fastcall

Process

1. First two arguments are saved into arg2: rbx, arg1: rcx
2. All subsequent arguments are pushed onto the stack
3. Function is called with call <location> & rip pushed
4. Space is allocated onto the stack for local variables
5. Function does work
6. Stack is restored
7. Returns by calling ret
8. Stack adjusted to remove arguments

Example

With C code:

__attribute__((fastcall)) void printnums(int num1, int num2, int num3){
	printf("The numbers you sent are: %d %d %d", num1, num2, num3);
}
 
int main(){
	printnums(1, 2, 3);
	return 0;
}

x86 decompilation of main() is:

main:
	; stack setup
	push ebp
	mov ebp, esp
	push 3 ; immediate 3 (third argument is pushed to the stack)
	mov edx, 0x2 ; immediate 2 (second argument) is copied to edx register.
	mov ecx, 0x1 ; immediate 1 (first argument) is copied to ecx register.
	call printnums
	mov eax, 0 ; return 0
	leave
	retn

x86 decompilation of printnums() is:

printnums:
	; stack setup
	push ebp
	mov ebp, esp
	sub esp, 0x08
	mov [ebp-0x04], ecx    ; in x86, ecx = first argument.
	mov [ebp-0x08], edx    ; arg2
	push [ebp+0x08]        ; arg3 is pushed to stack.
	push [ebp-0x08]        ; arg2 is pushed
	push [ebp-0x04]        ; arg1 is pushed
	push 0x8065d67         ; "The numbers you sent are %d %d %d"
	call printf
	; stack cleanup
	add esp, 0x10
	nop
	leave
	retn 0x04