very busy expressions - university of...

359CS 701 Fall 2008©

Very Busy ExpressionsThis is an interesting variant ofavailable expression analysis.An expression is very busy at a point ifit is guaranteed that the expressionwill be computed at some time in thefuture.Thus starting at the point in question,the expression must be reachedbefore its value changes.

Very busy expression analysis is abackward flow analysis, since itpropagates information about futureevaluations backward to “earlier”points in the computation.

360CS 701 Fall 2008©

The meet lattice is:

As initial values, at the end of all exitnodes, nothing is very busy. Hence,for a given expression,VeryBusyOut(blast) = F

T (Expression is Very Busy)

F (Expression is Not Very Busy)

361CS 701 Fall 2008©

The transfer function for e1 in block bis defined as:If e1 is computed in b before any of

its operandsThen VeryBusyIn(b) = TElsif any of e1’s operands are changed before e1 is computed Then VeryBusyIn(b) = FElse VeryBusyIn(b) = VeryBusyOut(b)

The meet operation (to combinesolutions) is:

VeryBusyOut(b) = ANDs ∈ Succ(b)

VeryBusyIn(s)

362CS 701 Fall 2008©

Example: e1=v+w

stop

v=2

w=5

v=3 x=v+w

u=v+w

F

F

F

F

T

T

363CS 701 Fall 2008©

stop

v=2

w=5

v=3 x=v+w

u=v+w

F

F

F

F

T

T

FF

FF

T

T

T

F

Move v+where?

Or here?

364CS 701 Fall 2008©

Identifying IdenticalExpressions

We can hash expressions, based onhash values assigned to operands andoperators. This makes recognizingpotentially redundant expressionsstraightforward.For example, if H(a) = 10, H(b) = 21and H(+) = 5, then (using a simpleproduct hash),H(a+b) = 10×21×5 Mod TableSize

365CS 701 Fall 2008©

Effects of Aliasing and CallsWhen looking for assignments tooperands, we must consider theeffects of pointers, formal parametersand calls.An assignment through a pointer(e.g, *p = val) kills all expressionsdependent on variables p might pointtoo. Similarly, an assignment to aformal parameter kills all expressionsdependent on variables the formalmight be bound to.A call kills all expressions dependenton a variable changeable during thecall.Lacking careful alias analysis,pointers, formal parameters and callscan kill all (or most) expressions.

366CS 701 Fall 2008©

Very Busy Expressions andLoop Invariants

Very busy expressions are idealcandidates for invariant loop motion.If an expression, invariant in a loop, isalso very busy, we know it must beused in the future, and henceevaluation outside the loop must beworthwhile.

367CS 701 Fall 2008©

for (...) {

if (...)

a=b+c;

else a=d+c;}

for (...) {

if (a>b+c)

x=1;

else x=0;}

t=b+c t=b+c

a=b+c a=d+c

a>b+c

T F

F

F

F T

b+c is not very busyat loop entrance

b+c is very busyat loop entrance

368CS 701 Fall 2008©

Reaching DefinitionsWe have seen reaching definitionanalysis formulated as a set-valuedproblem. It can also be formulated ona per-definition basis.That is, we ask “What blocks does aparticular definition to v reach?”This is a boolean-valued, forwardflow data flow problem.

369CS 701 Fall 2008©

Initially, DefIn(b0) = false.

For basic block b:DefOut(b) = If the definition being analyzed is the last definition to v in b Then TrueElsif any other definition to v occurs

in b Then False Else DefIn(b)The meet operation (to combinesolutions) is:

DefIn(b) =

To get all reaching definition, we do aseries of single definition analyses.

ORp ∈ Pred(b)

DefOut(p)

370CS 701 Fall 2008©

Live Variable AnalysisThis is a boolean-valued, backwardflow data flow problem.Initially, LiveOut(blast) = false.

For basic block b:LiveIn(b) = If the variable is used before it is defined in b Then True Elsif it is defined before it is used in b Then False Else LiveOut(b)The meet operation (to combinesolutions) is:

LiveOut(b) = ORs ∈ Succ(b)

LiveIn(s)

371CS 701 Fall 2008©

Bit Vectoring Data FlowProblems

The four data flow problems we havejust reviewed all fit within a singleframework.Their solution values are Booleans(bits).The meet operation is And or OR.The transfer function is of the generalform Out(b) = (In(b) - Killb) U Genb

or In(b) = (Out(b) - Killb) U Genb

where Killb is true if a value is “killed”within b and Genb is true if a value is“generated” within b.

372CS 701 Fall 2008©

In Boolean terms:Out(b) = (In(b) AND Not Killb) OR Genb

orIn(b) = (Out(b) AND Not Killb) OR Genb

An advantage of a bit vectoring dataflow problem is that we can do a seriesof data flow problems “in parallel” usinga bit vector.

Hence using ordinary word-level ANDs,ORs, and NOTs, we can solve 32 (or 64)problems simultaneously.

