to use division per digit (yes, division by constant which is not
power of two is a heavy operation).


> +noinline
> +char *_print_integer_u32(char *p, u32 x)
> +{
> +	do {
> +		*--p = '0' + (x % 10);
> +	} while (x /= 10);
> +	return p;
> +}

> +noinline
> +char *_print_integer_u64(char *p, u64 x)
> +{
> +	while (x >= 100 * 1000 * 1000) {
> +		u32 r;
> +
> +		x = div_u64_rem(x, 100 * 1000 * 1000, &r);
> +		p = memset(p - 8, '0', 8);
> +		(void)_print_integer_u32(p + 8, r);
> +	}
> +	return _print_integer_u32(p, x);
> +}

-- 
With Best Regards,
Andy Shevchenko