When using --!native on functions with many local variables combined with read-modify-write patterns, native code generation fails with an internal lowering error and the script falls back to interpreted mode.
Studio Version: 0.700.0.7000935
Affected Architectures: Both AMD and Intel (x64)
Error Message
Native code generation of script =ServerScriptService.Script failed:
Native code generation failed to compile some of the module functions:
Function ‘kMul’ at line 2 encountered an internal lowering failure
. Script will be interpreted.
Root Cause
Based on examining the Luau source code, this appears to be a bug in the dead store elimination (DSE) optimization pass in “OptimizeDeadStore.cpp”.
The DSE incorrectly marks STORE_DOUBLE instructions as dead when a variable is stored, then later updated using its own value. For example, “a00 = a00 + expr” generates the following IR:
STORE_DOUBLE R0, value1 -- first store
...
LOAD_DOUBLE R0 -- reads from R0
ADD_NUM -- computes new value
STORE_DOUBLE R0, result -- second store
The DSE sees two stores to R0 and incorrectly kills the first store, failing to recognize that the second store’s value depends on reading the first store’s result.
Repo
--!native
local function kMul(smallK)
local a00, a01, a02, a03, a04, a05, a06, a07, a08, a09, a10, a11 = 1,2,3,4,5,6,7,8,9,10,11,12
local c00, c01, c02, c03, c04, c05, c06, c07, c08, c09, c10, c11
a09 = a09 * smallK
a10 = a10 * smallK
a11 = a11 * smallK
c11 = a11 + 3 * 2 ^ 306 - 3 * 2 ^ 306
a00 = a00 + 19 / 2 ^ 255 * c11
c00 = a00 + 3 * 2 ^ 73 - 3 * 2 ^ 73
a01 = a01 + c00
c01 = a01 + 3 * 2 ^ 94 - 3 * 2 ^ 94
a02 = a02 + c01
c02 = a02 + 3 * 2 ^ 115 - 3 * 2 ^ 115
a03 = a03 + c02
c03 = a03 + 3 * 2 ^ 136 - 3 * 2 ^ 136
a04 = a04 + c03
c04 = a04 + 3 * 2 ^ 158 - 3 * 2 ^ 158
a05 = a05 + c04
c05 = a05 + 3 * 2 ^ 179 - 3 * 2 ^ 179
return {
a00 - c00 + 19 / 2 ^ 255 * c11,
a01 - c01,
a02 - c02,
a03 - c03,
a04 - c04,
a05 - c05,
a11 - c11,
}
end
print(kMul(121666))
Additional Context
This info was sent to me (not my own research) and apparently Keccak is also affected, though I was not able to reproduce the issue on my machine.
