Re: prologue length

["Vincent R." <forumer at smartmobili dot com>]

On Fri, 16 Jan 2009 08:37:20 -0800, Ian Lance Taylor <iant@google.com>
wrote:
> "Vincent R." <forumer@smartmobili.com> writes:
> 
>> I am working on a gcc-4.1.2 and I would like to know how the prologue
>> length from a function can be calculated.
> 
> The question is not well formed.  The instructions which are part of
> the prologue (e.g., saving callee-saved registers onto the stack) can
> be interspersed with instructions which are part of the function
> proper.  This happens routinely in gcc's second scheduling pass.
> 
Ok I will check.

> 
>> Indeed I am trying to evaluate what needs to be done to implement SEH
and
>> one requirement is to be able to
>> fill a structure holding information like function length and prologue
>> length.
> 
> I haven't looked at SEH.  How does it use this sort of information?
> What should happen when the prologue is not a unitary block of
> instructions?
> 
> Ian
>From msdn : 
To locate appropriate handlers when an exception occurs in Win32
environments other than x86, the system first determines the frames that
reside on the callstack, along with their associated functions in code.
Any function can have a handler associated with it. If so, the system gives
the handler associated with the function an opportunity to handle the
exception. As with x86, the system invokes handlers in reverse order; that
is, it first invokes the handler whose corresponding frames were most
recently pushed onto the stack. To determine the frames on the stack, the
system simulates the execution of a portion of each function's code in
reverse. This simulation creates a CPU context similar to the state the
real CPU context held at the point of entry to that function.
This process of reverse execution is known as Virtual Unwinding, because
the stack unwind is only being simulated, not actually performed.
The portion of the code that is reversed is known as the prolog of the
function. It consists of instructions that modify the stack pointer and set
up the stack frame immediately upon entry to the function. 
To virtually unwind, the system needs a small amount of information about
each function. This information is contained in data structures called
PDATA structures.

A PDATA structure marks where a function begins and ends in the code
stream, as well as the location of the function prolog. 

For instance here is the information associated to a simple seh example :

FuncStart 	PrologLen 	FuncLen 	ThirtyTwoBit 	ExceptionFlag 	Comment 
00011008 	0x4 	          0x15 	              1 	1 	         wmain() 
0001105c 	0x1 	          0x9 	              1 	0 	         filter_func() 
000110a0 	0x2 	          0x93 	              1 	0 	        
crtstart_ParseArgsWW 
000112f4 	0x4 	          0x30 	              1 	1 	        
mainWCRTStartup()

That's why I was asking about prolog length...