linux-kernel - [PATCH 05/12] lib: extend the scope of small_const

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Date:   Sun, 21 Mar 2021 14:54:50 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Yury Norov <yury.norov@...il.com>, linux-m68k@...ts.linux-m68k.org,
        linux-arch@...r.kernel.org, linux-sh@...r.kernel.org,
        Alexey Klimov <aklimov@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>, David Sterba <dsterba@...e.com>,
        Dennis Zhou <dennis@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Jianpeng Ma <jianpeng.ma@...el.com>,
        Joe Perches <joe@...ches.com>,
        John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Rich Felker <dalias@...c.org>,
        Stefano Brivio <sbrivio@...hat.com>,
        Wei Yang <richard.weiyang@...ux.alibaba.com>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>
Subject: [PATCH 05/12] lib: extend the scope of small_const_nbits() macro

find_bit would also benefit from small_const_nbits() optimizations.
The detailed comment is provided by Rasmus Villemoes.

Signed-off-by: Yury Norov <yury.norov@...il.com>
Suggested-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
 include/asm-generic/bitsperlong.h | 12 ++++++++++++
 include/linux/bitmap.h            |  8 --------
 2 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/include/asm-generic/bitsperlong.h b/include/asm-generic/bitsperlong.h
index 3905c1c93dc2..1023e2a4bd37 100644
--- a/include/asm-generic/bitsperlong.h
+++ b/include/asm-generic/bitsperlong.h
@@ -23,4 +23,16 @@
 #define BITS_PER_LONG_LONG 64
 #endif
 
+/*
+ * small_const_nbits(n) is true precisely when it is known at compile-time
+ * that BITMAP_SIZE(n) is 1, i.e. 1 <= n <= BITS_PER_LONG. This allows
+ * various bit/bitmap APIs to provide a fast inline implementation. Bitmaps
+ * of size 0 are very rare, and a compile-time-known-size 0 is most likely
+ * a sign of error. They will be handled correctly by the bit/bitmap APIs,
+ * but using the out-of-line functions, so that the inline implementations
+ * can unconditionally dereference the pointer(s).
+ */
+#define small_const_nbits(nbits) \
+	(__builtin_constant_p(nbits) && (nbits) <= BITS_PER_LONG && (nbits) > 0)
+
 #endif /* __ASM_GENERIC_BITS_PER_LONG */
diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
index 70a932470b2d..8c4adfa5cceb 100644
--- a/include/linux/bitmap.h
+++ b/include/linux/bitmap.h
@@ -222,14 +222,6 @@ extern int bitmap_print_to_pagebuf(bool list, char *buf,
 #define BITMAP_FIRST_WORD_MASK(start) (~0UL << ((start) & (BITS_PER_LONG - 1)))
 #define BITMAP_LAST_WORD_MASK(nbits) (~0UL >> (-(nbits) & (BITS_PER_LONG - 1)))
 
-/*
- * The static inlines below do not handle constant nbits==0 correctly,
- * so make such users (should any ever turn up) call the out-of-line
- * versions.
- */
-#define small_const_nbits(nbits) \
-	(__builtin_constant_p(nbits) && (nbits) <= BITS_PER_LONG && (nbits) > 0)
-
 static inline void bitmap_zero(unsigned long *dst, unsigned int nbits)
 {
 	unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
-- 
2.25.1