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# Comparing 'regular' length-prefixed strings.
== code
# instruction effective address register 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:
# run-tests()
e8/call run-tests/disp32 # 'run-tests' is a function created automatically by SubX. It calls all functions that start with 'test-'.
# syscall(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/exit
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
# 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
# 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
# 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
# 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
# . . vim:nowrap:textwidth=0
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