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. Compiler

Structured variables Structured variables: Arrays Records Strings Sets E.g., A: Array[1..10] of Integer … A[I] := 5 Compiler

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) Compiler

Structured variables (cont.) Compiler

Code generation for array reference Compiler

Code generation for array reference (cont.) Compiler

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 Compiler

Elimination the common subexpression Compiler

Elimination the common subexpression (cont.) Compiler

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. Compiler

Elimination the loop invariants (cont.) Compiler

Elimination the loop invariants (cont.) Compare the total number of quadruples operations among original codes and the code with loop invariant eliminated. Compiler

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] Compiler

Reduction in strength of an operation Compiler

Reduction in strength of an operation (cont.) Note: Addition is faster than multiplication on the target machine. Compiler

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 allocation It 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. Compiler

Recursive invocation of a procedure using static storage allocation Compiler

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. Compiler

Recursive invocation of a procedure using automatic storage allocation Compiler

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

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

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

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. Compiler

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) Compiler

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. Compiler

Nesting of blocks in a source program Compiler

Nesting of blocks in a source program (CONT.) Compiler

Use of display for above procedure Compiler

Use of display for above procedure (cont.) Compiler

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 * Z Case I : X, Y, Z are all Integer CaseII: Some of them are Integer variables, others are Real. Compiler

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. Compiler

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). Compiler

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. Compiler

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. Compiler

Compiler Design Options – Compiler-compilers (cont.) The advantages of using a compiler-compiler Ease of compiler construction. Ease of testing. Compiler