Output From a Production Compiler

We look at output from a production compiler to see how compiling is done in one real example.  The source Pascal code was entered on esus and compiled with the fpc compiler using the -al and -AAS flags.  For example:

fpc -al -AAS myprog.pas

The source code was in myprog.pas and the translated assembly code was in myprog.s

Source 1

program prog1(input, output);

  var
    a, b: Integer;

  begin {prog1}
    write('Please enter integer values for a and b: ');
    read(a, b);
    writeln;
    if a < b
      then
        a := b + 1
      else
        b := a - 1
    end.

Assembly 1

.text
        .balign 16
# [prog2.pas]
# [6] begin {prog1}
.globl  main
        .type   main,@function
main:
.globl  PASCALMAIN
        .type   PASCALMAIN,@function
PASCALMAIN:
.globl  program_init
        .type   program_init,@function
program_init:
        pushl   %ebp
        movl    %esp,%ebp
        subl    $4,%esp
        call    FPC_INITIALIZEUNITS
.L3:
# [7] write('Please enter integer values for a and b: ');
        leal    U_SYSLINUX_OUTPUT,%edi
        movl    %edi,-4(%ebp)
        pushl   $.L6
        pushl   -4(%ebp)
        pushl   $0
        call    FPC_WRITE_TEXT_SHORTSTR
        pushl   -4(%ebp)
        call    FPC_WRITE_END
        pushl   $.L3
        call    FPC_IOCHECK

.L7:
# [8] read(a, b);
        leal    U_SYSLINUX_INPUT,%edi
        movl    %edi,-4(%ebp)
        pushl   -4(%ebp)
        call    FPC_READ_TEXT_SINT
        movw    %ax,_A
        pushl   -4(%ebp)
        call    FPC_READ_TEXT_SINT
        movw    %ax,_B
        pushl   -4(%ebp)
        call    FPC_READ_END
        pushl   $.L7
        call    FPC_IOCHECK
.L8:
# [9] writeln;
        leal    U_SYSLINUX_OUTPUT,%edi
        movl    %edi,-4(%ebp)
        pushl   -4(%ebp)
        call    FPC_WRITELN_END
        pushl   $.L8
        call    FPC_IOCHECK
# [10] if a < b
        movswl  _A,%eax
        movswl  _B,%edx
        cmpl    %eax,%edx
        jg      .L9
        jmp     .L10
.L9:
# [12] a := b + 1
        movswl  _B,%eax
        incl    %eax
        movw    %ax,_A
        jmp     .L13
.L10:
# [14] b := a -1
        movswl  _A,%eax
        decl    %eax
        movw    %ax,_B
.L13:
# [15] end.
        call    FPC_DO_EXIT
        leave
        ret

.Le0:
        .size   main, .Le0 - main
        .balign 16

.data
# [16]
        .ascii  "FPC 1.0.6 [2002/05/23] for i386 - LINUX"

---

Program 2

    for a := 1 to 10 do
      b := b + a

Assembly 2

# [10] for a := 1 to 10 do
        movw    $1,_A
        .balign 4,144
.L11:
# [11] b := b + a
        movswl  _B,%eax
        movswl  _A,%edx
        addl    %eax,%edx
        movw    %dx,_B
        cmpw    $10,_A
        jge     .L10
        incw    _A
        jmp     .L11
.L10:

---

Program 3

    for i := 1 to n do
      a := a + 1

Assembly 3

# [10] for i := 1 to n do
        movw    _N,%di
        movw    %di,-4(%ebp)
        movw    $1,_I
        movw    _I,%ax
        cmpw    -4(%ebp),%ax
        jg      .L10
        .balign 4,144
.L11:
# [11] a := a + 1
        movswl  _A,%eax
        incl    %eax
        movw    %ax,_A
        movw    _I,%ax
        cmpw    -4(%ebp),%ax
        jge     .L10
        incw    _I
        jmp     .L11
.L10:

---

Program 4

    case choice of
      1:
       a := 3;
      2:
       a := 5;
      3:
        a := 10;
      4, 7:
        a := 100;
      6, 10:
        a := 1000
    end;

Assembly 4

# [10] case choice of
        movw    _CHOICE,%ax
        cmpw    $1,%ax
        jl      .L22
        decw    %ax
        jz      .L3
        decw    %ax
        jz      .L5
        decw    %ax
        jz      .L7
        decw    %ax
        jz      .L9
        subw    $2,%ax
        jz      .L12
        decw    %ax
        jz      .L9
        subw    $3,%ax
        jz      .L12
        jmp     .L22
.L12:
# [20] a := 1000
        movw    $1000,_A
        jmp     .L21
.L9:
# [18] a := 100;
        movw    $100,_A
        jmp     .L21
.L7:
# [16] a := 10;
        movw    $10,_A
        jmp     .L21
.L5:
# [14] a := 5;
        movw    $5,_A
        jmp     .L21
.L3:
# [12] a := 3;
        movw    $3,_A
        jmp     .L21
.L22:
.L21:
.L33: