path: root/subx/054string_equal.subx

# Comparing 'regular' length-prefixed strings.

== code
# instruction                     effective address                                                   operand     displacement    immediate
# op          subop               mod             rm32          base        index         scale       r32
# 1-3 bytes   3 bits              2 bits          3 bits        3 bits      3 bits        2 bits      2 bits      0/1/2/4 bytes   0/1/2/4 bytes

# main:
  e8/call  run-tests/disp32  # 'run-tests' is a function created automatically by SubX. It calls all functions that start with 'test-'.
  # exit(Num-test-failures)
  8b/copy                         0/mod/indirect  5/rm32/.disp32            .             .           3/r32/EBX   Num-test-failures/disp32          # copy *Num-test-failures to EBX
  b8/copy-to-EAX  1/imm32
  cd/syscall  0x80/imm8

string-equal:  # s : string, benchmark : string -> EAX : boolean
  # prolog
  55/push-EBP
  89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
  # save registers
  51/push-ECX
  52/push-EDX
  53/push-EBX
  56/push-ESI
  # pseudocode:
  #   if s.length != b.length return false
  #   for i = 0;  i < s.length;  ++i
  #     if s[i] != b[i] return false
  #   return true
  # registers:
  #   i: ECX
  #   s.length: EDX
  #   b.length: EBX
  #   b[i]: EBX
  #   s[i]: EAX
  #
  # var s/EAX : (address array byte)
  8b/copy                         1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           0/r32/EAX   8/disp8         .                 # copy *(EBP+8) to EAX
  # var benchmark/EBX : (address array byte)
  8b/copy                         1/mod/*+disp8   4/rm32/sib    5/base/EBP  4/index/none  .           3/r32/EBX   0xc/disp8       .                 # copy *(EBP+12) to EBX
  # if s.length != b.length return false
    # EDX = s.length
  8b/copy                         0/mod/indirect  0/rm32/EAX    .           .             .           2/r32/EDX   .               .                 # copy *EAX to EDX
    # compare s.length and b.length
  39/compare                      0/mod/indirect  3/rm32/EBX    .           .             .           2/r32/EDX   .               .                 # compare *EBX with EDX
  75/jump-if-not-equal  $string-equal:false/disp8
$string-equal:lengths:
  # var i/ECX : int = 0
  b9/copy-to-ECX  0/imm32
  # EBX = &b[i]
  43/inc-EBX
  # EAX = &s[i]
  40/inc-EAX
$string-equal:loop:
  # if i >= s.length return true
  39/compare                      3/mod/direct    1/rm32/ECX    .           .             .           2/r32/EDX   .               .                 # compare ECX with EDX
  7d/jump-if-greater-or-equal  $string-equal:true/disp8
  # if b[i] != s[i] return false
    # ESI = s[i]
  8b/copy                         0/mod/indirect  0/rm32/EAX    .           .             .           6/r32/ESI   .               .                 # copy *EAX to ESI
    # compare b[i] with ESI
  39/compare                      0/mod/indirect  3/rm32/EBX    .           .             .           6/r32/ESI   .               .                 # compare *EBX with ESI
  75/jump-if-not-equal  $string-equal:false/disp8
  # ++i
  41/inc-ECX
  40/inc-EAX
  43/inc-EBX
  # loop
  eb/jump  $string-equal:loop/disp8
$string-equal:true:
  # return true
  b8/copy-to-EAX  1/imm32
  eb/jump  $string-equal:end/disp8
$string-equal:false:
  # return false
  b8/copy-to-EAX  0/imm32
$string-equal:end:
  # restore registers
  5e/pop-to-ESI
  5b/pop-to-EBX
  5a/pop-to-EDX
  59/pop-to-ECX
  # epilog
  89/copy                         3/mod/direct    4/rm32/ESP    .           .             .           5/r32/EBP   .               .                 # copy EBP to ESP
  5d/pop-to-EBP
  c3/return

## tests

test-compare-empty-with-empty-string:
  # EAX = string-equal("", "")
    # push args
  68/push  ""/imm32
  68/push  ""/imm32
    # call
  e8/call  string-equal/disp32
    # discard args
  81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
  # call check-ints-equal(EAX, 1, msg)
    # push args
  68/push  "F - test-compare-empty-with-empty-string"/imm32
  68/push  1/imm32/true
  50/push-EAX
    # call
  e8/call  check-ints-equal/disp32
    # discard args
  81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
  c3/return

test-compare-empty-with-non-empty-string:  # also checks length-mismatch code path
  # EAX = string-equal("", "Abc")
    # push args
  68/push  "Abc"/imm32
  68/push  ""/imm32
    # call
  e8/call  string-equal/disp32
    # discard args
  81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
  # call check-ints-equal(EAX, 0, msg)
    # push args
  68/push  "F - test-compare-empty-with-non-empty-string"/imm32
  68/push  0/imm32/false
  50/push-EAX
    # call
  e8/call  check-ints-equal/disp32
    # discard args
  81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
  c3/return

test-compare-equal-strings:
  # EAX = string-equal("Abc", "Abc")
    # push args
  68/push  "Abc"/imm32
  68/push  "Abc"/imm32
    # call
  e8/call  string-equal/disp32
    # discard args
  81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
  # call check-ints-equal(EAX, 1, msg)
    # push args
  68/push  "F - test-compare-equal-strings"/imm32
  68/push  1/imm32/true
  50/push-EAX
    # call
  e8/call  check-ints-equal/disp32
    # discard args
  81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
  c3/return

test-compare-inequal-strings-equal-lengths:
  # EAX = string-equal("Abc", "Adc")
    # push args
  68/push  "Adc"/imm32
  68/push  "Abc"/imm32
    # call
  e8/call  string-equal/disp32
    # discard args
  81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
  # call check-ints-equal(EAX, 0, msg)
    # push args
  68/push  "F - test-compare-inequal-strings-equal-lengths"/imm32
  68/push  0/imm32/false
  50/push-EAX
    # call
  e8/call  check-ints-equal/disp32
    # discard args
  81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
  c3/return

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