README.md

ASM basics

• %r** and %e** are registers. Registers store output of instructions and can be used to give input to instructions
• when writing (%r**), it is accessing memory location given by value of register %r**. Its like pointer dereferencing. A number before a memory access applies an offset to the accessed location. -8(%rdi) would get value at memory location given by %rdi-8.
• %rdi register is used internally to pass reference of class instance to a member function of that class.
• %rbp register stores the location of start of stack frame of that function. That is why all temp values will be stored relative to it. Stacks grow downward (towards 0) hence you see so many negative offsets.
• movl %rs,%rd and movq %rs,%rd move 32 bit and 64 bit values from %rs (source register) to %rd (destination register). A dereferenced register notation moves data to and from memory location.
• leaq is a weird instruction to explain, it basically stores the address of memory access in 64-bit format. leaq -64(%rbp), %rax stores %rbp-64 in %rax without accessing that memory location.

scoo.asm

_ZN3bar4scooEv:
	pushq	%rbp
	movq	%rsp, %rbp
	pushq	%r12
	pushq	%rbx
	subq	$80, %rsp
	
	movq	%rdi, -72(%rbp) # pointer to bar instance was passed in %rdi
	
	movq	%fs:40, %rax
	movq	%rax, -24(%rbp)
	xorl	%eax, %eax
	
	# doo.gz()
	# %rbp    -> frame start
	# %rbp-72 -> pointer to bar::doo (doo is at 0 offset)
	# pass pointer to class instance in %rdi and call class member
	movq	-72(%rbp), %rax
	movq	%rax, %rdi
	call	_ZN3foo2gzEv # uses (%rdi+8) to return z value
	movl	%eax, %r12d

	# doo.gy()
	movq	-72(%rbp), %rax
	movq	%rax, %rdi
	call	_ZN3foo2gyEv
	movl	%eax, %ebx

	# doo.gx()
	movq	-72(%rbp), %rax
	movq	%rax, %rdi
	call	_ZN3foo2gxEv
	movl	%eax, %esi
	
	# foo(int, int, int)
	# args are passed in %ecx,%edx,%esi
	# %rdi points to new foo instance at %rbp-64
	leaq	-64(%rbp), %rax
	movl	%r12d, %ecx
	movl	%ebx, %edx
	movq	%rax, %rdi
	call	_ZN3fooC1Eiii # constructor call
	# new instance constructed at %rbp-64
	# move it to %rbp-48
	movq	-64(%rbp), %rax
	movq	%rax, -48(%rbp)
	movl	-56(%rbp), %eax
	movl	%eax, -40(%rbp)
	movq	-48(%rbp), %rdx
	movl	-40(%rbp), %eax

	# Store return value in %edx,%rax
	movq	%rdx, %rcx
	movl	%eax, %edx
	movq	%rcx, %rax
	
	addq	$80, %rsp
	popq	%rbx
	popq	%r12
	popq	%rbp
	ret

scoo_wrong.asm

_ZN3bar4scooEv:
	pushq	%rbp
	movq	%rsp, %rbp
	pushq	%r12
	pushq	%rbx
	subq	$80, %rsp

	movq	%rdi, -72(%rbp) # pointer to bar instance was passed in %rdi
	
	movq	%fs:40, %rax
	movq	%rax, -24(%rbp)
	xorl	%eax, %eax

	# doo.gz()
	# compare with scoo.asm
	# This implementation takes address of %rbp-64, new empty doo object
	# which is in scope of scoo
	leaq	-64(%rbp), %rax
	movq	%rax, %rdi
	call	_ZN3foo2gzEv
	movl	%eax, %r12d
	# because foo instance at %rbp-64 isnt properly constructed
	# function returns garbage. Same with rest two

	# doo.gy()
	leaq	-64(%rbp), %rax
	movq	%rax, %rdi
	call	_ZN3foo2gyEv
	movl	%eax, %ebx
	
	# doo.gx()
	leaq	-64(%rbp), %rax
	movq	%rax, %rdi
	call	_ZN3foo2gxEv
	movl	%eax, %esi

	# foo(int,int,int)
	# args are passed in %ecx,%edx,%esi (garbage values)
	# %rdi points to new foo instance at %rbp-64
	leaq	-64(%rbp), %rax
	movl	%r12d, %ecx
	movl	%ebx, %edx
	movq	%rax, %rdi
	call	_ZN3fooC1Eiii
	
	movq	-64(%rbp), %rax
	movq	%rax, -48(%rbp)
	movl	-56(%rbp), %eax
	movl	%eax, -40(%rbp)
	movq	-48(%rbp), %rdx
	movl	-40(%rbp), %eax
	movq	%rdx, %rcx
	movl	%eax, %edx
	movq	%rcx, %rax
	addq	$80, %rsp
	popq	%rbx
	popq	%r12
	popq	%rbp
	ret