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In compiler theory, a reaching definition for a given instruction is an earlier instruction whose target variable can reach (be assigned to) the given one without an intervening assignment. For example, in the following code: d1 : y := 3 d2 : x := y d1 is a reaching definition for d2. In the following, example, however:
Within computer science, a use-definition chain (or UD chain) is a data structure that consists of a use U, of a variable, and all the definitions D of that variable that can reach that use without any other intervening definitions. [1] [2] A UD Chain generally means the assignment of some value to a variable.
In compiler design, static single assignment form (often abbreviated as SSA form or simply SSA) is a type of intermediate representation (IR) where each variable is assigned exactly once. SSA is used in most high-quality optimizing compilers for imperative languages, including LLVM , the GNU Compiler Collection , and many commercial compilers.
The examples above are problems in which the data-flow value is a set, e.g. the set of reaching definitions (Using a bit for a definition position in the program), or the set of live variables. These sets can be represented efficiently as bit vectors , in which each bit represents set membership of one particular element.
In compiler theory, common subexpression elimination (CSE) is a compiler optimization that searches for instances of identical expressions (i.e., they all evaluate to the same value), and analyzes whether it is worthwhile replacing them with a single variable holding the computed value. [1]
Constant folding is the process of recognizing and evaluating constant expressions at compile time rather than computing them at runtime. Terms in constant expressions are typically simple literals, such as the integer literal 2, but they may also be variables whose values are known at compile time.
In compiler construction, a basic block is a straight-line code sequence with no branches in except to the entry and no branches out except at the exit. [1] [2] This restricted form makes a basic block highly amenable to analysis. [3] Compilers usually decompose programs into their basic blocks as a first step in the analysis process.
In computer science, instruction selection is the stage of a compiler backend that transforms its middle-level intermediate representation (IR) into a low-level IR. In a typical compiler, instruction selection precedes both instruction scheduling and register allocation; hence its output IR has an infinite set of pseudo-registers (often known as temporaries) and may still be – and typically ...