Code optimization techniques aim to improve the performance and efficiency of code. Some key techniques include peephole optimization, instruction selection, register allocation, code motion, strength reduction, and common subexpression elimination. Peephole optimization examines short instruction sequences to replace them with faster or shorter sequences. Instruction selection uses more efficient instructions to replace sequences. Register allocation assigns variables to registers to improve speed.

1. • Code optimization:
– A transformation to a program to make it run
faster and/or take up less space
– Optimization should be safe, preserve the
meaning of a program.
– Code optimization is an important component
in a compiler
– Example: peephole optimization.
• Peephole: a small moving window in the instruction
sequence
• Technique: examine a short sequence of target
instructions (peephole) and try to replace it with a
faster or shorter sequence

2. • Peephole optimization:
• Redundant instruction elimination
• Flow of control optimization
• Algebraic simplifications
• Instruction selection
• Examples:
– Redundant loads and stores
MOV R0, a
MOV a, R0
– Unreachable code
If debug = 1 goto L1
Goto L2
L1: print debugging info
L2:

3. – Examples:
• Flow of control optimization:
goto L1
…
L1: goto L2
goto L2
…
L1: goto L2
if a < b goto L1
…
L1: goto L2
if a<b goto L2
…
L1: goto L2
goto L1
…
L1: if a < b goto L2
if a < b goto L2
goto L3
…
L1:
L3:

4. • Algebraic simplification:
x : = x+0
x := x*1 == nop
• Reduction in strength
X^2  x * x
X * 4  x << 2
• Instruction selection
Sometimes some hardware instructions can
implement certain operation efficiently.

5. • Code optimization can either be high level
or low level:
– High level code optimizations:
• Loop unrolling, loop fusion, procedure inlining
– Low level code optimizations:
• Instruction selection, register allocation
– Some optimization can be done in both levels:
• Common subexpression elimination, strength
reduction, etc.
– Flow graph is a common intermediate
representation for code optimization.

6. • Basic block: a sequence of consecutive statements with
exactly 1 entry and 1 exit.
• Flow graph: a directed graph where the nodes are basic
blocks and block B1 block B2 if and only if B2 can be
executed immediately after B1:
• Algorithm to construct flow graph:
– Finding leaders of the basic blocks:
• The first statement is a leader
• Any statement that is the target of a conditional or
unconditional goto is a leader
• Any statement that immediately follows a goto or conditional
goto statement is a leader
– For each leader, its basic block consists all statements
up to the next leader.
– B1B2 if and only if B2 can be executed immediately
after B1.

7. • Example:
100: sum = 0
101: j = 0
102: goto 107
103: t1 = j << 2
104: t2 = addr(a)
105: t3 = t2[t1]
106: sum = sum + t3
107: if j < n goto 103

8. • Optimizations within a basic block is called local optimization.
• Optimizations across basic blocks is called global optimization.
• Some common optimizations:
– Instruction selection
– Register allocation
– Common subexpression elimination
– Code motion
– Strength reduction
– Induction variable elimination
– Dead code elimination
– Branch chaining
– Jump elimination
– Instruction scheduling
– Procedure inlining
– Loop unrolling
– Loop fusing
– Code hoisting

9. • Instruction selection:
– Using a more efficient instruction to replace a sequence of
instructions (space and speed).
– Example:
Mov R2, (R3)
Add R2, #1, R2
Mov (R3), R2  Add (R3), 1, (R3)

10. • Register allocation: allocate variables to registers (speed)
• Example:
M[R13+sum] = 0
M[R13+j] = 0
GOTO L18
L19:
R0 = M[R13+j] * 4
M[R13+sum] = M[R13+sum]
+M[R0+_a]
M[R13+j] = M[R13+j]+1
L18:
NZ = M[R13+j] - M[_n]
if NZ < 0 goto L19
R2 = 0
R1 = 0
GOTO L18
L19:
R0 = R1 * 4
R2 = R2+M[R0+_a]
R1 = R1+1
L18:
NZ = R1 - M[_n]
if NZ < 0 goto L19

11. • Code motion: move a loop invariant computation before
the loop
• Example:
R2 = 0
R1 = 0
GOTO L18
L19:
R0 = R1 * 4
R2 = R2+M[R0+_a]
R1 = R1+1
L18:
R4 = M[_n]
NZ = R1 – R4
if NZ < 0 goto L19
R2 = 0
R1 = 0
R4 = M[_n]
GOTO L18
L19:
R0 = R1 * 4
R2 = R2+M[R0+_a]
R1 = R1+1
L18:
NZ = R1 – R4
if NZ < 0 goto L19

12. • Strength reduction: replace expensive operation by
equivalent cheaper operations
• Example:
R2 = 0
R1 = 0
R4 = M[_n]
GOTO L18
L19:
R0 = R1 * 4
R2 = R2+M[R0+_a]
R1 = R1+1
L18:
NZ = R1 – R4
if NZ < 0 goto L19
R2 = 0
R1 = 0
R4 = M[_n]
R3 = _a
GOTO L18
L19:
R2 = R2+M[R3]
R3 = R3 + 4
R1 = R1+1
L18:
NZ = R1 – R4
if NZ < 0 goto L19

13. • Induction variable elimination: can induce value from
another variable.
• Example:
R2 = 0
R1 = 0
R4 = M[_n]
R3 = _a
GOTO L18
L19:
R2 = R2+M[R3]
R3 = R3 + 4
R1 = R1+1
L18:
NZ = R1 – R4
if NZ < 0 goto L19
R2 = 0
R4 = M[_n] << 2
R3 = _a
GOTO L18
L19:
R2 = R2+M[R3]
R3 = R3 + 4
L18:
NZ = R3 – R4
if NZ < 0 goto L19

14. • Common subexpression elimination:an expression was
previously calculated and the variables in the expression
have not changed. Can avoid recomputing the expression.
• Example:
R1 = M[R13+I] << 2
R1 = M[R1+_b]
R2 = M[R13+I] << 2;
R2 = M[R2+_b]
R1=M[R13+I] << 2
R1 = M[R1+_b]
R2 = R1