Question about x86 delegate calling convention

[James Dunne]

Jarrett Billingsley wrote:
> I was stepping through some assembly when debugging one of my programs, and 
> I noticed something a bit odd about how delegates were called.
> 
> Delegates consist basically of two things: a context pointer (either 'this' 
> for class members or the outer function's frame pointer for nested 
> functions), and the address of the actual code.  This is how D seems to call 
> delegates:
> 
> EAX = context ptr;
> EDX = code address;
> EBX = context ptr;
> EDX();
> 
> Inside the delegate, only the EAX context pointer is ever used.  'this' is 
> also passed in EAX when calling class methods directly (a.method()).
> 
> My question is: why is EBX filled with the context pointer?  It's never used 
> in the delegate; in fact, if the delegate has some complex code, and the 
> codegen needs another register, it will preserve EBX (pushing it) before 
> using it.
> 
> This happens even in release mode.
> 
> Is this a vestige of some older calling convention for delegates?
> 
> I also want to know this because I'm working on a (blatantly non-portable) 
> method of dynamically calling functions at run-time (for fun). 
> 
> 

A wild stab in the dark here, but perhaps the dup context ptr in EBX is 
coincidentally the same value used for a different feature 
implementation?  Try all the cases of delegate calling, nested function 
calling, virtual method calling, variadic function calling, etc. and 
check the value of EBX vs. EAX.  I have a suspicion your answer will lie 
in there somewhere. :)

Then again, perhaps it's just bad codegen... but let's hope not.  I 
don't think anyone could answer this 100% other than Walter.

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James Dunne