gradual programming: bridging the semantic gap bor-yuh evan chang bor-yuh evan chang amer diwan...

Gradual Programming: Gradual Programming: Bridging the Semantic GapBridging the Semantic Gap

Bor-Yuh Evan ChangBor-Yuh Evan Chang Amer Diwan Jeremy G. Siek

University of Colorado, Boulder

PLDI FIT 2009

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Have you noticed a time where your Have you noticed a time where your program is not optimized where you program is not optimized where you expect?expect?

Bor-Yuh Evan Chang, Amer Diwan, and Jeremy G. Siek - Gradual Programming

“I need a map data structure”

“I need a map data structure”

Load class fileRun class initializationCreate hashtable

Load class fileRun class initializationCreate hashtable

ProblemProblem: Tools (IDEs, checkers, optimizers) have no knowledge of what the programmer cares about

ProblemProblem: Tools (IDEs, checkers, optimizers) have no knowledge of what the programmer cares about

… hampering programmer productivity, software reliability, and execution efficiency

… hampering programmer productivity, software reliability, and execution efficiency

semantic gap

ObservationObservation: A disconnect between programmer intent and program meaning

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Example: Iteration OrderExample: Iteration Order

class OpenArray extends Object {private Double data[]; public boolean contains(Object

lookFor) {for (i = 0; i < data.length; i++) {

if (data[i].equals(lookFor)) return true;

}return false;

}}

Bor-Yuh Evan Chang, Amer Diwan, and Jeremy G. Siek - Gradual Programming

Compiler cannot choose a different iteration order (e.g., parallelparallel)

Compiler cannot choose a different iteration order (e.g., parallelparallel)

class OpenArray extends Object {private Double data[]; public boolean contains(Object

lookFor) {for (i = 0; i < data.length; i++) {

if (data[i].equals(lookFor)) return true;

}return false;

}}

Must specify an iteration Must specify an iteration orderorder even when it should not matter

Must specify an iteration Must specify an iteration orderorder even when it should not matter

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Wild and Crazy Idea: Use Non-Wild and Crazy Idea: Use Non-DeterminismDeterminism

• Programmer starts with a potentially non-deterministic program

• Analysis identifies instances of “under-determinedness”

• Programmer eliminates “under-determinedness”

Bor-Yuh Evan Chang, Amer Diwan, and Jeremy G. Siek - Gradual Programming

class OpenArray extends Object {private Double data[]; public boolean contains(Object lookFor)

{for (i = 0; i < data.length; i++) {

if (data[i].equals(lookFor)) return true;

}return false;

}}

class OpenArray extends Object {private Double data[]; public boolean contains(Object lookFor)

{for (i = 0; i < data.length; i++) {

if (data[i].equals(lookFor)) return true;

}return false;

}}

class OpenArray extends Object {private Double data[]; public boolean contains(Object lookFor)

{ i 0 .. data.length-1 {

if (data[i].equals(lookFor)) return true;

}return false;

}}

class OpenArray extends Object {private Double data[]; public boolean contains(Object lookFor)

{ i 0 .. data.length-1 {

if (data[i].equals(lookFor)) return true;

}return false;

}}

“over-determined”

“under-determined”

just right

starting point

QuestionQuestion: What does this mean? Is it “under-determined”?ResponseResponse: Depends, is the iteration order important?

QuestionQuestion: What does this mean? Is it “under-determined”?ResponseResponse: Depends, is the iteration order important?

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Let’s try a few program variantsLet’s try a few program variants

Bor-Yuh Evan Chang, Amer Diwan, and Jeremy G. Siek - Gradual Programming

public boolean contains(Object lookFor) {

for (i = data.length-1; i >= 0; i--) {if (data[i].equals(lookFor))

return true; }return false;

}

public boolean contains(Object lookFor) {

for (i = data.length-1; i >= 0; i--) {if (data[i].equals(lookFor))

return true; }return false;

}

public boolean contains(Object lookFor) {

for (i = 0; i < data.length; i++) {if (data[i].equals(lookFor))

return true; }return false;

}

public boolean contains(Object lookFor) {

for (i = 0; i < data.length; i++) {if (data[i].equals(lookFor))

return true; }return false;

}

public boolean contains(Object lookFor) {

parallel_for (0, data.length-1) i => {if (data[i].equals(lookFor))

return true; }return false;

}

public boolean contains(Object lookFor) {

parallel_for (0, data.length-1) i => {if (data[i].equals(lookFor))

return true; }return false;

}

Do they compute the same result?Do they compute the same result?

ApproachApproach: Try to verify equivalence of program variants up to a specification

ApproachApproach: Try to verify equivalence of program variants up to a specificationYes Pick any oneNo Ask user

Yes Pick any oneNo Ask user

What about here?What about here?

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Surprisingly, analysis says no. Surprisingly, analysis says no. Why? Why?

Exceptions!

Bor-Yuh Evan Chang, Amer Diwan, and Jeremy G. Siek - Gradual Programming

Need user interaction to refine specification that captures programmer intent

Need user interaction to refine specification that captures programmer intent

nulla.data =

a.contains( )left-to-right iteration

returns true

right-to-left iterationthrows

NullPointerException

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Scary Proposal?Scary Proposal?

• “Fix semantics per program”: Abstract constructs with many possible concrete implementations

• Apply program analysis to find inconsistent implementations

• Interact with the user to refine the specification

• Language designer role can enumerate the possible implementations

Bor-Yuh Evan Chang, Amer Diwan, and Jeremy G. Siek - Gradual Programming

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Bridging the Semantic GapBridging the Semantic Gap

Bor-Yuh Evan Chang, Amer Diwan, and Jeremy G. Siek - Gradual Programming

“I need a map data structure”

“I need a map data structure”

“Looks like iterator order matters for your program”

“Looks like iterator order matters for your program”

“Yes, I need iteration in sorted order”

“Yes, I need iteration in sorted order”

“Let’s use a balanced binary tree (TreeMap)”

“Let’s use a balanced binary tree (TreeMap)”