ch9 peephole

7/31/2019 Ch9 PeepHole

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CMPUT 680 - Compiler Designand Optimization 1

CH 9: CODE GENERATION

PEEP HOLE Optimization


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Peephole Optimization

Goals:- improve performance- reduce memory footprint

- reduce code sizeMethod:

1. Exam short sequences of target instructions2. Replacing the sequence by a more efficient one.

redundant-instruction elimination flow-of-control optimizations algebraic simplifications use of machine idioms


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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.


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CMPUT 680 - Compiler Design

and Optimization 4

Example

debug = 0. . .if(debug) {

print debugging information}

debug = 0

. . .if debug = 1 goto L1goto L2

L1: print debugging informationL2:

Source Code:

Intermediate

Code:


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and Optimization 5

Eliminate Jump after Jump

debug = 0. . .if debug = 1 goto L1

goto L2L1: print debugging informationL2:

Before:

debug = 0

. . .if debug 1 goto L2print debugging information

L2:

After:


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and Optimization 6

Constant Propagation

Before:

After:

debug = 0. . .

if debug

1 goto L2print debugging informationL2:

debug = 0. . .if 0 1 goto L2print debugging information

L2:


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and Optimization 7

Unreachable Code

(dead code elimination)

Before:

After:

debug = 0. . .

if debug

1 goto L2print debugging informationL2:

debug = 0. . .


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and Optimization 8

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. . .

L1: if a < b goto L2

L3:

if a < b goto L2goto L3. . .

L3:


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and Optimization 9

Transformations on Basic

Blocks

Structure-Preserving Transformations:

common subexpression elimination

dead code eliminationrenaming of temporary variables

interchange of two independent adjacent

statements


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and Optimization 10

Examples of Transformations

Common subexpression elimination:

a := b + cb := a - d

c := b + cd := a - d

a := b + cb := a - d

c := b + cd := b

Dead code elimination:

if x is never referenced after the statement x = y+z, the

statement can be safely eliminated.


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and Optimization 11


Interchange of statements:

t1 := b + ct2 := x + y

t2 := x + yt1 := 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.


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and Optimization 12


Algebraic transformations:

x := x + 0x := x * 1

x := y**2z := 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.

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