Re: [PATCH 4/5] lib/list_sort: Simplify and remove MAX_LIST_LENGTH_BITS

[George Spelvin]

On Thu, 14 Mar 2019 at 11:10:41 +0200, Andy Shevchenko wrote:
> On Tue, Mar 05, 2019 at 03:06:44AM +0000, George Spelvin wrote:
>> +		for (bit = 1; count & bit; bit <<= 1) {
>> +			cur = merge(priv, (cmp_func)cmp, pending, cur);
>> +			pending = pending->prev;  /* Untouched by merge() */
>>  		}
>
> Wouldn't be it the same to
> 
> 	bit = ffz(count);
> 	while (bit--) {
> 		...
> 	}
> ?
>
> Though I dunno which one is generating better code.

One question I should ask everyone: should "count" be 32 or 64 bits
on 64-bit machines?  That would let x86 save a few REX bytes.  (815
vs. 813 byte code, if anyone cares.)

Allegedy ARM can save a few pJ by gating the high 32
bits of the ALU.

Most other 64-bit processors would prefer 64-bit operations as
it saves masking operations.

If we never sort a list with more than 2^32 entries, it
makes no difference.

If we use a 32-bit count and we *do* sort a list with more than
2^32 entries, then it still sorts, but the performance degrades to
O((n/2^32)^2).

Just how often do we expect the kernel to face lists that long?
(Note that the old code was O((n/2^20)^2).)

In the code, I could do something like

#ifdef CONFIG_X86_64
/* Comment explaining why */
typedef uint32_t count_t;
#else
typedef size_t count_t;
#endif

...
	count_t count = 0;