https://github.com/akkartik/mu/blob/main/linux/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     (write-buffered Stderr "fn ")
     30     (write-buffered Stderr *(ebp+0x14))
     31     (write-buffered Stderr ": offset ")
     32     (write-int32-hex-buffered Stderr %eax)
     33     (write-buffered Stderr " is too large for array '")
     34     (write-buffered Stderr *(ebp+0x18))
     35     (write-buffered Stderr "'\n")
     36     (flush Stderr)
     37     # exit(1)
     38     bb/copy-to-ebx 1/imm32
     39     e8/call syscall_exit/disp32
     40     # never gets here
     41 $__check-mu-array-bounds:end:
     42     # . restore registers
     43     5a/pop-to-edx
     44     59/pop-to-ecx
     45     58/pop-to-eax
     46     # . epilogue
     47     89/<- %esp 5/r32/ebp
     48     5d/pop-to-ebp
     49     c3/return
     50
     51 __check-mu-array-bounds:overflow:
     52     # "fn " function-name ": offset to array '" array-name "' overflowed 32 bits\n"
     53     (write-buffered Stderr "fn ")
     54     (write-buffered Stderr *(ebp+0x14))
     55     (write-buffered Stderr ": offset to array '")
     56     (write-buffered Stderr *(ebp+0x18))
     57     (write-buffered Stderr "' overflowed 32 bits\n")
     58     (flush Stderr)
     59     # exit(1)
     60     bb/copy-to-ebx 1/imm32
     61     e8/call syscall_exit/disp32
     62     # never gets here
     63
     64 # potential alternative
     65
     66 #? __bounds-check:  # msg: (addr array byte)
     67 #?   (write-buffered Stderr "abort: array bounds exceeded in fn ")
     68 #?   8b/-> *(esp+4) 0/r32/eax  # we're going to abort, so just clobber away
     69 #?   (write-buffered Stderr %eax)
     70 #?   (write-buffered Stderr Newline)
     71 #?   # exit(1)
     72 #?   bb/copy-to-ebx 1/imm32
     73 #?   e8/call syscall_exit/disp32
     74
     75 # to be called as follows:
     76 #   var/reg <- index arr/rega: (addr array T), idx/regi: int
     77 #     | if size-of(T) is 1, 2, 4 or 8
     78 #         => # temporarily save array size to reg to check bounds
     79 #            "8b/-> *" rega " " reg "/r32"
     80 #            "c1/shift 5/subop/right %" reg " " log2(size-of(T)) "/imm32"
     81 #            "3b/compare " reg "/r32 *" rega
     82 #            "68/push \"" function "\"/imm32"  # pass function name to error message
     83 #            "0f 8d/jump-if->= __bounds_check/disp32"
     84 #            "81 0/subop/add %esp 4/imm32"  # drop function name
     85 #            # actually save the index addr in reg
     86 #            "8d/copy-address *(" rega "+" regi "<<" log2(size-of(T)) "+4) " reg "/r32"
     87
     88 __mu-abort-null-index-base-address:
     89     (write-buffered Stderr "null address in 'index'\n")
     90     (flush Stderr)
     91     # exit(1)
     92     bb/copy-to-ebx 1/imm32
     93     e8/call syscall_exit/disp32
     94     # never gets here
     95
     96 __mu-abort-null-get-base-address:
     97     (write-buffered Stderr "null address in 'get'\n")
     98     (flush Stderr)
     99     # exit(1)
    100     bb/copy-to-ebx 1/imm32
    101     e8/call syscall_exit/disp32
    102     # never gets here