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Index > Windows > How to align local variables ?

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Kuemmel



Joined: 30 Jan 2006
Posts: 200
Location: Stuttgart, Germany
Kuemmel
Hi people,

in our fractal thread we had an issue that might be interesing for others, too. The local alignment of variables. As far as I understand a proc ... with local variables is assembled regarding the variables and adjustment of the stack pointer to

push ebp
mov esp, ebp
sub esp, xx (depending on how much memory is needed)

I found executables that uses an extra

and esp,0xfffffff0 then to align the stack pointer, like align 16 or what ever value

How can this be done with FASM, even as a hack, but reliable in the 'right' way ?
Post 13 May 2006, 11:54
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f0dder



Joined: 19 Feb 2004
Posts: 3170
Location: Denmark
f0dder
First, the XX value for ESP must be large enough to accomodate both local variables and the alignment - that's easy enough. But now comes the issue of adressing... if EBP is aligned, locals will be aligned, but we can't access function arguments through EBP.

If ESP is aligned, we need to change LOCALs access to go through ESP instead of EBP (two bytes longer opcode for each instruction)...
Post 13 May 2006, 12:55
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Borsuc



Joined: 29 Dec 2005
Posts: 2466
Location: Bucharest, Romania
Borsuc
f0dder wrote:
If ESP is aligned, we need to change LOCALs access to go through ESP instead of EBP (two bytes longer opcode for each instruction)...
But you save one register.... ebp Wink
Post 13 May 2006, 13:15
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f0dder



Joined: 19 Feb 2004
Posts: 3170
Location: Denmark
f0dder
The_Grey_Beast wrote:
f0dder wrote:
If ESP is aligned, we need to change LOCALs access to go through ESP instead of EBP (two bytes longer opcode for each instruction)...
But you save one register.... ebp Wink


No, you still need EBP to preserve the original ESP value. And no, you cannot use a global variable for this since it will make the code non-reentrant.

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Post 13 May 2006, 13:17
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Borsuc



Joined: 29 Dec 2005
Posts: 2466
Location: Bucharest, Romania
Borsuc
Why do you need to preserve it? Are the locals of variable length?
If they're constant, it's simple to restore esp back through a constant add or somesuch... you just have to keep track of it. sorry for misunderstanding (if so)
Post 13 May 2006, 13:21
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f0dder



Joined: 19 Feb 2004
Posts: 3170
Location: Denmark
f0dder
The_Grey_Beast wrote:
Why do you need to preserve it? Are the locals of variable length?
If they're constant, it's simple to restore esp back through a constant add or somesuch... you just have to keep track of it. sorry for misunderstanding (if so)


The thing is that we need aligned locals - since a function can be called with just about any ESP value, the stack needs to be (re)aligned at runtime.

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Post 13 May 2006, 13:25
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r22



Joined: 27 Dec 2004
Posts: 805
r22
This is where having two stacks would be great.

But the global mem/data is a good enough option if ...
-Your function won't be called in a multithreaded environment
-Your function isn't recursive.

But in those cases you can just use aligned global memory for the local variabls themselves.

If you really need the extra register space, so that the ebp, esp, and FFFFFFF0h solution won't work for you then you can take up only HALF or a QUARTER of a register.

Say you only need BX not EBX but you need every other register.
Code:
Func:
push ebx
mov ebx,esp
sub esp, NUMLOCALS*?+16
;;if all locals need to be 16byte aligned ?=16 otherwise ?=4 dword
shl ebx,16
and esp, 0FFFFFFF0h
... mov BX, word[ blah ];; preserve top half of ebx
shr ebx, 16
and esp, 0FFFF0000h
or esp,ebx
pop ebx
ret
    
Post 13 May 2006, 19:25
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f0dder



Joined: 19 Feb 2004
Posts: 3170
Location: Denmark
f0dder
A generic solution would be preferable - and yeah, the code that brought this topic up is MT, so we can't do globals.
Post 13 May 2006, 19:34
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