------- Comment #49 from vvv at ru dot ru 2009-05-20 21:38 -------
(In reply to comment #48)
How this patches work? Is it required some special options?
# /media/disk-1/B/bin/gcc --version
gcc (GCC) 4.5.0 20090520 (experimental)
# cat test.c
void f(int i)
{
if (i == 1) F(1);
if (i == 2) F(2);
if (i == 3) F(3);
if (i == 4) F(4);
if (i == 5) F(5);
}
extern int F(int m);
void func(int x)
{
int u = F(x);
while (u)
u = F(u)*3+1;
}
# /media/disk-1/B/bin/gcc -o t test.c -O2 -c -mtune=k8
# objdump -d t
0000000000000000 <f>:
0: 83 ff 01 cmp $0x1,%edi
3: 74 1b je 20 <f+0x20>
5: 83 ff 02 cmp $0x2,%edi
8: 74 16 je 20 <f+0x20>
a: 83 ff 03 cmp $0x3,%edi
d: 74 11 je 20 <f+0x20>
f: 83 ff 04 cmp $0x4,%edi
12: 74 0c je 20 <f+0x20>
14: 83 ff 05 cmp $0x5,%edi
17: 74 07 je 20 <f+0x20>
19: f3 c3 repz retq
1b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
20: 31 c0 xor %eax,%eax
22: e9 00 00 00 00 jmpq 27 <f+0x27>
27: 66 0f 1f 84 00 00 00 nopw 0x0(%rax,%rax,1)
2e: 00 00
0000000000000030 <func>:
30: 48 83 ec 08 sub $0x8,%rsp
34: e8 00 00 00 00 callq 39 <func+0x9>
39: 85 c0 test %eax,%eax
3b: 89 c7 mov %eax,%edi
3d: 74 0e je 4d <func+0x1d>
3f: 90 nop
40: e8 00 00 00 00 callq 45 <func+0x15>
45: 8d 7c 40 01 lea 0x1(%rax,%rax,2),%edi
49: 85 ff test %edi,%edi
4b: 75 f3 jne 40 <func+0x10>
4d: 48 83 c4 08 add $0x8,%rsp
51: c3 retq
I can't see any padding in function f :(
PS. In file config/i386/i386.c (ix86_avoid_jump_mispredicts)
/* Look for all minimal intervals of instructions containing 4 jumps.
...
Not jumps, but _branches_ (CALL, JMP, conditional branches, or returns)
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39942
