lkml.org 
[lkml]   [2018]   [Aug]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH 3/7] linux/bitmap.h: remove redundant uses of small_const_nbits()
Date
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@rasmusvillemoes.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
\
 
 \ /
  Last update: 2018-08-18 15:17    [W:0.097 / U:2.508 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site