CS 208 s20 — Arithmetic in x86-64 Assembly

A C function with the signature long f(long *p, long i) compiled to the following assembly code:

f:
    movq    %rsi, %rax
    addq    (%rdi), %rax
    movq    %rax, (%rdi)
    ret

Write the C code for this function.

long f(long *p, long i) {
    *p += i;
    return *p;
}

How would the assembly change if the return statement were removed?

• "load effective address", but more often "lovely efficient arithmetic"
• instead of reading from the memory location given by the source operand, copies the effective address to the destination
  • generate pointers for later memory references
  • can also do a muliply and an addition in a single instruction
    • leaq 7(%rdx, %rdx, 4), %rax will set %rax equal to 5 * %rdx + 7
• destination must be a register
• must have the q size designation on a 64-bit system—why?
  • lea specifically works with a memory addresses, which will always by 8 bytes on a 64-bit system
• movq %rdx, %rax vs movq (%rdx), %rax vs leaq (%rdx), %rax
  • rdx holds 0x100, memory address 0x100 holds 0xab

Translate this C code to assembly

long arith(long x, long y, long z)
{
    long t1 = x + y;
    long t2 = z + t1;
    long t3 = x + 4;
    long t4 = y * 48;
    long t5 = t3 + t4;
    long rval = t2 * t5;
    return rval;
}

arith:
    leaq    (%rdi,%rsi), %rax
    addq    %rdx, %rax
    leaq    (%rsi,%rsi,2), %rcx
    salq    $4, %rcx
    leaq    4(%rdi,%rcx), %rcx
    imulq   %rcx, %rax
    ret

Examples on godbolt.org: https://godbolt.org/z/j_WZwW