Decision Structures: The Syntax Tree
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Decision Structures: The Syntax Tree
Lecture 23
Wed, Apr 14, 2004
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Reading
Read Section 8.4, Subsection Flow-of-Control Statements in Compilers: Principles, Techniques, and Tools.
Read Section 8.6, Backpatching.
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If Statements
We will consider two forms of the if statement.
stmt IF (cexpr) stmt;and
stmt IF (cexpr) stmt ELSE stmt; cexpr is a conditional expression. Recall that we have the productions
stmt {stmts}cexpr expr
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If Statements
For the time being, we will assume that cexpr is a numerical expression. Zero is interpreted as false. Any nonzero value is interpreted as true.
Thus, we do not need to be concerned with relational operators (==, !=, <, >, <=, >=) or boolean operators (&&, ||, !), yet.
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Flow of Control
Consider the flow of control for the production
stmt IF (cexpr) stmt1; If cexpr is non-zero, then execution must
jump to stmt1. If cexpr is zero, then execution must jump to
whatever follows stmt1. Problem: We do not yet know where that it.
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Flow of Control
We need an object associated with cexpr that will contain two destinations. The “true” destination. The “false” destination.
Such an object is called a backpatch node.
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The BackpatchNode Class
public class BackpatchNode{ LinkedList trueList; LinkedList falseList;}
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Backpatch Nodes
Each backpatch node contains A “true” reference to a linked list of labels. A “false” reference to a linked list of labels.
The nonterminal cexpr will represent a backpatch node.
From cexpr we set up a “true” destination label and a “false” destination label.
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Backpatching
The “true” destination and “false” destination labels are labels to be resolved once we know where the destinations are.
An unresolved label is called a backpatch label.
When we figure out the destination, we will “backpatch” the backpatch label to an actual label.
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Backpatching
An assembly language equate statement will accomplish this.
B6=L8
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Backpatching
The reason we keep a list of such labels is that there may be several branches in the program that all have the same destination.
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Example
Consider the following example.
if (a) b = 5;else b = 10;c = 2;
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Example
Consider the following example.
if (a) b = 5;else b = 10;c = 2;
Branch on true
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Example
Consider the following example.
if (a) b = 5;else b = 10;c = 2;
Branch on false
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Example
Consider the following example.
if (a) b = 5;else b = 10;c = 2;
UnconditionalBranch
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Example
Consider the following example.
if (a) b = 5;else b = 10;c = 2;
UnconditionalBranch
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Example
Notice that the unconditional branch from the true part and the unconditional branch from the false part have the same destination.
Thus, we build a list of two labels that will be resolved to the same destination.
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Example
Consider the example again.
if (a) b = 5;else b = 10;c = 2;
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Example
Consider the example again.
if (a) b = 5;else b = 10;c = 2;
Jump on trueto Label B1
T
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Example
Consider the example again.
if (a) b = 5;else b = 10;c = 2;
Jump on falseto Label B2FJump on true
to Label B1T
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Example
Consider the example again.
if (a) b = 5;else b = 10;c = 2;
Label L3 L3:
Jump on trueto Label B1
TJump on falseto Label B2F
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Example
Consider the example again.
if (a) b = 5;else b = 10;c = 2;
L3:
B1 = L3
Jump on trueto Label B1
TJump on falseto Label B2F
Label L3
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Example
Consider the example again.
if (a) b = 5;else b = 10;c = 2;
L3:
Jump on falseto Label B2F
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Example
Consider the example again.
if (a) b = 5;else b = 10;c = 2;
L3:Jump to Label B4
Jump on falseto Label B2F
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Example
Consider the example again.
if (a) b = 5;else b = 10;c = 2;
Label L5 L5:
Jump to Label B4
Jump on falseto Label B2F
L3:
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Example
Consider the example again.
if (a) b = 5;else b = 10;c = 2;
Label L5 L5:
Jump to Label B4
Jump on falseto Label B2F
L3:
B2 = L5
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Example
Consider the example again.
if (a) b = 5;else b = 10;c = 2;
L5:
Jump to Label B4
L3:
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Example
Consider the example again.
if (a) b = 5;else b = 10;c = 2;
L5:
Jump to Label B4
L3:
Jump to Label B6
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Example
Consider the example again.
if (a) b = 5;else b = 10;c = 2;
L5:
Jump to Label B4
L3:
Jump to Label B6
Merge B4 and B6
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Example
Consider the example again.
if (a) b = 5;else b = 10;c = 2;
L5:
Jump to Label B4
L3:
Label L7 L7:
Jump to Label B6
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Example
Consider the example again.
if (a) b = 5;else b = 10;c = 2;
L5:
Jump to Label B4
L3:
L7:
Jump to Label B6
Label L7
B4 = L7, B6 = L7
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Example
Consider the example again.
if (a) b = 5;else b = 10;c = 2;
L5:
L3:
L7:
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Example
Consider the example again.
if (a) b = 5;else b = 10;c = 2;
L5:
L3:
L7:
T F
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Example
Within the if statement, there are two “known” destinations and one unknown destination.
We can see that labels B4 and B6 will be resolved to the same destination eventually, but within the if statement that destination is unknown.
Therefore, in the meantime, we merge them together in a list of backpatch nodes to be resolved later to the same destination.
Once we learn the destination, all labels in the list are resolved to that location.
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Backpatch-Related Functions
We will create three functions to facilitate backpatching. makeList(labl)
Create a LinkedList containing the single Integer labl. Return a reference to the list.
merge(list1, list2) Merge the elements of list2 and list1. Return a reference to the merged list.
backpatch(list, labl) Insert labl as the target label for each backpatch label
in list.
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Labels and Jumps in the Grammar
We saw in the example that actual labels are needed within the if statement to serve as destinations.
We will incorporate the generation of these labels into the grammar.
The nonterminal m will create a label node which will serve as a known destination.
The nonterminal n will create a jump to an as-yet unknown destination.
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Labels and Jumps in the Grammar
m represents a destination. n represents a jump. The productions that involve these
nonterminals are stmts stmts m stmt
stmt IF ( cexpr ) m stmt
stmt IF ( cexpr ) m stmt n ELSE m stmt
func fbeg stmts RBRACE m
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Label Trees
LABEL6
A Label Tree
LABEL label=6
A Printed Label Tree
L6:
Assembly Code for a Label
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Jump Trees
JUMP
A Jump Tree
JUMP INT BLABEL blabel=6
jmp B6
Assembly Codefor a Jump
BLABEL6
A Printed JumpTree
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Equate Trees
EQU
An Equate Tree
EQU BLABEL blabel=6 LABEL label=8
B6=L8
Assembly Codefor an Equate
LABEL8
A Printed EquateTree
BLABEL6