Re: [Tarantool-patches] [PATCH luajit 2/3] DUALNUM: Fix narrowing of unary minus.

[Sergey Bronnikov via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

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Hi, Sergey,

thanks for the patch! See my comments.

Sergey

On 3/2/26 10:52, Sergey Kaplun wrote:
> From: Mike Pall <mike>
>
> Reported by Sergey Kaplun.
>
> (cherry picked from commit b1cd2f83b5d085bb71368b87c91a461be77d4364)
>
> `lj_opt_narrow_unm()` in the DUALNUM mode narrows doubles too
> optimistic, missing 0 check. In that case, the narrowing of 0 is
> incorrect. This leads to the assertion failure in `rec_check_slots()`
> for the string obtained from the corresponding number.
>
> This patch fixes it by restricting the check of the given TValue.
>
> Sergey Kaplun:
> * added the description and the test for the problem
>
> Part of tarantool/tarantool#12134
> ---
>   src/lj_opt_narrow.c                           |  4 +-
>   ...lj-1418-dualnum-narrowing-minus-0.test.lua | 49 +++++++++++++++++++
>   2 files changed, 51 insertions(+), 2 deletions(-)
>   create mode 100644 test/tarantool-tests/lj-1418-dualnum-narrowing-minus-0.test.lua
>
> diff --git a/src/lj_opt_narrow.c b/src/lj_opt_narrow.c
> index 6b6f20d3..6e3e9533 100644
> --- a/src/lj_opt_narrow.c
> +++ b/src/lj_opt_narrow.c
> @@ -553,9 +553,9 @@ TRef lj_opt_narrow_unm(jit_State *J, TRef rc, TValue *vc)
>     rc = conv_str_tonum(J, rc, vc);
>     if (tref_isinteger(rc)) {
>       uint32_t k = (uint32_t)numberVint(vc);
> -    if ((LJ_DUALNUM || k != 0) && k != 0x80000000u) {
> +    if ((tvisint(vc) || k != 0) && k != 0x80000000u) {
>         TRef zero = lj_ir_kint(J, 0);
> -      if (!LJ_DUALNUM)
> +      if (!tvisint(vc))
>   	emitir(IRTGI(IR_NE), rc, zero);
>         return emitir(IRTGI(IR_SUBOV), zero, rc);
>       }
> diff --git a/test/tarantool-tests/lj-1418-dualnum-narrowing-minus-0.test.lua b/test/tarantool-tests/lj-1418-dualnum-narrowing-minus-0.test.lua
> new file mode 100644
> index 00000000..84f17953
> --- /dev/null
> +++ b/test/tarantool-tests/lj-1418-dualnum-narrowing-minus-0.test.lua
> @@ -0,0 +1,49 @@
> +local tap = require('tap')
> +
> +-- This test demonstrates LuaJIT's incorrect narrowing
> +-- optimization in the DUALNUM mode for 0.
> +-- See alsohttps://github.com/LuaJIT/LuaJIT/issues/1418.
> +
> +local test = tap.test('lj-1418-dualnum-narrowing-minus-0'):skipcond({
> +  ['Test requires JIT enabled'] = not jit.status(),
> +})
> +

cannot reproduce an original bug with reverted fix.

CMake configuration: CFLAGS=-DDUALNUM cmake -S . -B build 
-DCMAKE_BUILD_TYPE=Debug


> +test:plan(2)
> +
> +local tostring = tostring
> +
> +local function test_const_on_trace(x)
> +  local zero = x % 1
> +  local mzero = -zero
> +  -- Bad IR slot with enabled optimizations.
> +  local res = tostring(mzero)
> +  return res
> +end
> +
> +local function test_non_const_on_trace(a, b)
> +  local mb_zero = a % b
> +  -- Too optimistic optimization without check for the 0 corner
> +  -- case.
> +  local mb_mzero = -mb_zero
> +  local res = tostring(mb_mzero)
> +  return res
> +end
> +
> +jit.opt.start('hotloop=1')
> +
> +-- Hot trace.
> +test_const_on_trace(1)
> +-- Compile trace.
> +test:is(test_const_on_trace(1), '-0', 'correct const value on trace')
> +
> +-- Reset hotcounts.
> +jit.opt.start('hotloop=1')
> +
> +-- Hot trace.
> +test_non_const_on_trace(2, 3)
> +-- Record trace, use non zero result value to record.
s/non zero/non-zero/
> +test_non_const_on_trace(2, 3)
> +-- Misbehaviour on trace with result zero value.
> +test:is(test_non_const_on_trace(2, 1), '-0', 'correct non-const value on trace')
> +
> +test:done(true)

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