linux-kernel - Re: [PATCH 0/5] simple sort swap function usage improvements

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Date:   Sat, 30 Mar 2019 17:16:02 GMT
From:   George Spelvin <lkml@....org>
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Subject: Re: [PATCH 0/5] simple sort swap function usage improvements

Great work; that is indeed a logical follow-on.

Reviewed by: George Spelvin <lkml@....org>

I you feel even more ambitious, you could try impementing Rasmus
Villemoes' idea of having generic *compare* functions.  (It's on
my to-do list, but I haven't made meaningful progress yet, and I'm
happy to pawn it off.)

A significant fraction of the time, the sort key is a 4- or 8-byte
integer field in a structure at a small offset from the base or
list_head.

A function pointer < 4096 could be interpreted as encoding:
- Key size (1 bit)
- Key signedness (1 bit)
- Sort direction (1 bit)
- Offset (9 bits; +/-256 words = +/-1024 bytes, or 0..511 words from start)

With the correct level of preprocessor hackery,
SIMPLE_CMP_ASCENDING(struct type, key_field)
SIMPLE_LIST_CMP_ASCENDING(struct type, list_field, key_field)
SIMPLE_CMP_DESCENDING(struct type, key_field)
SIMPLE_LIST_CMP_DESCENDING(struct type, list_field, key_field)

could encode all that and cause a compile-time error if the key
is the wrong type or the offset is out of range.