Code optimization:

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 lt b goto L1 L1 goto L2
if altb goto L2 L1 goto L2
goto L1 L1 if a lt b goto L2
if a lt b goto L2 goto L3
L1 L3
4

• Algebraic simplification
• x x0
• x x1 ? nop
• Reduction in strength
• X2 ? x x
• X 4 ? x ltlt 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 ltlt 2
• 104 t2 addr(a)
• 105 t3 t2t1
• 106 sum sum t3
• 107 if j lt 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

MR13sum 0 MR13j 0
GOTO L18 L19 R0 MR13j 4
MR13sum MR13sum
MR0_a MR13j
MR13j1 L18 NZ MR13j - M_n
if NZ lt 0 goto L19
R2 0 R1 0 GOTO
L18 L19 R0 R1 4 R2
R2MR0_a R1 R11 L18 NZ
R1 - M_n if NZ lt 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
R2MR0_a R1 R11 L18 R4
M_n NZ R1 R4 if NZ lt 0
goto L19
R2 0 R1 0 R4
M_n GOTO L18 L19 R0 R1 4
R2 R2MR0_a R1 R11 L18
NZ R1 R4 if NZ lt 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 R2MR0_a R1 R11 L18
NZ R1 R4 if NZ lt 0 goto L19
R2 0 R1 0 R4
M_n R3 _a GOTO L18 L19
R2 R2MR3 R3 R3 4 R1
R11 L18 NZ R1 R4 if NZ
lt 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 R2MR3 R3 R3 4 R1
R11 L18 NZ R1 R4 if NZ
lt 0 goto L19
R2 0 R4 M_n ltlt 2
R3 _a GOTO L18 L19 R2
R2MR3 R3 R3 4 L18 NZ
R3 R4 if NZ lt 0 goto L19
14

• Common subexpression eliminationan expression
  was previously calculated and the variables in
  the expression have not changed. Can avoid
  recomputing the expression.
• Example

R1 MR13I ltlt 2 R1 MR1_b R2 MR13I ltlt
2 R2 MR2_b
R1MR13I ltlt 2 R1 MR1_b R2 R1