C / C++ / MFC

CPallini15-Jul-08 21:53

CPallini

15-Jul-08 21:53

Shouting will not help you.
Smile | :)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.

This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong.
-- Iain Clarke

[My articles]

Blade_Bao15-Jul-08 22:25

Blade_Bao

15-Jul-08 22:25

Sorry, I'm not quite catch U...

CPallini15-Jul-08 22:30

CPallini

15-Jul-08 22:30

Using a bigger (or bolder) font than usual is called shouting and it's quite noisy.
Smile | :)

Blade_Bao15-Jul-08 22:45

Blade_Bao

15-Jul-08 22:45

Um,okay, here is the reason:
Cos I'm knew here, and I haven't got used to the rules here...
So, sorry for bothering. All I want is a little piece of advice.
TKS, though.

best,
Blade

Rajesh R Subramanian15-Jul-08 22:44

15-Jul-08 22:44

CPallini wrote:
Shouting will not help you.

Unless you're drowning into the sea. Smile | :)

Nobody can give you wiser advice than yourself. - Cicero
.·´¯`·->Rajesh<-·´¯`·.

Microsoft MVP - Visual C++[^]

Two basic questions about generated assembly

CPallini16-Jul-08 2:08

CPallini

16-Jul-08 2:08

Actually I'm drowning in a ocean of bytes, right now.
The (in)famous thread hijacker.

Big Grin | :-D

George_George15-Jul-08 21:02

15-Jul-08 21:02

Hello everyone,

Two questions after readnig this article,

http://www.microsoft.com/msj/0298/hood0298.aspx

1.

why using LEA to do multiplication is faster than using MUL?

"Using "LEA EAX,[EAX*4+EAX]" turns out to be faster than the MUL instruction."

2.

"The TEB's linear address can be found at offset 0x18 in the TEB." -- what means linear address? Something like array, which elements are put next to each other? What means non-linear address?

thanks in advance,
George

Rajesh R Subramanian15-Jul-08 21:28

15-Jul-08 21:28

George_George wrote:
why using LEA to do multiplication is faster than using MUL?

"Using "LEA EAX,[EAX*4+EAX]" turns out to be faster than the MUL instruction."

lea seems to be an addressing instruction, which means it probably must execute in a single cycle and therefore would surely be faster than mul. On the other side, because it is an addressing instruction, think if you will be able to multiply very large numbers this way. There must be surely limitations on that. mul can work with large numbers also. Smile | :)

Thanks for the question, the search for an answer got me an interesting read. Smile | :)

Link: Wikibooks->Reverse Engineering->CodeTransformations->Common instruction substitutions[^]

Warning: I'm not a full time assembly programmer and I may not be accurate.

My assumptions: An X86 Processor, pentium class.

Nobody can give you wiser advice than yourself. - Cicero
.·´¯`·->Rajesh<-·´¯`·.

Microsoft MVP - Visual C++[^]

George_George15-Jul-08 21:36

15-Jul-08 21:36

Thanks Rajesh!

1.

"it probably must execute in a single cycle and therefore would surely be faster than mul" -- do you have any documents to support this statement? Where to look for documents for cycles needed for a specific instruction (e.g. LEA and MUL)?

2.

Any ideas to my question #2? Smile | :)

regards,
George

Rajesh R Subramanian15-Jul-08 22:25

15-Jul-08 22:25

George_George wrote:
"it probably must execute in a single cycle and therefore would surely be faster than mul" -- do you have any documents to support this statement?

With the LEA instruction, the x86 processor can now perform a 3-number add, with something like a C expression "a = b + c + 10;" translating into EAX = EBX+ECX+10 and being coded into one instruction:

LEA EAX,[EBX+ECX+10]

Notice that no memory is actually referenced. LEA is used merely to calculate values by performing the addition of a base register (EBX) with an index register (ECX) with some constant displacement (10). This is what the address generation unit (AGU) does, allowing the processor to quickly calculate addresses of array elements, screen pixel locations, and do some basic arithmetic in one clock cycle. Source: http://www.emulators.com/docs/pentium_1.htm[^]

Refer to pages 6 & 7 in this PDF (Pentium: Not the same old song)[^]. This too supports my earlier statement.

Also, read "Handy info on speeding up integer instructions" in this page[^]

Nobody can give you wiser advice than yourself. - Cicero
.·´¯`·->Rajesh<-·´¯`·.

Microsoft MVP - Visual C++[^]

George_George16-Jul-08 21:19

16-Jul-08 21:19

Thanks Rajesh,

My question item 1 is answered. Any ideas to my question item 2? About what mean linear and non-linear address?

regards,
George

Whoever downvoted me

Rajesh R Subramanian15-Jul-08 21:44

15-Jul-08 21:44

I'll be happy to hear your feedback on "why" you voted the post down. Correct me if I said something wrong there, please. Smile | :)

Nobody can give you wiser advice than yourself. - Cicero
.·´¯`·->Rajesh<-·´¯`·.

