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Re: [Qemu-devel] [RFC] find_next_bit optimizations
From: |
Peter Lieven |
Subject: |
Re: [Qemu-devel] [RFC] find_next_bit optimizations |
Date: |
Mon, 11 Mar 2013 19:20:18 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20120827 Thunderbird/15.0 |
Am 11.03.2013 18:07, schrieb Paolo Bonzini:
> Il 11/03/2013 18:06, ronnie sahlberg ha scritto:
>> Even more efficient might be to do bitwise instead of logical or
>>
>>>> if (tmp | d1 | d2 | d3) {
>> that should remove 3 of the 4 conditional jumps
>> and should become 3 bitwise ors and one conditional jump
>
> Without any serious profiling, please let the compiler do that.
Paolo is right, i ran some tests with gcc 4.6.3 on x86_64 (with -O3) and tried
the
various ideas. They all made no significant difference. Even unrolling to 8
unsigned
longs didn't change anything.
What I tried is running 1^20 interations of find_next_bit(bitfield,4194304,0);
I choosed the bitfield to be 4MByte which equals a 16GB VM. The bitfield was
complete zeroed so find_next_bit had to run completely through the bitfield.
The original version took 1 minute and 10 seconds whereas all other took
approx. 37-38 seconds which is almost a 100% boost ;-)
So I think this here is the final version:
while (size >= 4*BITS_PER_LONG) {
unsigned long d1, d2, d3;
tmp = *p;
d1 = *(p+1);
d2 = *(p+2);
d3 = *(p+3);
if (tmp) {
goto found_middle;
}
if (d1 || d2 || d3) {
break;
}
p += 4;
result += 4*BITS_PER_LONG;
size -= 4*BITS_PER_LONG;
}
Peter
- Re: [Qemu-devel] [RFC] find_next_bit optimizations, (continued)
- Re: [Qemu-devel] [RFC] find_next_bit optimizations, Paolo Bonzini, 2013/03/11
- Re: [Qemu-devel] [RFC] find_next_bit optimizations, Peter Lieven, 2013/03/11
- Re: [Qemu-devel] [RFC] find_next_bit optimizations, Peter Lieven, 2013/03/11
- Re: [Qemu-devel] [RFC] find_next_bit optimizations, Paolo Bonzini, 2013/03/11
- Re: [Qemu-devel] [RFC] find_next_bit optimizations, Peter Lieven, 2013/03/11
- Re: [Qemu-devel] [RFC] find_next_bit optimizations, Paolo Bonzini, 2013/03/11
- Re: [Qemu-devel] [RFC] find_next_bit optimizations, Peter Lieven, 2013/03/11
- Re: [Qemu-devel] [RFC] find_next_bit optimizations, Paolo Bonzini, 2013/03/11
- Re: [Qemu-devel] [RFC] find_next_bit optimizations, ronnie sahlberg, 2013/03/11
- Re: [Qemu-devel] [RFC] find_next_bit optimizations, Paolo Bonzini, 2013/03/11
- Re: [Qemu-devel] [RFC] find_next_bit optimizations,
Peter Lieven <=
- [Qemu-devel] [PATCH] bitops: unroll while loop in find_next_bit()., Peter Lieven, 2013/03/12
- Re: [Qemu-devel] [RFC] find_next_bit optimizations, Peter Maydell, 2013/03/11
- Re: [Qemu-devel] [RFC] find_next_bit optimizations, Peter Maydell, 2013/03/11
- Re: [Qemu-devel] [RFC] find_next_bit optimizations, Peter Lieven, 2013/03/11
- Re: [Qemu-devel] [RFC] find_next_bit optimizations, Paolo Bonzini, 2013/03/11
- Re: [Qemu-devel] [RFC] find_next_bit optimizations, Peter Lieven, 2013/03/11
Re: [Qemu-devel] [RFC] find_next_bit optimizations, Stefan Hajnoczi, 2013/03/12