linux-kernel - [PATCH v3 0/4] lib: optimize find

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Date:   Sat, 27 Aug 2022 10:58:03 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Cc:     Yury Norov <yury.norov@...il.com>,
        Alexey Klimov <klimov.linux@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Andy Whitcroft <apw@...onical.com>,
        Catalin Marinas <catalin.marinas@....com>,
        David Laight <David.Laight@...LAB.COM>,
        Dennis Zhou <dennis@...nel.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Kees Cook <keescook@...omium.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Russell King <linux@...linux.org.uk>
Subject: [PATCH v3 0/4] lib: optimize find_bit() functions

In the recent discussion, it was noticed that find_next_bit() functions may
be improved by adding wrappers around common __find_next_bit() in .c file.

As suggested by Linus, I tried the meta-programming trick with the
EXPRESSION macro, which is passed from wrapper into find_bit()
helpers:

  #define BIT_FIND_BODY(addr, size, start, EXPRESSION)          \
        BIT_FIND_SETUP(addr, size, start)                       \
        BIT_FIND_FIRST_WORD(addr, size, start, EXPRESSION)      \
        BIT_WORD_LOOP(addr, size, idx, val, EXPRESSION)         \
        return size;                                            \
  found:        BIT_WORD_SWAB(val);                             \
        return min((idx)*BITS_PER_LONG + __ffs(val), size)

  unsigned long _find_next_and_bit(const unsigned long *addr1,
                                 const unsigned long *addr2,
                                 unsigned long size,
                                 unsigned long start)
  { BIT_FIND_BODY(addr, size, start, addr1[idx] & addr2[idx]); }

I appreciated the potential of how the EXPRESSION works, but I don't like
that the resulting macro is constructed from pieces because it makes it
harder to understand what happens behind the ifdefery. Checkpatch isn't
happy as well because the final macro contains 'return' statement; and I
would agree that it's better to avoid it.

I spined the idea one more time, trying to make FIND helper a more or
less standard looking macros.

This new approach saves 10-11K of Image size, and is 15% faster in the
performance benchmark. See the 3rd patch for some statistics.

v1: https://lore.kernel.org/all/20220728161208.865420-2-yury.norov@gmail.com/T/
v2: https://lore.kernel.org/lkml/YwaXvphVpy5A7fSs@yury-laptop/t/
v3:
 - add a MUNGE parameter to FIND_{FIRST,NEXT}_BIT and keep all machinery
   in lib/find_bit.c;
 - rename EXPRESSION to FETCH, and add comments;
 - sync tools.

Yury Norov (4):
  lib/find_bit: introduce FIND_FIRST_BIT() macro
  lib/find_bit: create find_first_zero_bit_le()
  lib/find_bit: optimize find_next_bit() functions
  tools: sync find_bit() implementation

 include/linux/find.h       |  46 +++++++---
 lib/find_bit.c             | 178 ++++++++++++++++++++++---------------
 tools/include/linux/find.h |  61 +++----------
 tools/lib/find_bit.c       | 149 ++++++++++++++-----------------
 4 files changed, 220 insertions(+), 214 deletions(-)

-- 
2.34.1