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5.3 Machine-Independent Compiler Features

Methods for handling structured variables such as array.

Code optimization. Storage allocation for the compiled program. Compiling a block-structured language.

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Structured variables: Arrays Records Strings Sets

E.g., A: Array[1..10] of Integer…A[I] := 5

Structured variables

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Structured variables (cont.)

One-dimension array declaration A : Array[1..10] of Integer A : Array[l..u] of Integer u-l+1 words of storage

Two-dimension array declaration B : Array[0..3, 1..6] of Integer B : Array [l1..u1, l2..u2] of Integer

the number of words to be allocated is given by

(u1-l1+1)*(u2-l2+1)

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Structured variables (cont.)

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Code generation for array reference

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Code generation for array reference (cont.)

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Machine-Independent Code Optimization

Concept illustrated by quadruple verbal description. - I #1 i1

Code optimization techniques Elimination the common subexpressions Elimination the loop invariants Reduction in strength

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Elimination the common subexpression

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Elimination the common subexpression (cont.)

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Elimination the loop invariants

Subexpressions within a loop whose values do not change from one iteration of the loop to the next.

Thus, their values can be computed once, before the loop is entered, rather than being recalculated for each iteration.

E.g., 2*J It can be detect by analyzing the flow graph.

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Elimination the loop invariants (cont.)

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Elimination the loop invariants (cont.)

Compare the total number of quadruples operations among original codes and the code with loop invariant eliminated.

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Another optimization techniques

Rewriting the source program. E.g.,

For I := 1 To 10 Do X[I , 2*J-1] := Y[I , 2*J]

T1 := 2*J;

T2 := T1 – 1;For I := 1 To 10 Do

X[I , T2] := Y[I , T1]

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Reduction in strength of an operation

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Reduction in strength of an operation (cont.)

Note: Addition is faster than multiplication on the target machine.

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Storage Allocation

Static allocation vs. dynamic allocation Static allocation

Temporary variables, including the one used to save the return address, were also assigned fixed addresses within the program. This type of storage assignment is called static allocation.

Dynamic allocationIt is necessary to preserve the previous values of any variables used by subroutine, including parameters, temporaries, return addresses, register save areas, etc.It can be accomplished with a dynamic storage allocation technique.

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Recursive invocation of a procedure using

static storage allocation

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Storage Allocation (cont.)

The dynamic storage allocation technique. Each procedure call creates an activation record that

contains storage for all the variables used by the procedure.

Activation records are typically allocated on a stack.

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Recursive invocation of a procedure using automatic storage allocation

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Recursive invocation of a procedure using automatic storage allocation (cont.)

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Recursive invocation of a procedure using automatic storage allocation (cont.)

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Recursive invocation of a procedure using automatic storage allocation (cont.)

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Storage Allocation (cont.)

The compiler must generate additional code to manage the activation records themselves.

Prologue At the beginning of each procedure there must be code

to create a new activation record, linking it to the previous one and setting the appropriate pointers.

Epilogue At the end of the procedure, there must be code to

delete the current activation record, resetting pointers as needed.

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Storage Allocation (cont.)

Other types of dynamic storage allocation. E.g.,

FORTRAN 90,ALLOCATE ( Matrix( Rows, Columns ))DEALLOCATE ( Matrix )

Pascal,NEW(P)DISPOSE(P)

C,MALLOC(size)FREE(P)

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Block-Structured Languages

A block is a portion of a program that has the ability to declare its own identifiers.

E.g., procedure Blocks may be nested within other blocks.

When a reference to an identifier appears in the source program, the compiler must first check the symbol table for a definition of that identifier by the current block. If no such definition is found, the compiler looks for a surrounds that, and so on.

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Nesting of blocks in a source program

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Nesting of blocks in a source program (CONT.)

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Use of display for above procedure

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Use of display for above procedure (cont.)

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Compiler Design Options – division into passes

Multi-pass compiler Code optimization forward reference problem (required multi-pass)

In Pascal, declarations of variables must appear in the program before the statements that use these variables.

In FORTRAN, declarations may appear at the beginning of the program; any variable that is not declared is assigned characteristic by default.

For example, X := Y * ZCase I : X, Y, Z are all IntegerCaseII: Some of them are Integer variables, others are Real.

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Compiler Design Options – division into passes (cont.)

Single-pass vs. Multi-pass compiler Speed of compilation is the major concerned one-pass Speed of execution is the major concerned multi-pass

The advantage of multi-pass compiler Could incorporate sophisticated code-optimization

techniques. Shorten the overall time required for compiler construction. Consumption limited system resources.

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Compiler Design Options – P-code compiler

P-code compiler (also called bytecode compiler) The source program is analyzed and converted into an

intermediate form, which is a machine language for a hypothetical computer (pseudo-machine or p-machine).

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Compiler Design Options – P-code compiler (cont.)

The advantages of P-code compiler Portability of software

It is not necessary for the compiler to generate different code for different computers.The source version of the compiler is compiled into p-code; this p-code can then be interpreted on another computer.

Easy to write a new compiler for each different machine. The p-code object program is much smaller than a

corresponding machine code program.

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Compiler Design Options – Compiler-compilers

A compiler-compiler is a software tool that can be used to help in the task of compiler construction.

Also called compiler generator or translator-writing system.

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Compiler Design Options – Compiler-compilers (cont.)

The advantages of using a compiler-compiler Ease of compiler construction. Ease of testing.