What!? Why does this happen....

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itsnobody

Joined: 01 Feb 2008
Posts: 93
Location: Silver Spring, MD

itsnobody 26 Mar 2008, 20:38

I was playing around with some speed tests and found something very strange...

This code gets me 4207 milliseconds:

Code:

include 'win32ax.inc'

.data
    start_time dd ?
    _output rb 6
    counter dd ?
.code
start:
            invoke GetTickCount
            mov [start_time],eax
            xor ecx,ecx
            .theloop:
            mov eax,[counter]
            add eax,1
            mov [counter],eax
            add ecx,1
            cmp ecx,1000000000
            jl .theloop
            .exit:
            invoke GetTickCount
            sub eax,[start_time]
            invoke wsprintf,_output,"%i",eax
            invoke MessageBox,NULL,_output,"Speed Test",MB_OK
            invoke ExitProcess,0
 .end start

But, when I add "mov [counter],ecx" to the code, things SPEED up to 3370 milliseconds:

Code:

include 'win32ax.inc'

.data
    start_time dd ?
    _output rb 6
    counter dd ?
.code
start:
            invoke GetTickCount
            mov [start_time],eax
            xor ecx,ecx
            .theloop:
            mov eax,[counter]
            add eax,1
            mov [counter],eax
            mov [counter],ecx
            add ecx,1
            cmp ecx,1000000000
            jl .theloop
            .exit:
            invoke GetTickCount
            sub eax,[start_time]
            invoke wsprintf,_output,"%i",eax
            invoke MessageBox,NULL,_output,"Speed Test",MB_OK
            invoke ExitProcess,0
 .end start

What's going on? Why do things speed up when I add the statement "mov [counter],ecx", I thought it might have modified ecx some how but it also speeds up with "mov [counter]," edx, and ebx

But what's happening, why do things run faster because of this?

26 Mar 2008, 20:38

rugxulo

Joined: 09 Aug 2005
Posts: 2341
Location: Usono (aka, USA)

rugxulo 26 Mar 2008, 22:26

Instruction pairing? What cpu type and specific OS flavor are you running?

26 Mar 2008, 22:26

AlexP

Joined: 14 Nov 2007
Posts: 561
Location: Out the window. Yes, that one.

AlexP 26 Mar 2008, 23:33

Just a wild guess, but maybe it was just enough to make the prediction work better Smile

, check alignment of loop too. Other than that, no clue.

26 Mar 2008, 23:33

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20338
Location: In your JS exploiting you and your system

revolution 27 Mar 2008, 01:24

Optimising for speed is always problematic with very tight loops. There are so many things that can affect the loop timing. Alignment in memory, number of instructions in the loop, CPU version, memory usage pattern, etc.

Out-of-order CPU's are very complex and to properly make sure you have really optimised a piece of code the only way to be sure is run it in many different code variants on many different system variants and compare the timings.

But the example above is of course useless and will tell you nothing about what to expect for a genuine function that actually does something useful.

27 Mar 2008, 01:24

AlexP

Joined: 14 Nov 2007
Posts: 561
Location: Out the window. Yes, that one.

AlexP 27 Mar 2008, 01:34

Quote:

But the example above is of course useless and will tell you nothing about what to expect for a genuine function that actually does something useful.

Optimist

27 Mar 2008, 01:34

r22

Joined: 27 Dec 2004
Posts: 805

r22 27 Mar 2008, 23:09

Quote:

add eax,1
mov [counter],eax
mov [counter],ecx

To a modern processor is just ...

Quote:

add eax,1
mov [counter],ecx

Thanks to dependency checking/optimizations and pipelines etc.

Now
add eax,1
mov [counter],eax
Has a partial stale because ADD is modifying eax and then MOV is writing it to memory. The processor has to wait for the ADD to finish before it can perform the MOV (in general don't start ripping this on semantics).

This stale doesn't exist in
add eax,1
mov [counter],ecx
The process can perform these two opcodes at the same time, because they are mutually exclusive.

Also it's not a loop alignment issue because the label
.theLoop: would represent the same memory location in both programs.

27 Mar 2008, 23:09

daniel.lewis

Joined: 28 Jan 2008
Posts: 92

daniel.lewis 28 Mar 2008, 08:01

Hrmm....

moving/adding/subbing things into eax has a shortcut form that takes fewer bytes, but this is out of eax, not into it. :p

28 Mar 2008, 08:01

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