Microsoft MVP - Visual C++[^]

George_George15-Jul-08 21:50

15-Jul-08 21:50

What means vote post down? I vote for 5 because I like your answer. You want 6? Smile | :)

regards,
George

Rajesh R Subramanian15-Jul-08 21:58

15-Jul-08 21:58

Nope, I knew it wouldn't be you. Someone has marked it as an unhelpful answer and I wanted their feedback in particular, so that I can know if I my answer was wrong or how can I make it any better. Smile | :)

Nobody can give you wiser advice than yourself. - Cicero
.·´¯`·->Rajesh<-·´¯`·.

Microsoft MVP - Visual C++[^]

George_George15-Jul-08 22:06

15-Jul-08 22:06

Take it easy, man! Smile | :)

Rajesh, just enjoy discussion with people around the world which have different options for the same question.

regards,
George

Rajesh R Subramanian15-Jul-08 22:11

15-Jul-08 22:11

The point was that there was nothing wrong in my answer (my opinion) and someone still voted it down. I just wanted their feedback in particular to know if anything was wrong with my answer, so that I can know something new. Not that I care for the vote. Smile | :)

Nobody can give you wiser advice than yourself. - Cicero
.·´¯`·->Rajesh<-·´¯`·.

Microsoft MVP - Visual C++[^]

ThatsAlok17-Jul-08 22:18

ThatsAlok

17-Jul-08 22:18

whoso ever downvoted you .. will be beaten with stick! any ways le me also square it of!

"Opinions are neither right nor wrong. I cannot change your opinion. I can, however, change what influences your opinion." - David Crow
Never mind - my own stupidity is the source of every "problem" - Mixture

cheers,
Alok Gupta
VC Forum Q&A :- I/IV
Support CRY- Child Relief and You/xml>

Rajesh R Subramanian17-Jul-08 23:07

17-Jul-08 23:07

Thanks man. You're kind. Smile | :)

Nobody can give you wiser advice than yourself. - Cicero
.·´¯`·->Rajesh<-·´¯`·.

Microsoft MVP - Visual C++[^]

Stephen Hewitt15-Jul-08 21:36

Stephen Hewitt

15-Jul-08 21:36

George_George wrote:
why using LEA to do multiplication is faster than using MUL?

You'd have to know about the internal architecture and circuitry of the CPU to answer that; I don't and I doubt there would be many people except for people that work (or have worked) at Intel that would.

George_George wrote:
"The TEB's linear address can be found at offset 0x18 in the TEB." -- what means linear address? Something like array, which elements are put next to each other? What means non-linear address?

To understand what's going on here you have to know a little about Intel CPUs and segment registers. Basically C/C++ has no concept of segment registers and such (it assumes a linear address space) so this is a page-table mapping trick done by the OS to make the TEB addressable in such an environment.

Steve

George_George15-Jul-08 21:42

15-Jul-08 21:42

Thanks Steve,

1.

Where to look for documents for cycles needed for a specific instruction (e.g. LEA and MUL)?

2.

"it assumes a linear address space" -- it you mean low layer CPU/register or high layer C/C++?

regards,
George

Stephen Hewitt15-Jul-08 21:46

Stephen Hewitt

15-Jul-08 21:46

George_George wrote:
Where to look for documents for cycles needed for a specific instruction (e.g. LEA and MUL)?

Download the datasheet for the CPU.

George_George wrote:
"it assumes a linear address space" -- it you mean low layer CPU/register or high layer C/C++?

It's complicated and very low level. I suggest you start reading something like this[^].

Steve

George_George16-Jul-08 0:49

16-Jul-08 0:49

Thanks Steve,

"datasheet for the CPU" -- could you give me some links or keywords to search? I am new to this topic. Smile | :)

regards,
George

Rajesh R Subramanian16-Jul-08 0:56

16-Jul-08 0:56

Have you Googled for "Pentium Datasheet"?

Nobody can give you wiser advice than yourself. - Cicero
.·´¯`·->Rajesh<-·´¯`·.

Microsoft MVP - Visual C++[^]

George_George16-Jul-08 1:09

16-Jul-08 1:09

Thanks, Rajesh!

I have found some from Intel website.

http://www.intel.com/design/Pentium4/documentation.htm#datasheets[^]

But, which one most likely contains the information of CPU cycles for a specific assembly instruction? I am new to this area and too long document list makes me confused. Any ideas?

regards,
George

George_George16-Jul-08 19:08