An important topic for Compiler Course.

1. Welcome to my
Presentation
Topic : Peephole Optimization
Samrin Ahmed
ID: 011142021

2. CONTENTS
Functioning of Peephole Optimization & Example
Replacement Rules
Working Flow of Peephole Optimization
Introduction to Peephole Optimization
What is an Optimization
2
Conclusion
References

3. What is Optimization ??
 Process of transforming a piece of code to make it more efficient (either in
terms of time or space) without changing its output or side-effects.
 Tries to minimize or maximize some attributes of an executable computer
program.

4. Introduction to Peephole
Optimization ??
 Optimization performed over a very small set of
instructions in a segment of generated code.
 Works by recognizing sets of instructions that can be
replaced by shorter or faster sets of instructions.
 Goals:
 improve performance
 reduce code size
 reduce memory footprint

5. Why it’s needed ??
After compilation, the code still has some flaws
 Scheduling & allocation really are NP-Complete
 Optimizer may not implement every needed transformation
Curing the problem
 More work on scheduling and allocation
 Implement more optimizations
— or —
 Optimize after compilation
 Peephole optimization
 Link-time optimization

6. Working Flow of Peephole
Optimization

7. Replacement Rules
Common techniques applied in peephole optimization:-
 Constant folding
– Evaluate constant sub-expressions in advance.
 Strength reduction
– Replace slow operations with faster equivalents.
 Null sequences
– Delete useless operations.
 Combine operations
– Replace several operations with one equivalent.
 Algebraic laws
– Use algebraic laws to simplify or reorder instructions.
 Special case instructions
– Use instructions designed for special operand cases.
 Address mode operations
– Use address modes to simplify code.

8. Functioning of Peephole
Optimization
 Replacing slow instructions with faster ones
 Removing redundant code
 Removing redundant stack instructions

9. Functioning (Contd…)
 Replacing slow instructions with faster ones
The following Java byteCode
...
load 1
load 1
mul
...
Can be replaced by
...
load 1
dup
mul
...

10. Functioning (Contd…)
 Removing redundant code
 Another example is to eliminate redundant load stores.
a = b + c;
d = a + e;
is straightforwardly implemented as
MOV b, R0 # Copy b to the register
ADD c, R0 # Add c to the register, the register is now b+c
MOV R0, a # Copy the register to a
MOV a, R0 # Copy a to the register
ADD e, R0 # Add e to the register, the register is now a+e [(b+c)+e]
MOV R0, d # Copy the register to d

11. Functioning (Contd…)
 Removing redundant code
but can be optimized to
MOV b, R0 # Copy b to the register
ADD c, R0 # Add c to the register, which is now b+c (a)
MOV R0, a # Copy the register to a
ADD e, R0 # Add e to the register, which is now b+c+e [(a)+e]
MOV R0, d # Copy the register to d

12. Functioning (Contd…)
 Removing redundant stack instructions
PUSH AF
PUSH BC
PUSH DE
PUSH HL
CALL _ADDR1
POP HL
POP DE
POP BC
POP AF
PUSH AF
PUSH BC
PUSH DE
PUSH HL
CALL _ADDR2
POP HL
POP DE
POP BC
POP AF

PUSH AF
PUSH BC
PUSH DE
PUSH HL
CALL _ADDR1
CALL _ADDR2
POP HL
POP DE
POP BC
POP AF

13. Functioning (Contd…)
Source Code:
If ( a<b ) {
a = a + b;
}
 Peephole optimization of jumps
 Eliminate jumps to jumps
 Eliminate jumps after conditional branches
“Adjacent” instructions = “Adjacent in control flow”
IL Code:
PUSH a
PUSH b
CMP a,b
JUMP L1
EXIT
L1:
ADD a,b


14. Other Considerations
Control-flow operations
 Can clear simplifier’s window at branch or label
 More aggressive approach: combine across branches
 Same considerations arise with predication
Physical versus logical windows
 Can run optimizer over a logical window
 Logical windows (within block) improve effectiveness

15. Conclusion
So…
 Peephole optimization remains viable
 Post allocation improvements
 Cleans up rough edges
 Peephole technology works for selection
 Description driven matchers
 Used in several important systems
All of this will work equally well in binary-to-binary translation

16. References
 https://en.wikipedia.org/wiki/Peephole_optimization
 http://www.iosrjournals.org/iosr-jce/papers/Vol9-Issue4/N0948086.pdf?id=255
 https://class.coursera.org/compilers/lecture/76
 https://www.slideshare.net/AnulChaudhary/peephole-optimization-techniques-in-
compiler-design

17. That’s all
Any Question