Assembly 2: Arithmetic, calling convention, control flow

Overview

We finish our discussion of assembly address modes and arithmetic instructions, then discuss calling conventions and control flow in machine code.

Full lecture notes on assembly — Textbook readings

Sidebar: Type-safe linkage and mangled names

• The name of a C++ function encodes the types of its arguments
• This makes C++ compilations safer and supports overloading (functions with different behavior based on argument types)
• Example: f(int) ⟶ _Z1fi
  • _Z: This is a mangled name
  • 1: Function name is 1 character long
  • f: Actual function name
  • i: First argument is int
• To demangle, try c++filt MANGLEDNAME

Data operands and address modes

• $X: an immediate value (a constant)
• %X: a register value
• a(%rip): a global symbol
• (%X): an indirect reference (dereferencing a “pointer”)
• 8(%X): an offset indirect reference (dereferencing a structure or array)
  • N(R) means dereference memory at address R + N

Arithmetic instructions

• General format: OP SRC, DST
  • This means DST := DST OP SRC
• addl %eax, %ebx
  • %ebx := %ebx + %eax
• subl %rdi, %r9
  • %r9 := %r9 - %rdi

f08.s

Arithmetic (computation) instructions

• xorl %eax, %eax means %eax := %eax ^ %eax
• Which means…

f09.s, f10.s

f11.s

Moving into register slices

• mov[SIGN][SRCSIZE][DSTSIZE]
  • SIGN is z (extend with zeros) or s (extend with sign bit)
  • SRCSIZE/DSTSIZE is b (byte), w (short), l (int), or q (long)

f12.s

f13.s, f14.s

f15.s, f16.s, f17.s

More data formats

• (%X,%Y,Z): an array indirect reference
  • Dereference memory at %X + %Y * %Z
• Full format: offset(base,index,scale)
  • offset + base + index * scale
  • offset must be a constant
  • scale must be 1, 2, 4, or 8
  • Default offset, base, and index are 0; default scale is 1

f18.s

The lea instruction

• lea stands for Load Effective Address
• It performs an address computation, but does not dereference
• Often used by compiler as a parsimonious alternative to array arithmetic
  • leal (%rdi,%rsi,8), %eax
  • movl %esi, %eax; shll $3, %eax; addl %edi, %eax

Calling convention

• Some aspects of machine code are fixed by the processor manufacturer
  • Intel decided 0xc3 is the representation of ret
• Some aspects of machine code are set by agreement among compiler and operating system developers
  • Intel did not decide which register holds return values
• We call this agreement the calling convention since it governs function calls
  • Think Geneva Convention, not Comic Convention
  • Different conventions can exist for the same processor (e.g., Unix vs. Windows)
  • Only codes with the same conventions can safely interact

Elements of a calling convention

• Function arguments
• Function return values
• Local variable storage
• Stack alignment
• Memory and processor state when the program begins

Let’s explore: cc01.cc–cc03.cc

Arguments

• Argument registers are %rdi, %rsi, %rdx, %rcx, %r8, %r9, in that order
• Large objects are passed in up to 2 registers if they fit, stack otherwise

Return values

• Return register is %rax
• Large objects are returned in %rax+%rdx if they fit, otherwise first argument points to space for return value