[fasmg] sad spinlock

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sylware

Joined: 23 Oct 2020
Posts: 478
Location: Marseille/France

sylware 24 Jan 2023, 14:16

no "pause" machine instruction in the standard distribution or I did grep like an animal?

24 Jan 2023, 14:16

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8367
Location: Kraków, Poland

Tomasz Grysztar 24 Jan 2023, 14:34

This one is really easy to implement, likely for everyone:

Code:

macro pause?
        db 0F3h, 90h
end macro

Also, it's actually the same as:

Code:

        rep nop

24 Jan 2023, 14:34

sylware

Joined: 23 Oct 2020
Posts: 478
Location: Marseille/France

sylware 24 Jan 2023, 14:39

Are you going to add "pause" to fasmg distribution?

24 Jan 2023, 14:39

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8367
Location: Kraków, Poland

Tomasz Grysztar 24 Jan 2023, 14:46

I don't know, there's lot of things I could add, I focus on ones I personally find the most valuable. The idea was, once everything is easily customizable, then it should be relatively easy for others to implement instruction sets they require when I'm not up to the task.

24 Jan 2023, 14:46

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20524
Location: In your JS exploiting you and your system

revolution 24 Jan 2023, 14:51

Code:

pause equ db 0xf3, 0x90

Solved.

24 Jan 2023, 14:51

sylware

Joined: 23 Oct 2020
Posts: 478
Location: Marseille/France

sylware 24 Jan 2023, 14:53

Well, if you don't use spin locks... that's not going to be very valuable Confused

24 Jan 2023, 14:53

sylware

Joined: 23 Oct 2020
Posts: 478
Location: Marseille/France

sylware 24 Jan 2023, 14:59

@revolution I was thinking it was mistakenly missing from the official distribution. Because using spinlocks is quite common in parallel programming and the "right" implementation on x86_64 uses the pause machine instruction.

24 Jan 2023, 14:59

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20524
Location: In your JS exploiting you and your system

revolution 24 Jan 2023, 15:07

PAUSE was first defined on the P4. So even 32-bit code can benefit from it.

Older CPUs just see rep nop, no benefit for them, but also no harm.

24 Jan 2023, 15:07

FlierMate11

Joined: 13 Oct 2022
Posts: 94

FlierMate11 24 Jan 2023, 15:45

I insert "rep nop" in FASM source file, but compiled program is disassembled as "pause".

So revolution is right.

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24 Jan 2023, 15:45

macomics

Joined: 26 Jan 2021
Posts: 1068
Location: Russia

macomics 24 Jan 2023, 15:54

Your debugger does not allow you to choose the disassembly mode. Open the program in IDA Pro and select different processors. Before Pentium 4, you will be issued a rep nop

ADD: You also assemble the sequence

Code:

rep xchg eax, eax

and it will also turn into pause

24 Jan 2023, 15:54

FlierMate11

Joined: 13 Oct 2022
Posts: 94

FlierMate11 24 Jan 2023, 16:12

macomics wrote:

ADD: You also assemble the sequence
Code:
rep xchg eax, eax    
and it will also turn into pause

Yeah, I see it now, is it designed this way?

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Viewed:	6791 Time(s)

24 Jan 2023, 16:12

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20524
Location: In your JS exploiting you and your system

revolution 24 Jan 2023, 16:23

Note that xchg eax,eax encoded as 0x90 is not valid for x64 code. It is a true nop and won't zero the upper 32-bits of rax.

In 64-bit code use db 0x87, 0xc0.

24 Jan 2023, 16:23

DimonSoft

Joined: 03 Mar 2010
Posts: 1228
Location: Belarus

DimonSoft 26 Jan 2023, 08:11

revolution wrote:

Code:

pause equ db 0xf3, 0x90

Solved.

Wouldn’t a macro be a better solution? pause probably shouldn’t be used everywhere, just in places where an instruction name is allowed. (I completely ignore the fasmg branch, so may miss something here.)

26 Jan 2023, 08:11

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20524
Location: In your JS exploiting you and your system

revolution 26 Jan 2023, 08:17

I suppose a macro would be better. But I don't know the fasmg syntax for macros so I just took the easy path and posted what I do know.

26 Jan 2023, 08:17

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8367
Location: Kraków, Poland

Tomasz Grysztar 26 Jan 2023, 09:04

revolution wrote:

I suppose a macro would be better. But I don't know the fasmg syntax for macros so I just took the easy path and posted what I do know.

This actually wouldn't work with fasmg, for a reason very similar to why you cannot do "mc equ macro" in fasm 1. In fasmg this extends to all instructions and therefore EQU becomes limited to replacements in the expressions arguments. In fasmg's terminology it's the separation of symbols into three classes: instruction class, expression class, and also "labeled instruction" class for things like structure macros (but also EQU itself is a "labeled instruction").

26 Jan 2023, 09:04

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20524
Location: In your JS exploiting you and your system

revolution 26 Jan 2023, 09:11

Okay. I cheated to took the 8087.inc as a template and made this:

Code:

macro pause?
        db 0xf3, 0x90
end macro

I have no idea if it works or not. Embarassed

What is the ? for?

26 Jan 2023, 09:11

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8367
Location: Kraków, Poland

Tomasz Grysztar 26 Jan 2023, 09:19

revolution wrote:

I have no idea if it works or not.

You actually reproduced the macro I posted in the beginning of this thread.

revolution wrote:

What is the ? for?

It defines the symbol as case-insensitive.

PS. Also, usually you can adapt fasmg to work the way you need:

Code:

pause? equ db 0xf3, 0x90

calminstruction ? line&
        transform line
        assemble line
end calminstruction

pause

But this like opening a can of... bugs. Good luck if you need to re-define an equate etc.

PPS. Actually, I forgot I have a better option now:

Code:

calminstruction ?? line&
        transform line
        assemble line
end calminstruction

pause? equ db 0xf3, 0x90

pause   

pause? equ db 0F3h, 90h

pause

Last edited by Tomasz Grysztar on 26 Jan 2023, 09:48; edited 2 times in total

26 Jan 2023, 09:19

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20524
Location: In your JS exploiting you and your system

revolution 26 Jan 2023, 09:25

Tomasz Grysztar wrote:

You actually reproduced the macro I posted in the beginning of this thread.

Oh, yeah I did (but with better hexification Razz

). That'll teach me for not properly reveiwing the thread. Embarassed

Tomasz Grysztar wrote:

revolution wrote:
What is the ? for?
It defines the symbol as case-insensitive.

I like that feature. Smile

fasm also needs that. Very Happy

26 Jan 2023, 09:25

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