Lecture 005

Processor States (x86-64)

Temporary data: %rax, ... Location of runtime stack: %rsp Location of current code control point: %rip, ... Status of recent tests: CF, ZF, SF, OF (condition codes)

Condition Codes

Implicit Setting when Arithmetic

Condition Codes: single bit storage

• CF: Carry Flag (for unsigned)

  • if if carry / borrow out from most significant bit (unsigned overflow)
  • set when carry or borrow

• SF: Sign Flag (for signed)

  • if t < 0 (as signed)
  • if sum is negative (ignored when doing unsigned addition)

• ZF: Zero Flag

  • if t == 0
  • if the sum is all 0

• OF: Overflow Flag (for signed)

  • if two's-complement overflow: (a > 0 && b > 0 && t < 0) || (a < 0 && b < 0 && t >= 0)

Note: leaq instruction does not set flags

Explicit Setting Condition Codes

cmpq Src2, Src1: computing a-b without setting destination

• CF: unsigned comparison, if carry/borrow from most significant bit

• ZF: a == b

• SF: (a-b) < 0 (or a < b, as signed)

• OF: if signed overflow

testq Src2, Src1: computing a&b without setting destination

• ZF: a&b == 0

• SF: a&b < 0

• use case: masking, testq %rax, %rax check whether %tax is 0

Explicit Reading Condition Codes

setX Dest: set lowest byte to 0 or 1 based on some condition flags

cmp    %rsi, %rdi # compare x and y
setg   %al        # lowest byte of %rax set to 1 when >
movzbl %al, %eax  # move %al to %eax (both part of %rax) with zero extend from 1 byte quantity to 4 byte quantity (actually zero padding both 4 byte quantity and 8 byte quantity)

jX Dest (or bX sometimes): jump to part of code depending on condition codes

• Note: ja (unsigned) may be optimized to do if (x > 6 || x < 0) when x is signed.

Conditional Moves (cmov..): val = Test ? Then_Expr : Else_Expr (compiler only do it when it is safe)

• calculate both resulting branches before testing

• instead of guessing the testing result (very inefficient when guessed wrong)

• 

• Bad use cases

  • expensive computation
  • risky de-reference
  • illegal transformation: computation with side effects

test: logical AND cmp: subtraction

Loops

While

While Loop in C

while (Test)
  Body

While Loop in Goto C

  goto test;
loop:
  Body
test:
  if (Test)
    goto loop;
done:

Do-While

Do-While Loop in C

do
  Body
  while (Test);

Do-While Loop in Goto C

loop:
  Body
  if (Test)
  goto loop

For

For Loop in C

for (Init; Test; Update)
  Body

For Loop in Goto C

Init;
while (Test) {
  Body
  Update
}

Jump Table

Example: jmp *.L4 (, %rdi, 8)

• 

• 

  • Notice the breake jump here is optimized to return because nothing after break

• 

•