Windows32 API: “mov edi,edi” on function entry?

Question

I'm stepping through Structured Error Handling recovery code in Windows 7 (e.g, what happens after SEH handler is done and passes back "CONTINUE" code). Here's a function which is called:

 7783BD9F  mov         edi,edi  
 7783BDA1  push        ebp 
 7783BDA2  mov         ebp,esp   
 7783BDA4  push        1  
 7783BDA6  push        dword ptr [ebp+0Ch]  
 7783BDA9  push        dword ptr [ebp+8]  
 7783BDAC  call        778692DF  
 7783BDB1  pop         ebp  
 7783BDB2  ret         8  

I'm used to the function prolog of "push ebp/mov ebp,esp". What's the purpose of the "mov edi,edi"?

Answer 1

Raymond Chen (one of the Microsoft developers) has answered this exact question:

• Why do Windows functions all begin with a pointless MOV EDI, EDI instruction?

And he links an even earlier reference:

• Why does the compiler generate a MOV EDI, EDI instruction at the beginning of functions?

Basically, it leaves space for a jump instruction to be inserted during hot patching.