Hi!
The function f() is deterministic, but its result is changing when run on
LuaJIT x86.
local ffi = require "ffi"
local dwords = ffi.typeof("uint32_t[?]")
local ror = bit.ror
local function f()
local arr = dwords(100, 0xDEADBEEF)
for i = 2, 99 do
arr[i] = arr[i-2] + ror(arr[i-1], 7)
end
local a, b = 0, 0
for i = 0, 99 do
a, b = arr[i], a
end
return a + b
end
jit.off()
print(f())
jit.on()
f() -- warming up
print(f())
The output on LuaJIT x64 (both Windows/Linux):
5730702483
5730702483
On LuaJIT x86 the output is a pair of unequal numbers:
Output on Windows (LuaJIT 2.0/2.1) x86 and Linux (LuaJIT 2.0) x86
5730702483
2764532822
Output on Linux (LuaJIT 2.1) x86
6102788609
2764532822
*Question*:
Does function f() exhibit some kind of "unspecified behavior"?
Or is it a bug?
This feature/bug was discovered here:
https://github.com/catwell/cw-lua/issues/1
P.S.
A small "bonus" (unrelated):
Compilation warning is displayed when building latest LuaJIT 2.1 under
VS2010:
lib_base.c(302) : warning C4146: unary minus operator applied to
unsigned type, result still unsigned
The code in lib_base.c:
unsigned long ul;
...
if (neg) ul = -ul; // line 302
-- Egor
