24 Mar
2005
24 Mar
'05
6 a.m.
BSR has a latency of 8-12 Cycles on Athlon/P3 but can be pipelined. Worse
(up to ~80 cycles) on Pentium and other older CPUs.
http://www.amd.com/us-en/Processors/TechnicalResources/0,,30_182_739_3748,0…
Maximum latency on a Pentium 4 is eight clock cycles.
But its throughput
is one, which means it is fully pipelined. So when you start this
instruction eight clock cycles before you need the result, it behaves as
if it only takes one clock cycle. You're not going to be able to beat that
with any other code. The closest you can get is to convert the integer to
float, then extract the exponent. That could be done in less than eight
clock cycles but throughput will be lower.
http://www.flipcode.com/cgi-bin/fcmsg.cgi?thread_show=16986&msg=113105
Dont know about A64 - maybe someone can test BSR with A64?
I'm very disappointed by its peformance on my K7 system - i might have
messed something up thought.
You could save on the Pushing / Popping but that would be kinda like
cheating and if you do it dirty/lazy I wont get the right returns either.
It doesnt make much sense to put the optimized ASM in there, neither is much
hope of GCC having a good day and doing a lot of optimisation.
So far the best option would be the macro with a lookup table (only one
global kernel table tho).
Here are the updated STATS
also available at http://hackersquest.org/kerneltest.html
result orig function 46ffffe9
it took 1526862 18%
result orig function inlined 46ffffe9
it took 1041460 12%
result second proposal inlined 46ffffe9
it took 1248990 15%
result optimized asm 46ffffe9
it took 1321532 16%
result lookup inlined 46ffffe9
it took 682264 8%
result bsr inlined 46ffffe9
it took 1751088 21%
result macro 46ffffe9
it took 653692 7%
http://wohngebaeudeversicherung.einsurance.de/
----- Original Message -----
From: "Alex Ionescu" <ionucu(a)videotron.ca>
To: "ReactOS Development List" <ros-dev(a)reactos.com>
Sent: Thursday, March 24, 2005 3:57 AM
Subject: Re: [ros-dev] Speed Tests (was: ping Alex regarding log2()
forscheduler)
Ash wrote:
Hello,
I'd like to provide a small VS.NET project file with 5 different tests.
- 46ffffe9 tells everything is alright with the function calculation
- times are measured with QueryPerformanceCounter
- loop run cnt: 0x2ffffff
result orig function 46ffffe9
it took 1491052
result orig function inlined 46ffffe9
it took 1035547
result second proposal inlined 46ffffe9
it took 1244434
result optimized asm 46ffffe9
it took 1338367
result debug asm 46ffffe9
it took 8774815
The second proposal is the original proposal but more shifts - still
slower tho.
Interesting is the inlined version generated by MSVC, shaving off almost
1/3 of the overall time.
Also stared as "optimized asm" - no gurantee on register safety tho ;)
For portability and performance sake it should be considered to create a
compiler macro.
This function is terribly small, any optimisations inside are outweighted
by the calling overhead in this case.
The most impressive one is the original function inlined, althought the
ASM would only work on x86.
Please do not think about using 64k tables, thats what, 1/2 of a Sempron
L2 cache?
It would really really trash performance.
Hi Ash,
Thanks a lot for your tests. I don't have much time tonight, but if you'd
like/can, can you add two more tests?
One using "bsr", the intel opcode. It does all the work for you and
returns the index.
i think it's as simple as "bsr ecx, eax" where "ecx" is the mask
and eax
is the returned index. Or it might be backwards.
The second test is using a 256-byte log2 table:
const char LogTable256[] = {
0, 0, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3,
4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7
};
and a lookup code like this:
Addon = 16;
if ((IntMask = Mask >> 16)) {
Addon = 0;
IntMask = Mask;
}
if (IntMask && 0xFFFFFF00) {
Addon += 8;
}
HighBit = LogTable256[(Mask >> Addon)] + Addon;
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