CS 403 Compiler Construction Lecture 8 Syntax Tree and Intermediate Code Generation [Based on Chapter 6 of Aho2] This Lecture

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1 CS 403 Compiler Construction Lecture 8 Snta Tree and Intermediate Code Generation [Based on Chapter 6 of Aho2] 1 This Lecture 2 1

2 Remember: Phases of a Compiler This lecture: Intermediate Code This lecture: Snta Tree F means floating point 3 What is a Snta Tree? = Parse Tree Snta tree for Snta tree for 952 Snta trees are similar to parse trees. In parse tree, internals nodes are nonterminals. In snta tree, internal nodes are programming constructs (that means, operations) 4 2

3 Parse Tree Vs Snta Tree Parse Trees: The are generated b parser/leical analser to check the grammar for snta error Helpful for grammar checking. If no parse tree possible, then snta error If more than one parse tree possible, then the grammar is ambiguous and wrong But, parse trees have too man internal node, too man information. Snta trees: The are generated b snta analser and are used for both snta and smantec errors (such as tpe mismatch, wrong number of parameters, etc) Snta trees are intermediate representation before the code generation. The are used in intermediate code generation. Snta trees represent the hierarchical sntactic structure of the source program. Snta trees are simple, short. 5 Parse Tree Vs Snta Tree Parse Tree for nodes! Big! Smbol E, id are not in the original epression. The are etra. Snta Tree for 952 Onl 5 nodes! Small! No etra smbols 6 3

4 Snta Tree For Epression In the snta tree for epression, internal nodes are operators, and leafs are ids or terminals. General structures for an internal node in a snta tree for epression E 1 op E 2 op: operator (, /, ) E 1, E 2 : epressions. Can have their own snta trees T 1 and T 2 7 Snta Tree For Epression Snta tree for (ab)c(de) 8 4

5 Snta Tree For Epression Snta tree for a = bcbc This is same as a = b(c) b(c) Unar minus (onl one operator) 9 Snta Tree Helps in Error Checking When compiler creates a snta tree for an epression, it can check some error. Eample 1: Check lvalue error: lvalue means left value In an epression, left side cannot be a constant, because a constant does have lvalue, so error. For eample, 5 = a b is wrong. When the compiler creates the snta tree for this epression, it checks that the left child is a constant, so, it should be an error of lvalue. 5 = a b 10 5

6 Snta Tree Helps in Error Checking When compiler creates a snta tree for an epression, it can check some error. Eample 2: Check tpe mismatch error: For some operation, left and right node should have certain tpes. For eample, following code has tpe mismatch error in the second line, because left side is an arra and right side is integer. int a[10], b; a = 5 b; a = 5 b 11 DAG: Another Tpe of Snta Tree Snta tree b DAG (Directed Acclic Graph): Similar to snta tree: Internal nodes are operators or program constructs, leafs are ids or terminals. The difference is that a node in DAG can have more than one parent when it is repeated more than once in the statement or epression. For repeated epression, onl one node is created. So, it is small and takes small memor. Eample: 12 6

7 DAG Eample Eample: DAG for Eample: DAG for DAG Eample 14 7

8 DAG is short, snta tree is big. Eample: DAG vs Snta Tree DAG: Onl 9 nodes. Snta Tree: 19 nodes. 15 How Compiler Creates and Stores DAG in Memor DAG can be stored in an arra. One node in one arra position. Eample: in the Smbol Table Compiler creates DAG b storing the nodes in the arra. 16 8

9 Snta Tree For Statement Snta tree can be constructed for an statement, such as while, dowhile, if, ifelse, Eample: Snta tree for while statement. The general structure of a while statement has three parts as follows: the while construct, the argument and the statement The general structure of a snta tree for a while statement is: while epr stmt 17 Snta Tree For Statement Eample: Snta tree for the do while statement: The general structure of a dowhile statement has three parts as follows: the dowhile construct, the statement, and the epression do (stmt) while (epr) dowhile = stmt epr 18 9

10 Snta Directed Translation The overall procedure for creating snta tree is called sntadirected translation. We onl saw some part of it and some eamples. Detail is ver length procedure. 19 Intermediate Code Generation (Three Address Code) 20 10

11 What is Three Address Code? Assembllike instructions Each instruction has at most three operands and two operators At most one operator in the right side. Thus no chance of ambiguit. Temporar variables are generated to hold temporar values Three Address Codes are used for code optimization and for finding assembl codes (target code) 21 What is Three Address Code? Moreover: 22 11

12 Eamples of Three Address Code Eample: Threeaddress code for ijk Eample: Threeaddress code for 2a[i] Eample: Threeaddress code for a[i]=2a[jk] 23 What is Three Address Code? 24 12

13 What is Three Address Code? 25 Eamples of Three Address Code Eample: Threeaddress code for 26 13

14 Eamples of Three Address Code Compiler uses go to in three address codes. But, using go to statements is not a good programming technique in high level languages like Java/C/C/Pascal. Because, when ou use go to statements, ou cannot keep track of the program sequence. It will make difficult to trace the program, where to go, where did ou come from, etc. and ou will be lost! 27 Eamples of Three Address Code Program code Threeaddress code 28 14

15 Three Address Code from DAG Threeaddress code can be generated from DAG easil b this algorithm: Use one temporar variable for one interior node and compute from bottom to top. Eample: Below, we have 5 interior nodes, so we need 5 temporar variables: t1, t2, t3, t4, t5 t5 t3 t2 t4 t1 29 Eample: Three Address Code from DAG 2 t3 t1 2 t2 t1 = 2 t2 = t1 t3 = t1t2 Threeaddress code DAG 30 15

16 Eample: Three Address Code from DAG t5 t4 t3 t1 t2 t1 = t2 = t3 = t1t2 t4 = t1t3 t5 = t4t3 Threeaddress code DAG 31 Quadruples: How Compiler Implements ThreeAddress Codes and Stores in Memor Compiler can implement the threeaddress instructions as objects (in object oriented programing like Java, C) or as records (in procedural programming like Pascal, C) Each record is called quadruples, which is defined as follows: 32 16

17 Quadruples: Eample 33 Triples: Another wa to Implement and store ThreeAddress Codes in Memor Triples: Similar to quadruples, but have no result field. When result is needed from the previous instruction, the address is used. Similar to DAG representation