Re: Stack trace on SIGSEGV?
- From: Mike Pall <mike-1210@xxxxxxxxxx>
- To: luajit@xxxxxxxxxxxxx
- Date: Mon, 8 Oct 2012 16:38:04 +0200
Luke Gorrie wrote:
> Can someone tell me if this is a good implementation, or how to
> do it better?
local function protected(type, base, offset, size)
type = ffi.typeof(type)
local bound = (size + 0ULL) / ffi.sizeof(type)
local tptr = ffi.typeof("$ *", type)
local wrap = ffi.metatype(ffi.typeof("struct { $ _ptr; }", tptr), {
__index = function(w, idx)
assert(idx < bound)
return w._ptr[idx]
end,
__newindex = function(w, idx, val)
assert(idx < bound)
w._ptr[idx] = val
end,
})
return wrap(ffi.cast(tptr, ffi.cast("uint8_t *", base) + offset))
end
This generates machine code that would be identical with e.g.
C++ bounds checking templates.
Here's a simple test:
local mem = ffi.new("int32_t[400]")
local t = protected("int32_t", mem, 0, 400*4)
for i=0,399 do t[i] = i end
for i=0,399 do assert(t[i] == i) end
And here's the machine code of the two inner loops:
->LOOP:
394cffe0 cmp rbp, 0x190 Bounds check
394cffe7 jnb 0x394c0024 ->5
394cffed mov [rax+rbp*4], ebp Store
394cfff0 add ebp, +0x01 Loop counter
394cfff3 cmp ebp, 0x18f
394cfff9 jle 0x394cffe0 ->LOOP
->LOOP:
394cff20 cmp rbp, 0x190 Bounds check
394cff27 jnb 0x394c002c ->7
394cff2d cmp ebp, [rcx+rbp*4] Load + assert
394cff30 jnz 0x394c0030 ->8
394cff36 add ebp, +0x01 Loop counter
394cff39 cmp ebp, 0x18f
394cff3f jle 0x394cff20 ->LOOP
Note that the array bound has been constified and that a single
unsigned compare is sufficient.
[BTW: Avoid uint32_t as far as possible in FFI code. int32_t is
almost always a better choice.]
--Mike
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