https://github.com/akkartik/mu/blob/main/313index-bounds-check.subx

     1 # Helper to check an array's bounds, and to abort if they're violated.
     2 # Really only intended to be called from code generated by mu.subx.
     3
     4 == code
     5
     6 __check-mu-array-bounds:  # index: int, elem-size: int, arr-size: int, function-name: (addr array byte), array-name: (addr array byte)
     7     # . prologue
     8     55/push-ebp
     9     89/<- %ebp 4/r32/esp
    10     # . save registers
    11     50/push-eax
    12     51/push-ecx
    13     52/push-edx
    14     # . not bothering saving ebx; it's only clobbered if we're going to abort
    15     # ecx = arr-size
    16     8b/-> *(ebp+0x10) 1/r32/ecx
    17     # var overflow/edx: int = 0
    18     ba/copy-to-edx 0/imm32
    19     # var offset/eax: int = index * elem-size
    20     8b/-> *(ebp+8) 0/r32/eax
    21     f7 4/subop/multiply-eax-with *(ebp+0xc)
    22     # check for overflow
    23     81 7/subop/compare %edx 0/imm32
    24     0f 85/jump-if-!= __check-mu-array-bounds:overflow/disp32
    25     # check bounds
    26     39/compare %eax 1/r32/ecx
    27     0f 82/jump-if-unsigned< $__check-mu-array-bounds:end/disp32  # negative index should always abort
    28     # abort if necessary
    29     (draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0 "fn " 3 0)  # 3=cyan
    30     (draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0 *(ebp+0x14) 3 0)  # 3=cyan
    31     (draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0 ": offset " 3 0)  # 3=cyan
    32     (draw-int32-hex-wrapping-right-then-down-from-cursor-over-full-screen 0 %eax 3 0)  # 3=cyan
    33     (draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0 " is too large for array '" 3 0)  # 3=cyan
    34     (draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0 *(ebp+0x18) 3 0)  # 3=cyan
    35     (draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0 "'" 3 0)  # 3=cyan
    36     {
    37       eb/jump loop/disp8
    38     }
    39     # never gets here
    40 $__check-mu-array-bounds:end:
    41     # . restore registers
    42     5a/pop-to-edx
    43     59/pop-to-ecx
    44     58/pop-to-eax
    45     # . epilogue
    46     89/<- %esp 5/r32/ebp
    47     5d/pop-to-ebp
    48     c3/return
    49
    50 __check-mu-array-bounds:overflow:
    51     (draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0 "fn " 3 0)  # 3=cyan
    52     (draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0 *(ebp+0x14) 3 0)  # 3=cyan
    53     (draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0 ": offset to array '" 3 0)  # 3=cyan
    54     (draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0 *(ebp+0x18) 3 0)  # 3=cyan
    55     (draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0 "' overflowed 32 bits" 3 0)  # 3=cyan
    56     {
    57       eb/jump loop/disp8
    58     }
    59     # never gets here