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Tuesday, December 11, 2007

Calling Conventions

This is the outcome of today's presentation by Anand - our tech lead.
There are four calling conventions, and the Microsoft terminologies for them are:
  1. __stdcall
  2. __cdecl
  3. __thiscall
  4. __fastcall
As its known, the calling conventions will tell who, amongst the caller and callee, will clear the parameters from the top of the stack when the function call completes.

__cdecl is the old 'C' way where the stack is cleared by the calling function. Clearing the stack is nothing but having a statement to move the stack pointer up by some number (which is generally the number of arguments). So when a call is placed, immediately after instruction to call, there will be an instruction to move the stack pointer. This way we have an extra instruction associated with every function call instruction. This leads to the increased size of the executable, because of one extra instruction for every function call.

__stdcall is the new way where every function is responsible for clearing the stack allocations made for its parameters. As in, the callee is the one who will rewind the stack and move the stack pointer. In this approach before the return statement of the function a statement to move the stack pointer is introduced. This way the number of instructions will not increase with the number of the function calls.

__thiscall is the object-oriented way of calling functions. Here the first parameter passed is always the "this" pointer. So any instance function member of a class will follow this calling convention. Unlike other parameters, which go to the stack top, the "this" parameter which is also passed is stored in "ECX" register. Now this is how the "this" pointer is implicitly available to all the instance member functions. No function can be explicitly qualified with this calling convention. Any instance member function will get this implicitly, but it can be over-ridden with any other calling convention.

__fastcall is an approach where in the system will try to push the parameters on to Registers instead of the stack just to get a performance boost. But as with the variable qualifier "register" , if the registers are not available then again stack is used for the parameters.

There are a couple of other posts regarding the specifics of the calling conventions and comparison. If this post made any sense then check out those also.

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