On Friday 03 February 2006 12:58, Andreas Mohr wrote:
> - adev->tx_head = (head + 1) % TX_CNT;
> + /* slower: adev->tx_head = (head + 1) % TX_CNT; */
> + adev->tx_head = head + 1;
> + if (adev->tx_head >= TX_CNT)
> + adev->tx_head = 0;
struct a {
int tx_head;
};
#define TX_CNT 16
void f(struct a *adev, int head) {
adev->tx_head = (head + 1) % TX_CNT;
}
void g(struct a *adev, int head) {
adev->tx_head = head + 1;
if (adev->tx_head >= TX_CNT)
adev->tx_head = 0;
}
gcc -Os -S t.c -fomit-frame-pointer -falign-functions=1 -falign-labels=1
-falign-loops=1 -falign-jumps=1 -mtune=i386 -march=i386
produces:
f:
movl 8(%esp), %eax
incl %eax
movl $16, %edx
movl %edx, %ecx
cltd
idivl %ecx
movl 4(%esp), %eax
movl %edx, (%eax)
ret
.size f, .-f
.globl g
.type g, @function
g:
movl 4(%esp), %edx
movl 8(%esp), %eax
incl %eax
movl %eax, (%edx)
cmpl $15, %eax
jle .L6
movl $0, (%edx)
.L6:
ret
.size g, .-g
.ident "GCC: (GNU) 4.0.0"
Well, gcc obviously failed to realize that "% 16" == "& 15".
I'll file a bug. Meanwhile, with & 15 f() is better:
f:
movl 8(%esp), %eax
incl %eax
andl $15, %eax
movl 4(%esp), %edx
movl %eax, (%edx)
ret
.size f, .-f
.globl g
.type g, @function
g:
movl 4(%esp), %edx
movl 8(%esp), %eax
incl %eax
movl %eax, (%edx)
cmpl $15, %eax
jle .L6
movl $0, (%edx)
.L6:
ret
.size g, .-g
.ident "GCC: (GNU) 4.0.0"
--
vda
-
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