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Code generation (compiler) | Wikipedia audio article


In computing, code generation is the process
by which a compiler’s code generator converts some intermediate representation of source
code into a form (e.g., machine code) that can be readily executed by a machine. Sophisticated compilers typically perform
multiple passes over various intermediate forms. This multi-stage process is used because many
algorithms for code optimization are easier to apply one at a time, or because the input
to one optimization relies on the completed processing performed by another optimization. This organization also facilitates the creation
of a single compiler that can target multiple architectures, as only the last of the code
generation stages (the backend) needs to change from target to target. (For more information on compiler design,
see Compiler.) The input to the code generator typically
consists of a parse tree or an abstract syntax tree. The tree is converted into a linear sequence
of instructions, usually in an intermediate language such as three-address code. Further stages of compilation may or may not
be referred to as “code generation”, depending on whether they involve a significant change
in the representation of the program. (For example, a peephole optimization pass
would not likely be called “code generation”, although a code generator might incorporate
a peephole optimization pass.)==Major tasks in code generation==
In addition to the basic conversion from an intermediate representation into a linear
sequence of machine instructions, a typical code generator tries to optimize the generated
code in some way. Tasks which are typically part of a sophisticated
compiler’s “code generation” phase include: Instruction selection: which instructions
to use. Instruction scheduling: in which order to
put those instructions. Scheduling is a speed optimization that can
have a critical effect on pipelined machines. Register allocation: the allocation of variables
to processor registers Debug data generation if required so the code
can be debugged.Instruction selection is typically carried out by doing a recursive postorder
traversal on the abstract syntax tree, matching particular tree configurations against templates;
for example, the tree W :=ADD(X,MUL(Y,Z)) might be transformed into a linear sequence
of instructions by recursively generating the sequences for t1 :=X and t2 :=MUL(Y,Z),
and then emitting the instruction ADD W, t1, t2. In a compiler that uses an intermediate language,
there may be two instruction selection stages — one to convert the parse tree into intermediate
code, and a second phase much later to convert the intermediate code into instructions from
the instruction set of the target machine. This second phase does not require a tree
traversal; it can be done linearly, and typically involves a simple replacement of intermediate-language
operations with their corresponding opcodes. However, if the compiler is actually a language
translator (for example, one that converts Eiffel to C), then the second code-generation
phase may involve building a tree from the linear intermediate code.==Runtime code generation==
When code generation occurs at runtime, as in just-in-time compilation (JIT), it is important
that the entire process be efficient with respect to space and time. For example, when regular expressions are
interpreted and used to generate code at runtime, a non-deterministic finite state machine is
often generated instead of a deterministic one, because usually the former can be created
more quickly and occupies less memory space than the latter. Despite its generally generating less efficient
code, JIT code generation can take advantage of profiling information that is available
only at runtime.==Related concepts==
The fundamental task of taking input in one language and producing output in a non-trivially
different language can be understood in terms of the core transformational operations of
formal language theory. Consequently, some techniques that were originally
developed for use in compilers have come to be employed in other ways as well. For example, YACC (Yet Another Compiler Compiler)
takes input in Backus-Naur form and converts it to a parser in C. Though it was originally
created for automatic generation of a parser for a compiler, yacc is also often used to
automate writing code that needs to be modified each time specifications are changed.Many
integrated development environments (IDEs) support some form of automatic source code
generation, often using algorithms in common with compiler code generators, although commonly
less complicated. (See also: Program transformation, Data transformation.)===
Reflection===In general, a syntax and semantic analyzer
tries to retrieve the structure of the program from the source code, while a code generator
uses this structural information (e.g., data types) to produce code. In other words, the former adds information
while the latter loses some of the information. One consequence of this information loss is
that reflection becomes difficult or even impossible. To counter this problem, code generators often
embed syntactic and semantic information in addition to the code necessary for execution.==See also==
Automatic programming Comparison of code generation tools
Source to source compilation: automatic translation of a computer program from one programming
language to another

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