373CS 701 Fall 2008©

Example Do live variable analysis for u and v,using a 2 bit vector:

We expect no variable to be live atthe start of b0. (Why?)

v=1

u=0

a=u v=2

print(u,v)

Gen=0,0Kill=0,1

Gen=0,0

Gen=1,0 Gen=0,0

Gen=1,1

Kill=1,0

Kill=0,0 Kill=0,1

Kill=0,0

Live=0,0

Live=0,1

Live=1,1 Live=1,0

Live=1,1

374CS 701 Fall 2008©

Reading Assignment• Read pages 31-62 of “Automatic

Program Optimization,” by Ron Cytron.(Linked from the class Web page.)

375CS 701 Fall 2008©

Depth-First Spanning TreesSometimes we want to “cover” thenodes of a control flow graph with anacyclic structure.This allows us to visit nodes once,without worrying about cycles orinfinite loops.Also, a careful visitation order canapproximate forward control flow(very useful in solving forward dataflow problems).A Depth-First Spanning Tree (DFST) isa tree structure that covers the nodesof a control flow graph, with the startnode serving as root of the DFST.

376CS 701 Fall 2008©

Building a DFSTWe will visit CFG nodes in depth-firstorder, keeping arcs if the visited nodehasn’t be reached before.To create a DFST, T, from a CFG, G:

1. T ← empty tree2. Mark all nodes in G as “unvisited.”3. Call DF(start node)

DF (node) {1. Mark node as visited.2. For each successor, s, of node in G:

If s is unvisited (a) Add node → s to T (b) Call DF(s)

377CS 701 Fall 2008©

ExampleA

B

C

D

E F

G

H

I J

Visit order is A, B, C, D, E, G, H, I, J, F

378CS 701 Fall 2008©

The DFST is

A

B

C

D

E F

G

H

I J

379CS 701 Fall 2008©

Categorizing Arcs using aDFST

Arcs in a CFG can be categorized byexamining the corresponding DFST.An arc A→B in a CFG is(a) An Advancing Edge if B is a proper descendent of A in the DFST.(b) A Retreating Edge if B is an ancestor of A in the DFST. (This includes the A→A case.)(c) A Cross Edge if B is neither a descendent nor an ancestor of A in the DFST.

380CS 701 Fall 2008©

ExampleA

B

C

D

E F

G

H

I J

aa

a

a

a a

a

a

a a

r

r

r

r

c

381CS 701 Fall 2008©

Depth-First OrderOnce we have a DFST, we can labelnodes with a Depth-First Ordering(DFO).Let i = the number of nodes in a CFG(= the number of nodes in its DFST).DFO(node) { For (each successor s of node) do DFO(s); Mark node with i; i--;}

383CS 701 Fall 2008©

Application of Depth-FirstOrdering• Retreating edges (a necessary component

of loops) are easy to identify: a→b is a retreating edge if and only if dfo(b) ≤ dfo(a)

• A depth-first ordering in an excellentvisit order for solving forward data flowproblems. We want to visit nodes inessentially topological order, so that allpredecessors of a node are visited (andevaluated) before the node itself is.

384CS 701 Fall 2008©

DominatorsA CFG node M dominates N(M dom N) if and only if all pathsfrom the start node to N must passthrough M.A node trivially dominates itself.Thus (N dom N) is always true.

A CFG node M strictly dominates N(M sdom N) if and only if(M dom N) and M ≠ N.A node can’t strictly dominates itself.Thus (N sdom N) is never true.

386CS 701 Fall 2008©

Immediate DominatorsIf a CFG node has more than onedominator (which is common), thereis always a unique “closest”dominator called its immediatedominator.(M idom N) if and only if

(M sdom N) and(P sdom N) ⇒ (P dom M)

To see that an immediate dominatoralways exists (except for the startnode) and is unique, assume thatnode N is strictly dominated by M1,M2, ..., Mp, P ≥ 2.

By definition, M1, ..., Mp must appearon all paths to N, including acyclicpaths.

387CS 701 Fall 2008©

Look at the relative ordering amongM1 to Mp on some arbitrary acyclicpath from the start node to N.Assume that Mi is “last” on that path(and hence “nearest” to N).

If, on some other acyclic path,Mj ≠ Mi is last, then we can shortenthis second path by going directlyfrom Mi to N without touching anymore of the M1 to Mp nodes.

But, this totally removes Mj from thepath, contradicting the assumptionthat (Mj sdom N).

388CS 701 Fall 2008©

Dominator TreesUsing immediate dominators, we cancreate a dominator tree in which A→Bin the dominator tree if and only if(A idom B).

A

B C

D

E

F

Start

End

A

B C D

E

F

Start

End

Control Flow Graph

Dominator Tree

389CS 701 Fall 2008©

Note that the Dominator Tree of aCFG and its DFST are distinct trees(though they have the same nodes).

A

B C

D

E

F

Start

End

A

B C D

E

F

Start

End

Dominator Tree

Depth-First Spanning Tree

390CS 701 Fall 2008©

A Dominator Tree is a compact andconvenient representation of both thedom and idom relations.A node in a Dominator Treedominates all its descendents in thetree, and immediately dominates allits children.

very busy expressions - university of...

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