1 files changed, 86 insertions, 0 deletions
diff --git a/linux/313index-bounds-check.subx b/linux/313index-bounds-check.subx
new file mode 100644
index 00000000..615935b3
--- /dev/null
+++ b/linux/313index-bounds-check.subx
@@ -0,0 +1,86 @@
+# Helper to check an array's bounds, and to abort if they're violated.
+# Really only intended to be called from code generated by mu.subx.
+
+== code
+
+__check-mu-array-bounds:  # index: int, elem-size: int, arr-size: int, function-name: (addr array byte), array-name: (addr array byte)
+    # . prologue
+    55/push-ebp
+    89/<- %ebp 4/r32/esp
+    # . save registers
+    50/push-eax
+    51/push-ecx
+    52/push-edx
+    # . not bothering saving ebx; it's only clobbered if we're going to abort
+    # ecx = arr-size
+    8b/-> *(ebp+0x10) 1/r32/ecx
+    # var overflow/edx: int = 0
+    ba/copy-to-edx 0/imm32
+    # var offset/eax: int = index * elem-size
+    8b/-> *(ebp+8) 0/r32/eax
+    f7 4/subop/multiply-eax-with *(ebp+0xc)
+    # check for overflow
+    81 7/subop/compare %edx 0/imm32
+    0f 85/jump-if-!= __check-mu-array-bounds:overflow/disp32
+    # check bounds
+    39/compare %eax 1/r32/ecx
+    0f 82/jump-if-unsigned< $__check-mu-array-bounds:end/disp32  # negative index should always abort
+    # abort if necessary
+    (write-buffered Stderr "fn ")
+    (write-buffered Stderr *(ebp+0x14))
+    (write-buffered Stderr ": offset ")
+    (write-int32-hex-buffered Stderr %eax)
+    (write-buffered Stderr " is too large for array '")
+    (write-buffered Stderr *(ebp+0x18))
+    (write-buffered Stderr "'\n")
+    (flush Stderr)
+    # exit(1)
+    bb/copy-to-ebx 1/imm32
+    e8/call syscall_exit/disp32
+    # never gets here
+$__check-mu-array-bounds:end:
+    # . restore registers
+    5a/pop-to-edx
+    59/pop-to-ecx
+    58/pop-to-eax
+    # . epilogue
+    89/<- %esp 5/r32/ebp
+    5d/pop-to-ebp
+    c3/return
+
+__check-mu-array-bounds:overflow:
+    # "fn " function-name ": offset to array '" array-name "' overflowed 32 bits\n"
+    (write-buffered Stderr "fn ")
+    (write-buffered Stderr *(ebp+0x14))
+    (write-buffered Stderr ": offset to array '")
+    (write-buffered Stderr *(ebp+0x18))
+    (write-buffered Stderr "' overflowed 32 bits\n")
+    (flush Stderr)
+    # exit(1)
+    bb/copy-to-ebx 1/imm32
+    e8/call syscall_exit/disp32
+    # never gets here
+
+# potential alternative
+
+#? __bounds-check:  # msg: (addr array byte)
+#?   (write-buffered Stderr "abort: array bounds exceeded in fn ")
+#?   8b/-> *(esp+4) 0/r32/eax  # we're going to abort, so just clobber away
+#?   (write-buffered Stderr %eax)
+#?   (write-buffered Stderr Newline)
+#?   # exit(1)
+#?   bb/copy-to-ebx 1/imm32
+#?   e8/call syscall_exit/disp32
+
+# to be called as follows:
+#   var/reg <- index arr/rega: (addr array T), idx/regi: int
+#     | if size-of(T) is 1, 2, 4 or 8
+#         => # temporarily save array size to reg to check bounds
+#            "8b/-> *" rega " " reg "/r32"
+#            "c1/shift 5/subop/right %" reg " " log2(size-of(T)) "/imm32"
+#            "3b/compare " reg "/r32 *" rega
+#            "68/push \"" function "\"/imm32"  # pass function name to error message
+#            "0f 8d/jump-if->= __bounds_check/disp32"
+#            "81 0/subop/add %esp 4/imm32"  # drop function name
+#            # actually save the index addr in reg
+#            "8d/copy-address *(" rega "+" regi "<<" log2(size-of(T)) "+4) " reg "/r32"