CMPUT680 - Winter 2006
Topic 5: Peep Hole Optimization
José Nelson Amaral
http://www.cs.ualberta.ca/~amaral/courses/680
CMPUT 680 - Compiler Design
and Optimization
1
Peephole “Optimization”
Goals:
- improve performance
- reduce memory footprint
- reduce code size
Method:
1. Exam short sequences of target instructions
2. Replacing the sequence by a more efficient one.
•
•
•
•
redundant-instruction elimination
flow-of-control optimizations
algebraic simplifications
use of machine
idiomsDesign
CMPUT 680 - Compiler
and Optimization
2
Redundant Load and Stores
(1) LOAD R0, a
(2) STORE a, R0
(1) LOAD R0, a
If there are no changes to a between (1) and (2),
then the store (2) is redundant.
CMPUT 680 - Compiler Design
and Optimization
3
Example
Source Code:
Intermediate
Code:
debug = 0
...
if(debug) {
print debugging information
}
debug = 0
...
if debug = 1 goto L1
goto L2
L1: print debugging information
L2:
CMPUT 680 - Compiler Design
and Optimization
4
(Aho-Sethi-Ullman,pp. 555)
Eliminate Jump after Jump
Before:
debug = 0
...
if debug = 1 goto L1
goto L2
L1: print debugging information
L2:
After:
L2:
debug = 0
...
if debug  1 goto L2
print debugging information
CMPUT 680 - Compiler Design
and Optimization
5
(Aho-Sethi-Ullman,pp. 555)
Constant Propagation
Before:
L2:
debug = 0
...
if debug  1 goto L2
print debugging information
After:
L2:
debug = 0
...
if 0  1 goto L2
print debugging information
CMPUT 680 - Compiler Design
and Optimization
6
(Aho-Sethi-Ullman,pp. 555)
Unreachable Code
(dead code elimination)
Before:
L2:
After:
debug = 0
...
if debug  1 goto L2
print debugging information
debug = 0
...
CMPUT 680 - Compiler Design
and Optimization
7
(Aho-Sethi-Ullman,pp. 555)
Flow-of-control optimizations
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
if a < b goto L2
...
goto L3
L1: if a < b goto L2
...
CMPUT 680 - Compiler Design
L3:
L3:
and Optimization
8
(Aho-Sethi-Ullman,pp. 556)
Transformations on Basic
Blocks
Structure-Preserving Transformations:
common subexpression elimination
dead code elimination
renaming of temporary variables
interchange of two independent adjacent
statements
CMPUT 680 - Compiler Design
and Optimization
9
Examples of Transformations
Common subexpression elimination:
a := b + c
b := a - d
c := b + c
d := a - d
a := b + c
b := a - d
c := b + c
d := b
Dead code elimination:
if x is never referenced after the statement x = y+z, the
statement can be safely eliminated.
CMPUT 680 - Compiler Design
and Optimization
10
Examples of Transformations
Interchange of statements:
t1 := b + c
t2 := x + y
t2 := x + y
t1 := b + c
Renaming temporary variables:
if there is a statement t := b + c, we can change it to u := b + c
and change all uses of t to u.
CMPUT 680 - Compiler Design
and Optimization
11
Examples of Transformations
Algebraic transformations:
x := x + 0
x := x * 1
x := y**2
z := 2*x
x := y*y
z := x + x
Changes such as y**2 into y*y and 2*x into x+x
are also known as strength reduction.
CMPUT 680 - Compiler Design
and Optimization
12