A compiler is a computer program (or set of programs) that transforms source code written in a computer language (the source language) into another computer language (the target language, often having a binary form known as object code). The most common reason for wanting to transform source code is to create an executable program.
The Basic Structure of a Compiler
The five stages of a compiler combine to translate a high level language to a low level language, generally closer to that of the target computer. Each stage, or sub-process, fulfills a single task and has one or more classic techniques for implementation.
Analyzes the Source Code
| Finite State Machines LEX | |
Amalgamates symbols into syntactic groups Tags groups with type information | Top-down analyzers Bottom-up analyzers Expression analyzers YACC | |
Fills in assumed or missing information Tags groups with meaning information | Ad hoc analyzers | |
Loop unrolling Operator reduction etc. |
** The Symbol Table is the data structure that all elements of the compiler use to collect and share information about symbols and groups of symbols in the program being translated.
*Alternative* Answer:-
Question - How does a language compiler work? For example, what is
the mechanism behind the compiling process of a program in a specific
language?
The question was about compilers, so I will explain how a compiler works,
rather than the process of converting a source program into an executable
program. The first question involves only a compiler, while in the second
process, a compiler is only one of the programs involved.
A compiler for a language generally has several different stages as it
processes the input.
These are:
1. Preprocessing
2. Lexical analysis
3. Syntactical analysis
4. Semantical analysis
5. Intermediate code generation
6. Code optimization
7. Code generation
Most of theses stages occur during a single pass or reading of the source
files. In other words, for example, the preprocessing stage is usually
reads only slightly ahead of the lexical analysis stage, which is usually
one world ahead of the syntactical analysis stage.
1. Preprocessing
During the preprocessing stage, comments, macros, and directives are
processed.
Comments are removed from the source file. This greatly simplifies the
later stages.
If the language supports macros, the macros are replaced with the equivalent
text.
For example, C and C++ support macros using the #define directive. So if a
macro were defined for pi as:
#define PI 3.1415927
Any time the preprocessor encountered the word PI, it would replace PI with
3.1415927 and process the resulting text.
The preprocessor also handles preprocessor directives. These are most often
include statements. In C and C++, an include statement looks like either:
#include
#include "file"
These lines are replaced by the actual file and the resulting text
processed.
The preprocessor may also replace special strings with other characters. In
C and C++, the preprocessor recognizes the \ character as an escape code,
and will replace the escape sequence with a special character. For example
\t is the escape code for a tab, so \t would be replaced at this stage with
a tab character.
2. Lexical analysis is the process of breaking down the source files into
key words, constants, identifiers, operators and other simple tokens. A
token is the smallest piece of text that the language defines.
A. Key words are words the language defines, and which always have specific
meaning in the language. In C and C++ some of these key words are:
if
else
int
char
do
while
for
struct
return
B. Constants are the literal valued items that the language can recognize.
Often these are numbers, strings, and characters:
i. Numbers are the types of numbers that may be used in expressions: 3.14,
5, 12, 0. But, usually negative numbers (-17) are processes as an operator
(-) and a number (17)
ii. Strings are text items the language can recognize. In C or C++ a string
is enclosed by double quotes: "This is a string"
iii. Characters are single letters. In C or C++, a character is enclosed by
single quotes: 'c'
C. Identifiers are names the programmer has given to something. These
include variables, functions, classes, enumerations, etc. Each language has
rules for specifying how these names can be written.
D. Operators are the mathematical, logical, and other operators that the
language can recognize. Each language generally has the standard operators
+, -, *, /, and often defines many other operators as well. For example
some of the additional C and C++ define are:
% modulo
-- decrement
++ increment
E. Other tokens are things not covered by any of the above items. Often
these will produce errors, but depending on the compiler, things like
{ ( ) } may be valid in the language, but not treated as a key word or
operator.
3. Syntactical analysis is the process of combining the tokens into
well-formed expressions, statements, and programs. Each language has
specific rules about the structure of a program--called the grammar or
syntax. Just like English grammar, it specifies how things may be put
together. In English, a simple sentence is: subject, verb, predicate.
In C or C++ an if statement is:
if ( expression ) statement
The syntactical analysis checks that the syntax is correct, but doesn't
enforce that it makes sense. In English, a subject could be: Pants, the
verb: are, the predicate: a kind of car. This would yield: Pants are a kind
of car. Which is a sentence, but doesn't make much sense.
In C or C++, a constant can be used in an expression: so the expression:
float x = "This is red"++
Is syntactically valid, but doesn't make sense because a float number can
not have string assigned to it, and a string can not be incremented.
4. Semantic analysis is the process of examining the types and values of the
statements used to make sure they make sense. During the semantic
analysis, the types, values, and other required information about statements
are recorded, checked, and transformed as appropriate to make sure the
program makes sense.
For C/C++ in the line:
float x = "This is red"++
The semantic analysis would reveal the types do not match and can not be
made to match, so the statement would be rejected and an error reported.
While in the statement:
float y = 5 + 3.0;
The semantical analysis would reveal that 5 is an integer, and 3.0 is a
double, and also that the rules for the language allow 5 to be converted to
a double, so the addition could be done, so the expression would then be
transformed to a double and the addition performed. Then, the compiler
would recognize y as a float, and perform another conversion from the double
8.0 to a float and process the assignment.
5. Intermediate code generation
Depending on the compiler, this step may be skipped, and instead the program
may be translated directly into the target language (usually machine object
code). If this step is implemented, the compiler designers also design a
machine independent language of there own that is close to machine language
and easily translated into machine language for any number of different
computers.
The purpose of this step is to allow the compiler writers to support
different target computers and different languages with a minimum of effort.
The part of the compiler which deals with processing the source files,
analyzing the language and generating the intermediate code is called the
front end, while the process of optimizing and converting the intermediate
code into the target language is called the back end.
6. Code optimization
During this process the code generated is analyzed and improved for
efficiency. The compiler analyzes the code to see if improvements can be
made to the intermediate code that couldn't be made earlier. For example,
some languages like Pascal do not allow pointers, while all machine
languages do. When accessing arrays, it is more efficient to use pointers,
so the code optimizer may detect this case and internally use pointers.
7. Code generation
Finally, after the intermediate code has been generated and optimized, the
compiler will generated code for the specific target language. Almost
always this is machine code for a particular target machine.
Also, it us usually not the final machine code, but is instead object code,
which contains all the instructions, but not all of the final memory
addresses have been determined.
A subsequent program, called a linker is used to combine several different
object code files into the final executable program.
ALTERNATIVE ANSWER
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