Basic Blocks and Flow Graphs
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The document discusses basic blocks and flow graphs in program representation. It defines basic blocks as straight-line code segments with a single entry and exit point. To construct the representation: 1. The program is partitioned into basic blocks 2. A flow graph is created where basic blocks are nodes and edges show control flow between blocks The flow graph explicitly represents all execution paths between basic blocks. Loops in the flow graph are identified by having a single loop entry node with all paths from the start going through it, and all nodes inside the loop reaching the entry.
Basic Blocks and Flow Graphs
- 2. Construction of the Representation 1. Partition the intermediate code into basic blocks 2. The basic blocks become the nodes of a flow graph and the edges indicate the flow (which blocks follow which)
- 3. Basic Blocks *a piece of straight line code i.e. there are no jumps in or out of the middle of a block 1. the flow of control can only enter the basic block through the first instruction 2. control will leave the block without halting or branching, except possibly, at the last instruction in the block
- 4. Constructing Basic Blocks 1) Determine a set of leaders, the first instruction of blocks 2) A basic block consists of a leader and all the following instructions until the next leader
- 5. Constructing Basic Blocks Determine a set of leaders 1) The first instruction is a leader 2) Instruction L is a leader if there is an instruction a or a 3) Instruction L is a leader if it immediately follows an instruction a or a if … goto L goto L if … goto B goto B goto B
- 6. Flow Graph Once an intermediate-code program is partitioned into basic blocks, we represent the flow of control between them by a flow graph.
- 7. Constructing the Flow Graph There is an edge from block B to block C iff it is possible for the first instruction in block C to immediately follow the last instruction in block B 1. There is a conditional or unconditional jump from the end of B to the beginning of C
- 8. Constructing the Flow Graph There is an edge from block B to block C iff it is possible for the first instruction in block C to immediately follow the last instruction in block B 2. C immediately follows B in the original order of the three-address instructions, and B does not end in an unconditional jump
- 9. Constructing the Flow Graph Often we add two nodes, called the entry and exit. 1. There is an edge from the entry to the first executable node.
- 10. Constructing the Flow Graph Often we add two nodes, called the entry and exit. 2. There is an edge to the exit from any basic block that contains an instruction that could bet eh last executed instruction of the program
- 11. goto B
- 12. goto B
- 13. Loops We say that a set of nodes L in a flow graph is a loop if 1. There is a node in L called the loop entry with the property that no other node in L has a predecessor outside L.That is , every path from the entry of the entire flow graph to any node in L goes through the loop entry.
- 14. Loops We say that a set of nodes L in a flow graph is a loop if 2. Every node in L has a nonempty path, completely within L, to the entry of L.
- 15. Fin
Editor's Notes
- #2: This section introduces a graph presentation of intermediate code (although not constructed by a code-generation algo) which is is helpful for discussing code generation Better register allocation, if we know how values are defined and used
- #7: The nodes of the flow graph are the basic blocks
- #9: We say that B is a predecessor of C , and C is a successor of B .
- #11: If the final instruction of the program is not an unconditional jump, then the block containing the final instruction of the program is one predecessor of the exit, but so is any basic block that has a jump to code that is not part of the program.
- #13: Note from that in the flow graph, it is normal to replace the jumps to instruction numbers or labels by jumps to basic blocks
- #14: If the final instruction of the program is not an unconditional jump, then the block containing the final instruction of the program is one predecessor of the exit, but so is any basic block that has a jump to code that is not part of the program.
- #15: If the final instruction of the program is not an unconditional jump, then the block containing the final instruction of the program is one predecessor of the exit, but so is any basic block that has a jump to code that is not part of the program.