induction variables
DESCRIPTION
Induction Variables. Region-Based Analysis Meet and Closure of Transfer Functions Affine Functions of Reference Variables. Jeffrey D. Ullman Stanford University. Regions. A set of nodes N and edges E is a region if: There is a header h in N that dominates all nodes in N. - PowerPoint PPT PresentationTRANSCRIPT
Induction VariablesRegion-Based AnalysisMeet and Closure of Transfer FunctionsAffine Functions of Reference VariablesJeffrey D. UllmanStanford University
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Regions A set of nodes N and edges E is a region if:
1. There is a header h in N that dominates all nodes in N.
2. If n ≠ h is in N, then all predecessors of n are also in N.
I.e., the header is the sole entry into the region.
3. E consists of all edges between nodes in N, possibly excluding nodes that enter h from somewhere in N.
Note: Every node is a region by itself.
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T1-T2 Reduction For reducible flow graphs, we can “reduce” the
graph by two region-creating transformations. T1: Remove a self-loop from a node. T2: combine two nodes n and m such that m’s only
predecessor is n, and m is not the entry. Say “n consumes m.” n
m
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Example: T1-T2 Reduction
1
35
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T2
T2
5
Example: T1-T2 Reduction
14
5
23 T1
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Example: T1-T2 Reduction
14
5
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T2
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Example: T1-T2 Reduction
1234
5
T2
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Example: T1-T2 Reduction
12345 T1
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Example: T1-T2 Reduction
12345
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Regions Constructed During T1-T2 Reduction As we reduce, each node represents a region of
the original flow graph, and each edge represents one or more edges of the original flow graph.
T2: Take the union of the two regions represented by the two nodes, plus all edges represented by the edge between them.
T1: Add (to the region represented by the node) those edges represented by the loop that was removed.
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Regions Constructed – (2) Easy inductive proof: every set of nodes and
edges constructed is a region. Key point: When you use T2, the header of the
resulting region is the header of the region that consumes the other.
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Example: T1-T2 Reduction
1
3
5
24
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Region-Based Analysis First, compute for each region, smallest-to-
largest, some transfer functions:1. For region R constructed by T2 from S consuming
T: fR,IN[S] = identity function; fR,IN[T] = meet over paths from the beginning of the header of R to the beginning of the header of T, going only through S.
2. For region R with header h constructed from S by T1: fR,IN[S] = meet over paths within R from h to h.
3. For each region R with exit block ( = has an out-edge not in R) B, fR,OUT[B] = meet over paths within R from header of R to end of B.
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Example: Transfer Functions
1
3
5
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U
S
fU,IN[S] = f1
fS,OUT[2] = f2(f3f2)*
fU,OUT[2] = f1f2(f3f2)*f U,OUT[4] = f1f4
Like regular expressions.= f2 ∧ f2f3f2 ∧ f2f3f2f3f2 ∧ .. . .
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Additional Assumptions We can compute the meet of transfer functions:
[f ∧ g](x) = f(x) ∧ g(x). Needed when we combine transfer functions from
header to several exits. We can compute the closure f* for each transfer
function f, given by f* = f0 ∧ f1 ∧ f2 ∧ … Note f0 = identity, f1 = f, f2 = f composed with f, etc.. Needed when we apply T1 to consume a back edge.
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Example: RD’s Meet of transfer functions f(x) = (x - K) ∪ G and
g(x) = (x - K’) ∪ G’ corresponds to the paths of f and g in parallel.
Thus, [f∧g](x) = (x – (K ∩ K’)) ∪ (G ∪ G’). f*(x) = x ∪ f(x) ∪ f(f(x)) ∪ … = x ∪ (x – K) ∪ G ∪
((x - K ∪ G) – K) ∪ G … = x ∪ G. f(x)
f(f(x))
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Example: Meet
1
3
5
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UV
fU,OUT[2] = f1f2(f3f2)*fU,OUT[4] = f1f4
fV,IN[5] = f1f4 ∧ f1f2(f3f2)*Region U consumesnode 5 to makeregion V.
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Region-Based Analysis: Basis If R is a region consisting of a single node
(block), and f is the transfer function for that block, then fR,OUT[R] = f. Note: we use the same name for the block and the
region consisting of only that block.
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Region-Based Analysis: Induction Apply T2: region R consumes region S to make T.
1. fT,IN(R) = the identity function.2. fT,IN(S) = meet of fR,OUT[B] for all predecessors B of the
header of S.3. For blocks B in R, fT,OUT[B] = fR,OUT[B].4. For blocks B in S, fT,OUT[B] = fT,IN(S) fS,OUT[B].
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Induction – (2) Apply T1: add back edges to region S to make R.
1. Compute g = meet of fS,OUT[B] for all predecessors B of the header of S.
2. fR,IN[S] = g*. Notice that the headers of R and S are the same node, but
there are paths within R that are not in S, yet take you to the header of S.
