Pass
- In an implementation of a compiler, portion of one or more phases are combined into a module called pass
- It is a group of phases implemented together
- A pass reads the source program of the output of the previous pass, makes the transformations specified by its phases, and writes output into an intermediate file, which is read by a subsequent pass
- In pass, the operations of phases may be interleaved
- Generally, the lexical analysis, scanner, semantic analysis and intermediate code generation are grouped into one pass whereas the remaining phases into another pass
- Number of passes to keep is generally dependent upon the structure of the source language and the environment in which the compiler must operate
- It is better to have less number of passes as it takes time to read and write intermediate files
- However it is not always possible to have a single pass instead of several passes, as sometimes there may be a need to keep the entire program in memory because one phase may need information in a different order than a previous phase produces
Single pass: All phases are combined into single
Multi pass: More than one phase are combined into a number of groups called multi pass
Difference between Multi pass and Single pass
Multi pass
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Single pass
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Uses less space than single pass compilers,
since the space occupied by the compiler for one pass can be reused by
another pass
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Uses more space than multi pass compilers
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Slower than single pass compiler, due to reading
and writing of intermediate files
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Faster than multi pass as there is no reading
and writing of intermediate files
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Works fine with small memory
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Works fine with large memory
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Difference between a Phase and Pass of a Compiler:
Phase
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Pass
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In compilers, the process of compilation can be
carried out with the help of various phases such as lexical analysis, syntax
analysis, intermediate code generation, code optimization and code generation
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In case of passes, various phases are combined
together to form a single pass. An input program can be compiled using a
single pass or using two passes
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Different groups of phase from corresponding
front end and back ends of the compilers
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Compiling a source program into single pass is
difficult because of forward reference that may occur in the program.
Similarly if we group all the phases into a single pass then we may be forced
to keep the entire program in the memory
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