From: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
Subject: 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
Link: http://lkml.kernel.org/r/20180818131623.8755-4-linux@xxxxxxxxxxxxxxxxxx
Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Andy Shevchenko <andy.shevchenko@xxxxxxxxx>
Cc: Yury Norov <ynorov@xxxxxxxxxxxxxxxxxx>
Cc: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
Cc: Sudeep Holla <sudeep.holla@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/bitmap.h | 24 ++++++------------------
1 file changed, 6 insertions(+), 18 deletions(-)
--- a/include/linux/bitmap.h~linux-bitmaph-remove-redundant-uses-of-small_const_nbits
+++ a/include/linux/bitmap.h
@@ -214,33 +214,21 @@ extern int bitmap_print_to_pagebuf(bool
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);
}
/*
_
