add trick

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BOTOKILLER

Joined: 07 Jan 2011
Posts: 154
Location: Ukraine

BOTOKILLER 21 Jul 2011, 13:19

Hi everyone!

Code:

add cl, cl

Will it work???

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NSOS

21 Jul 2011, 13:19

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20589
Location: In your JS exploiting you and your system

revolution 21 Jul 2011, 13:24

What is supposed to be the trick?

Will what work? The add will proceed as expected: cl = cl + cl. But you could've verified that yourself in just a few seconds with a test program.

21 Jul 2011, 13:24

BOTOKILLER

Joined: 07 Jan 2011
Posts: 154
Location: Ukraine

BOTOKILLER 21 Jul 2011, 13:51

then what will be faster

Code:

shl cl, 1

Code:

add cl, cl

???

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NSOS

21 Jul 2011, 13:51

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20589
Location: In your JS exploiting you and your system

revolution 21 Jul 2011, 14:07

BOTOKILLER wrote:

then what will be faster
Code:
shl cl, 1    
or
Code:
add cl, cl    
???

Nobody knows. Different CPU's, and different surrounding code, will affect the results.

21 Jul 2011, 14:07

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20589
Location: In your JS exploiting you and your system

revolution 21 Jul 2011, 14:08

Moving to Main.

21 Jul 2011, 14:08

typedef

Joined: 25 Jul 2010
Posts: 2909
Location: 0x77760000

typedef 21 Jul 2011, 14:12

i think

Code:

 shl cl,1

21 Jul 2011, 14:12

Madis731

Joined: 25 Sep 2003
Posts: 2139
Location: Estonia

Madis731 22 Jul 2011, 07:03

The correct answer is: they both take the same time on pretty much any architecture and surrounding code, because they are really simple instructions. Not much to optimize there. 1 uop.

The only difference that you can measure is maybe power Smile

because shifting in bit logic is a matter of wiring neighbouring bits, but adding needs some fancy xor tricks and ripple carry and what not.

So get your gear out and start measuring those precious pW (pico-Watts) you're saving Wink

EDIT: Maybe its an order of magnitude bigger (the power saved I mean)

22 Jul 2011, 07:03

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