11.2.5 Optimization and Code Generation
The syntax tree is a useful intermediate representation (IR) that is independent of both the source language and the target ISA. It contains information about the sequencing and grouping of operations that isn't apparent in individual machine language instructions. And it allows frontends for different source languages to share a common backend targeting a specific ISA. As we'll see, the backend processing can be split into two sub-phases. The first performs machine-independent optimizations on the IR. The optimized IR is then translated by the code generation phase into sequences of instructions for the target ISA. A common IR is to reorganize the syntax tree into what's called a control flow graph (CFG). Each node in the graph is a sequence of assignment and expression evaluations that ends with a branch. The nodes are called "basic blocks" and represent sequences of operations that are executed as a unit. Once the first operation in a basic block ...