# 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
Entry: # run all tests
#? e8/call test-compare-equal-strings/disp32
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 : (address string), benchmark : (address string) -> EAX : boolean
# pseudocode:
# lens = s->length
# if (lens != benchmark->length) return false
# i = 0
# currs = s->data
# currb = benchmark->data
# while i < s->length
# c1 = *currs
# c2 = *currb
# if (c1 != c2) return false
# ++i, ++currs, ++currb
# return true
#
# registers:
# i: ECX
# lens: EDX
# currs: ESI
# currb: EDI
# c1: EAX
# c2: EBX
#
# . 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
57/push-EDI
# ESI = s
8b/copy 1/mod/*+disp8 5/rm32/EBP . . . 6/r32/ESI 8/disp8 . # copy *(EBP+8) to ESI
# EDI = benchmark
8b/copy 1/mod/*+disp8 5/rm32/EBP . . . 7/r32/EDI 0xc/disp8 . # copy *(EBP+12) to EDI
# lens/EDX = s->length
8b/copy 0/mod/indirect 6/rm32/ESI . . . 2/r32/EDX . . # copy *ESI to EDX
$string-equal?:lengths:
# if (lens != benchmark->length) return false
39/compare 0/mod/indirect 7/rm32/EDI . . . 2/r32/EDX . . # compare *EDI and EDX
75/jump-if-not-equal $string-equal?:false/disp8
# currs/ESI = s->data
81 0/subop/add 3/mod/direct 6/rm32/ESI . . . . . 4/imm32 # add to ESI
# currb/EDI = benchmark->data
81 0/subop/add 3/mod/direct 7/rm32/EDI . . . . . 4/imm32 # add to EDI
# i/ECX = c1/EAX = c2/EBX = 0
31/xor 3/mod/direct 1/rm32/ECX . . . 1/r32/ECX . . # clear ECX
31/xor 3/mod/direct 0/rm32/EAX . . . 0/r32/EAX . . # clear EAX
31/xor 3/mod/direct 3/rm32/EBX . . . 3/r32/EBX . . # clear EBX
$string-equal?:loop:
# if (i >= lens) 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
# c1 = *currs
8a/copy-byte 0/mod/indirect 6/rm32/ESI . . . 0/r32/AL . . # copy byte at *ESI to AL
# c2 = *currb
8a/copy-byte 0/mod/indirect 7/rm32/EDI . . . 3/r32/BL . . # copy byte at *EDI to BL
# if (c1 != c2) return false
39/compare 3/mod/direct 0/rm32/EAX . . . 3/r32/EBX . . # compare EAX and EBX
75/jump-if-not-equal $string-equal?:false/disp8
# ++i
41/increment-ECX
# ++currs
46/increment-ESI
# ++currb
47/increment-EDI
eb/jump $string-equal?:loop/disp8
$string-equal?:true:
b8/copy-to-EAX 1/imm32
eb/jump $string-equal?:end/disp8
$string-equal?:false:
b8/copy-to-EAX 0/imm32
$string-equal?:end:
# . restore registers
5f/pop-to-EDI
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