linux-kernel - [PATCH 3/7] linux/bitmap.h: remove redundant uses of small_const

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Date:   Sat, 18 Aug 2018 15:16:19 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Yury Norov <ynorov@...iumnetworks.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel@...r.kernel.org
Subject: [PATCH 3/7] linux/bitmap.h: remove redundant uses of small_const_nbits()

In the _zero, _fill and _copy functions, the small_const_nbits
branch is redundant. If nbits is small and const, gcc knows full well
that BITS_TO_LONGS(nbits) is 1, so len is also a compile-time
constant (sizeof(long)), and calling memset or memcpy with a length
argument of sizeof(long) makes gcc generate the expected code anyway:

#include <string.h>
void a(unsigned long *x) { memset(x, 0, 8); }
void b(unsigned long *x) { memset(x, 0xff, 8); }
void c(unsigned long *x, const unsigned long *y) { memcpy(x, y, 8); }

turns into

0000000000000000 <a>:
   0:   48 c7 07 00 00 00 00    movq   $0x0,(%rdi)
   7:   c3                      retq
   8:   0f 1f 84 00 00 00 00    nopl   0x0(%rax,%rax,1)
   f:   00

0000000000000010 <b>:
  10:   48 c7 07 ff ff ff ff    movq   $0xffffffffffffffff,(%rdi)
  17:   c3                      retq
  18:   0f 1f 84 00 00 00 00    nopl   0x0(%rax,%rax,1)
  1f:   00

0000000000000020 <c>:
  20:   48 8b 06                mov    (%rsi),%rax
  23:   48 89 07                mov    %rax,(%rdi)
  26:   c3                      retq

Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
 include/linux/bitmap.h | 24 ++++++------------------
 1 file changed, 6 insertions(+), 18 deletions(-)

diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
index b91a6b5d3e78..a7a8b5017d62 100644
--- a/include/linux/bitmap.h
+++ b/include/linux/bitmap.h
@@ -206,33 +206,21 @@ extern int bitmap_print_to_pagebuf(bool list, char *buf,
 
 static inline void bitmap_zero(unsigned long *dst, unsigned int nbits)
 {
-	if (small_const_nbits(nbits))
-		*dst = 0UL;
-	else {
-		unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
-		memset(dst, 0, len);
-	}
+	unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
+	memset(dst, 0, len);
 }
 
 static inline void bitmap_fill(unsigned long *dst, unsigned int nbits)
 {
-	if (small_const_nbits(nbits))
-		*dst = ~0UL;
-	else {
-		unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
-		memset(dst, 0xff, len);
-	}
+	unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
+	memset(dst, 0xff, len);
 }
 
 static inline void bitmap_copy(unsigned long *dst, const unsigned long *src,
 			unsigned int nbits)
 {
-	if (small_const_nbits(nbits))
-		*dst = *src;
-	else {
-		unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
-		memcpy(dst, src, len);
-	}
+	unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
+	memcpy(dst, src, len);
 }
 
 /*
-- 
2.16.4