> Ohh... I used to think I know something about optimization. I tried build
> Rasmus' 'find_last_bit' against mine on MIPS, and found that sizes are as
> 268 vs 320 bytes. I think the best version is suggested by George, both
> readable, and effective. What about using it? If no objections, I'll
> gather all fixes and upload new patchset this weekend.
I'll happily ack whichever you prefer. Tightening the code to the
maximum possible fun exercise, but not essential. However, I finally
got GCC to generate reasonable code with the following:
size_t find_last_bit3(const unsigned long *addr, size_t size)
{
if (size) {
unsigned long val = LAST_WORD_MASK(size);
size_t idx = (size-1) / BITS_PER_LONG;
do {
val &= addr[idx];
if (val)
return idx * BITS_PER_LONG + __fls(val);
val = ~0ul;
} while (idx--);
}
return size;
}
size_t find_last_bit4(const unsigned long *addr, size_t size)
{
if (size) {
unsigned long val = LAST_WORD_MASK(size);
size_t idx = DIV_ROUND_UP(size, BITS_PER_LONG);
do {
val &= addr[idx];
if (val)
return idx * BITS_PER_LONG + __fls(val);
val = ~0ul;
} while (--idx);
}
return size;
}
Which produced, respectively, 76 and 68-byte functions:
0000000000000000 <find_last_bit3>:
0: 31 c0 xor %eax,%eax
2: 48 85 f6 test %rsi,%rsi
5: 74 44 je 4b <find_last_bit3+0x4b>
7: 48 8d 46 ff lea -0x1(%rsi),%rax
b: 89 f1 mov %esi,%ecx
d: 48 c7 c2 ff ff ff ff mov $0xffffffffffffffff,%rdx
14: f7 d9 neg %ecx
16: 48 c1 e8 06 shr $0x6,%rax
1a: 48 d3 ea shr %cl,%rdx
1d: eb 11 jmp 30 <find_last_bit3+0x30>
1f: 90 nop
20: 48 83 e8 01 sub $0x1,%rax
24: 48 c7 c2 ff ff ff ff mov $0xffffffffffffffff,%rdx
2b: 48 39 d0 cmp %rdx,%rax
2e: 74 18 je 48 <find_last_bit3+0x48>
30: 48 23 14 c7 and (%rdi,%rax,8),%rdx
34: 74 ea je 20 <find_last_bit3+0x20>
36: 48 c1 e0 06 shl $0x6,%rax
3a: 48 0f bd d2 bsr %rdx,%rdx
3e: 48 01 d0 add %rdx,%rax
41: c3 retq
42: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
48: 48 89 f0 mov %rsi,%rax
4b: c3 retq
4c: 0f 1f 40 00 nopl 0x0(%rax)
0000000000000050 <find_last_bit4>:
50: 31 c0 xor %eax,%eax
52: 48 85 f6 test %rsi,%rsi
55: 74 3c je 93 <find_last_bit4+0x43>
57: 48 8d 46 3f lea 0x3f(%rsi),%rax
5b: 89 f1 mov %esi,%ecx
5d: 48 c7 c2 ff ff ff ff mov $0xffffffffffffffff,%rdx
64: f7 d9 neg %ecx
66: 48 c1 e8 06 shr $0x6,%rax
6a: 48 d3 ea shr %cl,%rdx
6d: eb 0d jmp 7c <find_last_bit4+0x2c>
6f: 90 nop
70: 48 c7 c2 ff ff ff ff mov $0xffffffffffffffff,%rdx
77: 48 01 d0 add %rdx,%rax
7a: 74 14 je 90 <find_last_bit4+0x40>
7c: 48 23 14 c7 and (%rdi,%rax,8),%rdx
80: 74 ee je 70 <find_last_bit4+0x20>
82: 48 c1 e0 06 shl $0x6,%rax
86: 48 0f bd d2 bsr %rdx,%rdx
8a: 48 01 d0 add %rdx,%rax
8d: c3 retq
8e: 66 90 xchg %ax,%ax
90: 48 89 f0 mov %rsi,%rax
93: c3 retq
The second one omits all the unwanted tests & compares, and even does
something clever with the -1 mask that I didn't think of.
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