Introduction to Optimization · Introduction to Optimization1 Kristoffer H. Rose...

Basic Block DAG Peephole Optimization Thanks! HACS Q&A

Introduction to Optimization1

Kristoffer H. [email protected]

Compiler ConstructionCSCI-GA.2130-001/Fall 2013

NYU Courant Institute

December 5, 2013

1ALSU §§8.5,7Introduction to Optimization Kris Rose

Context

sourceprogram

FrontEnd// Code

OptimizerIR// Code

GeneratorIR// target

program//

Introduction to Optimization Kris Rose

1 Basic Block DAG

2 Peephole Optimization

3 Thanks!

4 HACS Q&A

DAG – Directed Acyclic Graph

I One node per initial value.I One node per statement, with an edge to last node

observing every parameter.I Node labeled by operator and list of variables not used

further.I Output nodes are those with live exit variables.

DAG uses

I Local common subexpressions.I Dead code elimination.I Apply albegraic simplifications.I Reorder statements to reduce variable count.

Local Common Subexpressions

1 a = b + c2 b = a - d3 c = b + c4 d = a - d

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Local Common Subexpressions

1 a = b + c2 b = a - d3 c = b + c4 d = a - d

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Dead Code Elimination

Inputs: b,c,d1 a = b + c2 b = b - c3 c = c + d4 e = b + c

Outputs: a,bb0 c0 d0

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{{ ��

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Dead Code Elimination

Inputs: b,c,d1 a = b + c2 b = b - c3 c = c + d4 e = b + c

Outputs: a,bb0 c0 d0

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{{ ��

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Algebraic Identities

x + 0 = 0 + x = x x − 0 = xx × 1 = 1× x = x x/1 = x

EXPENSIVE CHEAPER

x2 = x × x2× x = x + x

x/2 = x × 0.5

x + 0 = 0 + x = x x − 0 = xx × 1 = 1× x = x x/1 = x

EXPENSIVE CHEAPER

x2 = x × x2× x = x + x

x/2 = x × 0.5

I Constant foldingI Commutativity with Local Common SubexpressionsI Associativity with composite expressions

1 a = c + b2 e = c + d + b

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1 a = c + b2 e = c + d + b

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1 a = c + b2 e = c + d + b

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Array References

1 x = a[i]2 a[j] = y3 z = a[i]

a0 i0 j0 y0

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xKilled!

tt��

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Array References

1 x = a[i]2 a[j] = y3 z = a[i]

a0 i0 j0 y0

��

xKilled!

tt��

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No Pointing!

1 x = *p2 *q = y

Needs pointer (“points-to”) analysis!

No Pointing!

1 x = *p2 *q = y

Needs pointer (“points-to”) analysis!

Back to Code

��

1 a = b + c2 b = a - d3 c = b + c4 d = b

Respect ordering, pay special attention to overlapping sideeffects!

Back to Code

��

1 a = b + c2 b = a - d3 c = b + c4 d = b

Back to Code

��

1 a = b + c2 b = a - d3 c = b + c4 d = b

1 Basic Block DAG

3 Thanks!

4 HACS Q&A

Redundant Load&Store

1 LD R0, a2 ST a, R0

1 LD R0, a

Redundant Load&Store

1 LD R0, a2 ST a, R0

1 LD R0, a

Unreachable Code

1 if debug == 1 goto L12 goto L23 L1: compute-and-print-really-big-expensive-stuff4 L2:

1 if debug != 1 goto L22 L1: compute-and-print-really-big-expensive-stuff3 L2:

With debug = 0,1 L2:

Unreachable Code

Flow of Control

1 goto L12 . . .3 L1: goto L2

1 goto L22 . . .3 L1: goto L2

Flow of Control

1 goto L12 . . .3 L1: goto L2

1 goto L22 . . .3 L1: goto L2

Flow of Control

1 goto L12 . . .3 L1: if a < b goto L24 L3:

1 if a < b goto L22 goto L33 . . .4 L3:

Flow of Control

1 goto L12 . . .3 L1: if a < b goto L24 L3:

1 if a < b goto L22 goto L33 . . .4 L3:

Just the Beginning. . .

I Peephole vs Good Code GeneratorI Everything above has better analysis-based solutions

1 Basic Block DAG

3 Thanks!

4 HACS Q&A

Finally. . .

I Evaluations!I Project Milestone 3 on Sunday.I Exam:

I 5:10 pm–7:00 pm, December 12, 2013, Room WWH 312.I Closed book.I All ALSU sections quoted on class web page.

1 Basic Block DAG

3 Thanks!

4 HACS Q&A

The End

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