3. For all blocks B in S (and therefore in R) let fR,OUT[B] = fR,IN[S]fS,OUT[B].
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Now We Work Large-to-Small Let e be the header of the region that is the
entire flow graph. Start with IN(e) = vENTRY. Proceed inward, from larger regions to smaller.
1. If R was constructed from S by T1, and both have header h, replace IN(h) by fR,IN(S)(IN(h)).
2. If T, with header h’ was constructed from R and S, with R consuming S, and S has header h, initialize IN(h) to fT,IN(S)(IN(h’).
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Examples: Top-Down Finale
1
3
5
24R
IN[1] = fR,IN[V](vENTRY) = f*V,OUT[5] ](vENTRY)
UV
IN[5] = fV,IN[5](IN(1))
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New Topic: Symbolic Analysis Values V are mappings from variables to
expressions involving reference variables. Example: m(a) = 2i; m(b) = i + j + 1.
i and j are the reference variables. We are going to deal with two complicated
types of functions:1. Mappings from variables to expressions, as above.2. Transfer functions from mappings (in) to mappings
(out).
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Application to Induction Variables
Reference variables are basic induction variables = counts of the number of times around some loop.
This count is well-defined for “natural loops.” Set of values V for the framework = affine
mappings = each variable is mapped to either:1. A linear function of the reference variables, or2. NAA = “Not an affine” mapping, or3. UNDEF = top element = “nothing known.”
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Example: Affine Mappingm(a) = 2i + 3j + 4m(b) = NAAm(c) = 4i + 1m(d) = UNDEF
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Lattice for Each Variable
UNDEF
All affine functions of reference variables
NAA
LessIgnorance
Note: The lattice for V is the productof one of these for each variable.
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Induction Variable Discovery Natural application of region-based analysis. Each loop is a region, and its induction variables
can be discovered by a framework based on affine mappings.
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Example: A Transfer Function Let f be the transfer function associated with a
block containing only a = a+10. Let m be the input mapping for the block; then
f(m) = m’, where:
m’(a) = m(a) + 10. m’(x) = m(x) for all x ≠ a.
Book uses the notation f(m)(a) = m(a) + 10, etc., to avoid having to name f(m).
a = a + 10
m
m'
Question: What if m(a)is NAA or UNDEF?
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The Meet Operator (f ∧ g)(m)(x) =
f(m)(x) if f(m)(x) = g(m)(x). f(m)(x) if g(m)(x) = UNDEF. g(m)(x) if f(m)(x) = UNDEF. NAA otherwise.
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Example: Some Analysis
a=b+3
b=a+2
a=a+5
B1
B5
B2
B4
B3 fB3(m)(a) = m(a)(b) = m(a)+2
fB2(m)(a) = m(a)+5(b) = m(b)
fB4(m)(a) = m(b)+3(b) = m(b)
R
fR,OUT[B4](m)(a) = m(a)+5(b) = m(a)+2
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Example: Meet
a=b+3
b=a+2
a=a+5
B1
B5
B2
B4
B3
fB2(m)(a) = m(a)+5(b) = m(b)
R
fR,OUT[B4](m)(a) = m(a)+5(b) = m(a)+2
S
fS,OUT[B5](m)(a) = m(a)+5(b) = NAA
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Handling Loop Regions Treat the iteration count i as a basic induction
variable. If f(m)(x) = m(x)+c, then fi(m)(x) = m(x)+ci. Some other cases, e.g., where m(x) is an affine
function of basic induction variables, in book.
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Example: The Entire Loop
a=b+3
b=a+2
a=a+5
B1
B5
B2
B4
B3
S
fS,OUT[B5](m)(a) = m(a)+5(b) = NAA
TfS,OUT[B5](m)(a) = m(a)
+5i(b) = NAA
i
OUTi[B3](m)(a) = m(a)+5i(b) = m(a)+5i+2
OUTi[B4](m)(a) = m(a)+5i+5(b) = m(a)+5i+2
OUTi[B2](m)(a) = m(a)+5i+5(b) = NAA
OUTi[B5](m)(a) = m(a)+5i+5(b) = NAA
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Taking Advantage of Affine Expressions
Replace the loop counter variable by one of the induction variables (variables that are mapped to an affine expression of the loop count at the point were the loop count is tested).
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Example: Assume i is Loop Counter
i<10?
a=b+3
b=a+2
a=a+5
i=i+1
B1
B5
B2
B4
B3
i=0 a=30 Here, the value of a is
m(a)+5i, where m(a) is thevalue of a before enteringthe loop, and i is the numberof iterations of the loop so far.
Since a=30 when i=0,replace “i<10?” by“a<80?”.
If i is dead on exit fromthe loop, delete “i=i+1”and “i=0”.
And since i is an inductionvariable, if not dead we canjust assign i = 10 at the exit.
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Example: Revised Code
a<80?
a=b+3
b=a+2
a=a+5
B1
B5
B2
B4
B3
a=30