On Tue, 1 Feb 2005, linux-os wrote:
On Tue, 1 Feb 2005, Andreas Gruenbacher wrote:
On Mon, 2005-01-31 at 18:30, Paulo Marques wrote:
Andreas Gruenbacher wrote:
[...]
static inline void swap(void *a, void *b, int size)
{
if (size % sizeof(long)) {
char t;
do {
t = *(char *)a;
*(char *)a++ = *(char *)b;
*(char *)b++ = t;
} while (--size > 0);
} else {
long t;
do {
t = *(long *)a;
*(long *)a = *(long *)b;
*(long *)b = t;
size -= sizeof(long);
} while (size > sizeof(long));
You forgot to increment a and b, and this should be "while (size);", no?
Correct, yes.
Or better yet,
static inline void swap(void *a, void *b, int size)
{
long tl;
char t;
while (size >= sizeof(long)) {
tl = *(long *)a;
*(long *)a = *(long *)b;
*(long *)b = tl;
a += sizeof(long);
b += sizeof(long);
size -= sizeof(long);
}
while (size) {
t = *(char *)a;
*(char *)a++ = *(char *)b;
*(char *)b++ = t;
size--;
}
}
This works better if the size is not a multiple of sizeof(long), but is
bigger than a long.
In case size is not a multiple of sizeof(long) here, you'll have
unaligned memory accesses most of the time anyway, so it probably
doesn't really matter.
Thanks,
--
Andreas Gruenbacher <[EMAIL PROTECTED]>
SUSE Labs, SUSE LINUX GMBH
This uses an GNU-ISM where you are doing pointer arithmetic
on a pointer-to-void. NotGood(tm) this is an example of
where there is absolutely no rationale whatsoever for
writing such code.
Here is swap with no games. Plus it handles the case where
sizeof(long) is not the same as sizeof(int).
void swap(void *a, void *b, size_t len)
{
while(len >= sizeof(long))
{
long one, two;
long *p0 = a;
long *p1 = b;
one = *p0;
two = *p1;
*p1++ = one;
*p0++ = two;
len -= sizeof(long);
}
while(len >= sizeof(int)) // Compiler may even ignore
{
int one, two;
int *p0 = a;
int *p1 = b;
one = *p0;
two = *p1;
*p1++ = one;
*p0++ = two;
len -= sizeof(int);
}
while(len--)
{
char one, two;
char *p0 = a;
char *p1 = b;
one = *p0;
two = *p1;
*p1++ = one;
*p0++ = two;
}
}
//----------------------------------------------
And here is the output. You can make it in-line of you want,
but you really need to make in ANSI first.
.file "xxx.c"
.text
.p2align 2,,3
.globl swap
.type swap, @function
swap:
pushl %ebp
movl %esp, %ebp
movl 16(%ebp), %ecx
pushl %esi
cmpl $3, %ecx
pushl %ebx
movl 8(%ebp), %esi
movl 12(%ebp), %ebx
jbe .L17
.p2align 2,,3
.L5:
subl $4, %ecx
movl (%esi), %eax
movl (%ebx), %edx
cmpl $3, %ecx
movl %eax, (%ebx)
movl %edx, (%esi)
ja .L5
.L17:
decl %ecx
cmpl $-1, %ecx
je .L19
.p2align 2,,3
.L13:
decl %ecx
movb (%esi), %al
movb (%ebx), %dl
cmpl $-1, %ecx
movb %al, (%ebx)
movb %dl, (%esi)
jne .L13
.L19:
popl %ebx
popl %esi
leave
ret
.size swap, .-swap
.section .note.GNU-stack,"",@progbits
.ident "GCC: (GNU) 3.3.3 20040412 (Red Hat Linux 3.3.3-7)"
A lot of folks don't realilize that adding a new program-unit
after a conditional expression doesn't necessarily add any code.
In this case, it simply tells the compiler what we want to do.
Cheers,
Dick Johnson
Penguin : Linux version 2.6.10 on an i686 machine (5537.79 BogoMips).
Notice : All mail here is now cached for review by Dictator Bush.
98.36% of all statistics are fiction.
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