Course Notes on Formal Languages and Compilers

7. SIMPLIFYING THE PROCESS OF BUILDING A COMPILER
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A great deal of effort is needed to build a compiler, and we shall present two
methods for saving much effort.
Notation
Let us use the notation L C M A to denote a compiler which translates from
language L to language M and is written in language A. (Yes - writing a
compiler may involve you working with three languages.)
When this compiler is executing and translating a program P L in language L
to the same program P M but in language M we shall write
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P L → L C M A →P M
Bootstrapping a compiler
(The boxed item is being executed.)
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Suppose we wish to build the compiler L C A A from high level language L to
assembly or machine language A.
The following approach will save much effort.
(i) Identify a subset S of the language L which is easier to compile.
(ii) Write the compiler S C A A .
(iii) Write the compiler L C A S .
(iv) Execute the following. .
L C A S → S C A A → L C A A
Cross compilation
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Here are two uses of this technique.
1) If the machine (e.g. microcontroller) with language M does not have
the resources to do the compilation itself then we can use another
machine with language A and run the compiler on machine with
language A and just transfer the translated program onto the machine
with language M. For example when L C M A runs on machine with
language A then execute
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P L → L C M A → P M
and transfer P M to run on machine with language M. This technique
allows the use of high level languages on machines with very limited
resources by using an auxiliary computer to do the compilation.
2) Another use of this concept is to create a compiler for a new machine
by using a compiler for an existing machine. Suppose a machine
exists which can run L C A A where L is a high level language. Suppose