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* gencode() will generate Assembly code w.r.t. * nameofoperation() will return opcode w.r.t. * function push() will increment top by one and will insert element at top position of Register stack */ * function pop() will remove and return topmost element of stack */ * function swap() will swap the top and second top elements of Register stack R */ Solution- Directed Acyclic Graph for the given basic block is- In this code fragment, 4 x I is a common sub-expression. After eliminating the common sub-expressions, re-write the basic block. Printf("%c with Label %d\n",tree->data,tree->label) Draw a directed acyclic graph and identify local common sub-expressions. Here we are also printing label of each node of tree(DAG) */ * function print_inorder() will print inorder of nodes. DAG can be understood here: Leaf nodes represent identifiers, names or constants. DAG provides easy transformation on basic blocks. If(tree->left->label > tree->right->label) Directed Acyclic Graph (DAG) is a tool that depicts the structure of basic blocks, helps to see the flow of values flowing among the basic blocks, and offers optimization too. If(tree->left->label = tree->right->label) * findinteriornodelabel() will find out the label of interior nodes of tree(DAG) */ If(tree->left != NULL & tree->right !=NULL) Void findleafnodelabel(node *tree,int val) * findleafnodelabel() will find out the label of leaf nodes of tree(DAG) */ Printf("\nEnter Right Child of %c:",val) Printf("\nEnter number of children of %c:",val) * insertnode() and insert() functions are for adding nodes to tree(DAG) */ * op will be used for opcode name w.r.t. * We will implement DAG as Strictly Binary Tree where each node has zero or two children */ Continue : A DAG may be used to represent common subexpressions in an optimizing compiler. The most important aspect of constructing a causal DAG is to include on the DAG any common cause of any other 2 variables on the DAG. A rooted tree is a special kind of DAG and a DAG is a special kind of direct. Print "op Node's right child's data,R"Ĭase 2: else if Node's left child's label >= 1 & Node's right child's label = 0Ĭase 3: else if Node's left child's label = Node's right child's labelĮlse if Node is leaf node and it is left child of it's immediate parent Construction of DAGs should not be limited to measured variables from available data they must be constructed independent of available data. If Node is intermediate node of tree(DAG)Ĭase 1: if Node's left child's label = 1 & Node's rightĬhild's label = 0 & Node's left child is leaf node & (Say, R is a Stack consists of registers.) As indices start from 0, index of topmost element(i.e. Number of registers in stack=Label of Root of DAG.
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Node's Label = (Left Child's Label OR Right Child's Label) + 1.įollowing Algorithm for Generating Assembly Code requires a Stack of registers. Node's Label = Maximum among Children's Labels.Ĭase 2: If Node's children's labels are same, then These algorithms takeĪt the end of this post, I have provided two examples.Īlgorithm for labeling the nodes of tree(DAG):Īssign label 1 to left child node and label 0 to right child node.Ĭase 1: If Node's children's labels are different, then We will see algorithms for Labeling the nodes of tree(DAG) and Code Generation using DAG /Labeled treeįollowed by their implementation in C language. (For implementation in C++, check Next Post).Ĭode Generation is the last phase among the six phases of compilation. If n is an interior node, it will be an operator node labeled by op with the children n 1 and n 2, and n 2 is a simple operand and not a root of the subtree, as shown in Figure 5.įigure 5: The node n is an operand and not a subtree root.In this post, we will see C language implementation of Code Generation using DAG / Labeled tree. If n is a leaf node and is the left-most child of its parent, then gencode() generates a load instruction for loading the top register of RSTACK by the label of node n: Depending upon the type of node n with which gencode() is called, gencode() performs the following: It also makes use of TSTACK to allocate temporary memory locations. The resulting code computes the value of the tree in the top register of RSTACK. A call to gencode() may find a subset of registers, perhaps in a different order in RSTACK, but when gencode() returns, it leaves the registers in RSTACK in the same order in which they were found.
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We assume the order of the registers to be R0, R1, …, from top to bottom. Initially, RSTACK contains all available registers. This procedure makes use of RSTACK to allocate registers. The algorithm uses a recursive procedure, gencode( n), to generate the code for evaluating into a register a subtree that has its root in node n. We assume that only binary operators are used in the tree. The content of R0 can then be stored in the appropriate memory location. We will now examine an algorithm that traverses the labeled tree and generates machine code to evaluate the tree in the register R0. 3 Code Generation by Traversing the Labeled Tree