computer programming Computer programming or coding is the composition of sequences of instructions, called computer program, programs, that computers can follow to perform tasks. It involves designing and implementing algorithms, step-by-step specifications of proc ...

, a one-pass compiler is a

compiler In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primaril ...

that processes each compilation unit only once, sequentially translating each source statement or declaration into something close to its final machine code. This is in contrast to a

multi-pass compiler A multi-pass compiler is a type of compiler that processes the source code or abstract syntax tree of a program several times. This is in contrast to a one-pass compiler, which traverses the program only once. Each pass takes the result of the prev ...

which converts the program into one or more

intermediate representation An intermediate representation (IR) is the data structure or code used internally by a compiler or virtual machine to represent source code. An IR is designed to be conducive to further processing, such as optimization and translation. A "good" ...

s in steps between

source code In computing, source code, or simply code or source, is a plain text computer program written in a programming language. A programmer writes the human readable source code to control the behavior of a computer. Since a computer, at base, only ...

and machine code, and which reprocesses the entire compilation unit in each sequential pass. A one-pass compiler initially has to leave addresses for forward jumps unresolved. These can be handled in various ways (e.g. tables of forward jumps and targets) without needing to have another complete pass. Some nominally one-pass compilers effectively 'pass the buck' by generating assembly language and letting the assembler sort out the forward references, but this requires one or more passes in the assembler. One-pass compilers are smaller and faster than multi-pass compilers. One-pass compilers are unable to generate as efficient programs as multi-pass compilers due to the limited scope of available information. Many effective

compiler optimization An optimizing compiler is a compiler designed to generate code that is optimized in aspects such as minimizing program execution time, memory usage, storage size, and power consumption. Optimization is generally implemented as a sequence of op ...

s require multiple passes over a

basic block 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. This restricted form makes a basic block highly amenable to analysis. Compilers usually decom ...

, loop (especially nested loops), subroutine, or entire module. Some require passes over an entire program.

Difficulties

A core requirement for any

programming language A programming language is a system of notation for writing computer programs. Programming languages are described in terms of their Syntax (programming languages), syntax (form) and semantics (computer science), semantics (meaning), usually def ...

intended for one-pass compilation is that all user-defined identifiers, except possibly labels, must be declared before use: * Pascal contains a forward specification to allow routines to be mutually recursive. * PL/I allows data declarations to be placed anywhere within a program, specifically, ''after'' some references to the not-yet-declared items, so one pass is needed to resolve data declarations, followed by one or more passes to generate code. Any language which allows preprocessing such as PL/1, C, or C++, must have at least two passes, one pass for preprocessing and one or more passes for translation. Some computer hardware (e.g. x86) may have short and long branch instructions: short if the destination is within about 127 bytes, and long otherwise. A one-pass compiler must assume that all jumps are long, whereas a multi-pass compiler can check on jump distance and generate possibly shorter code. Languages which rely on context for statement identification rather than using reserved or stropped keywords may also cause problems. The following examples are from Fortran 77: IF (B) l1,l2 ! two-way branch, where B is a boolean/logical expression IF (N) l1,l2,l3 ! three-way branch, where N is a numeric expression IF (B) THEN ! start conditional block IF (B) THEN = 3.1 ! conditional assignment to variable THEN IF (B) X = 10 ! single conditional statement IF (B) GOTO l4 ! conditional jump IF (N) = 2 ! assignment to a subscripted variable named IF DO 12 I = 1,15 ! start of a count-controlled loop DO 12 I = 1.15 ! assignment of the value 1.15 to a variable called DO12I The entire statement must be scanned to determine what sort of statement it is; only then can translation take place. Hence any potentially ambiguous statements must be processed at least twice.

References

{{DEFAULTSORT:One-Pass Compiler Compilers Articles with example Pascal code

Difficulties

See also

References