[Tarantool-patches] [PATCH] Mark CONV as non-weak, to prevent elimination of its side-effect.
Sergey Bronnikov
sergeyb at tarantool.org
Mon Nov 20 11:24:11 MSK 2023
Hello, Max
thanks for the patch! LGTM
On 7/17/23 18:11, Maxim Kokryashkin wrote:
> Hi!
> It’s silly how it is escaped from my attention, that this patch
> is essentially relevant only for `DUALNUM` mode. This issue
> reproduces on arm64, or if you have LuaJIT built with
> `-DLUJAIT_NUMMODE=2` option. I’ve altered the commit message
> a bit to mention the mode name explicitly. Also, I’ve added an
> additional comment to the test case itself.
> Here is the diff:
> ===============================================
> diff --git a/test/tarantool-tests/mark-conv-non-weak.test.lua
> b/test/tarantool-tests/mark-conv-non-weak.test.lua
> index aad39058..9a325202 100644
> --- a/test/tarantool-tests/mark-conv-non-weak.test.lua
> +++ b/test/tarantool-tests/mark-conv-non-weak.test.lua
> @@ -10,6 +10,10 @@ local sum = 0
> jit.opt.start('hotloop=1', 'hotexit=1')
> +-- XXX: The test fails before the patch only
> +-- for `DUALNUM` mode. All of the IRs below are
> +-- produced by the corresponding LuaJIT build.
> +
> -- When the last trace is recorded, the traced bytecode
> -- is the following before the patch:
> -- ---- TRACE 4 start 2/3 test.lua:6
> ===============================================
> And here is the altered commit message:
> ===============================================
> Mark CONV as non-weak, to prevent elimination of its side-effect.
> An unused guarded CONV int.num cannot be omitted in general.
> (cherry-picked from commit 881d02d3117838acaf4fb844332c8e33cc95c8c5)
> In some cases, an unused `CONV int.num` omission in `DUALNUM` mode
> may lead to a guard absence, resulting in invalid control flow
> branching and undefined behavior. For a comprehensive example of
> the described situation, please refer to the comment
> in `test/tarantool-tests/mark-conv-non-weak.test.lua`.
> Maxim Kokryashkin:
> * added the description and the test for the problem
> Part of tarantool/tarantool#8825
> ===============================================
> --
> Best regards,
> Maxim Kokryashkin
>
> Четверг, 13 июля 2023, 15:23 +03:00 от Sergey Bronnikov
> <sergeyb at tarantool.org>:
> Hi, Max!
>
>
> thanks for the patch!
>
> Links to the branch and PR are missed, but I found them:
>
> branch:
> https://github.com/tarantool/luajit/tree/fckxorg/mark-conv-non-weak
>
> PR: https://github.com/tarantool/tarantool/pull/8871
>
>
> Test is passed after reverting the patch with fix.
>
>
> Sergey
>
>
> On 7/12/23 12:52, Maksim Kokryashkin wrote:
> > From: Mike Pall <mike>
> >
> > An unused guarded CONV int.num cannot be omitted in general.
> >
> > (cherry-picked from commit 881d02d3117838acaf4fb844332c8e33cc95c8c5)
> >
> > In some cases, an unused `CONV` omission may lead to a guard
> > absence, resulting in invalid control flow branching and
> > undefined behavior. For a comprehensive example of
> > the described situation, please refer to the comment
> > in `test/tarantool-tests/mark-conv-non-weak.test.lua`.
> >
> > Maxim Kokryashkin:
> > * added the description and the test for the problem
> >
> > Part of tarantool/tarantool#8825
> > ---
> > src/lj_ir.h | 2 +-
> > .../mark-conv-non-weak.test.lua | 58 +++++++++++++++++++
> > 2 files changed, 59 insertions(+), 1 deletion(-)
> > create mode 100644 test/tarantool-tests/mark-conv-non-weak.test.lua
> >
> > diff --git a/src/lj_ir.h b/src/lj_ir.h
> > index e8bca275..bf9b9292 100644
> > --- a/src/lj_ir.h
> > +++ b/src/lj_ir.h
> > @@ -132,7 +132,7 @@
> > _(XBAR, S , ___, ___) \
> > \
> > /* Type conversions. */ \
> > - _(CONV, NW, ref, lit) \
> > + _(CONV, N , ref, lit) \
> > _(TOBIT, N , ref, ref) \
> > _(TOSTR, N , ref, lit) \
> > _(STRTO, N , ref, ___) \
> > diff --git a/test/tarantool-tests/mark-conv-non-weak.test.lua
> b/test/tarantool-tests/mark-conv-non-weak.test.lua
> > new file mode 100644
> > index 00000000..aad39058
> > --- /dev/null
> > +++ b/test/tarantool-tests/mark-conv-non-weak.test.lua
> > @@ -0,0 +1,58 @@
> > +local tap = require('tap')
> > +local test = tap.test('mark-conv-non-weak'):skipcond({
> > + ['Test requires JIT enabled'] = not jit.status(),
> > +})
> > +
> > +test:plan(1)
> > +
> > +local data = {0.1, 0, 0.1, 0, 0 / 0}
> > +local sum = 0
> > +
> > +jit.opt.start('hotloop=1', 'hotexit=1')
> > +
> > +-- When the last trace is recorded, the traced bytecode
> > +-- is the following before the patch:
> > +-- ---- TRACE 4 start 2/3 test.lua:6
> > +-- 0018 ADDVV 1 1 6
> > +-- 0019 ITERC 5 3 3
> > +-- 0000 . FUNCC ; ipairs_aux
> > +-- 0020 JITERL 5 1
> > +-- 0021 GGET 2 7 ; "assert"
> > +-- 0022 ISEQV 1 1
> > +-- 0023 JMP 4 => 0026
> > +-- 0024 KPRI 4 1
> > +-- 0025 JMP 5 => 0027
> > +-- 0027 CALL 2 1 2
> > +-- 0000 . FUNCC ; assert
> > +--
> > +-- And the following after the patch:
> > +-- ---- TRACE 4 start 2/2 test.lua:5
> > +-- 0016 ISNEV 6 6
> > +-- 0017 JMP 7 => 0019
> > +-- 0019 ITERC 5 3 3
> > +-- 0000 . FUNCC ; ipairs_aux
> > +-- 0020 JITERL 5 1
> > +-- 0021 GGET 2 7 ; "assert"
> > +-- 0022 ISEQV 1 1
> > +-- 0023 JMP 4 => 0026
> > +-- 0026 KPRI 4 2
> > +-- 0027 CALL 2 1 2
> > +-- 0000 . FUNCC ; assert
> > +-- 0028 RET0 0 1
> > +--
> > +-- The crucial difference here is the abscent
> > +-- `ISNEV` in the first case, which produces the
> > +-- desired guarded `CONV`, when translated to IR.
> > +--
> > +-- Since there is no guard, NaN is added to the sum,
> > +-- despite the test case logic.
> > +
> > +for _, val in ipairs(data) do
> > + if val == val then
> > + sum = sum + val
> > + end
> > +end
> > +
> > +test:ok(sum == sum, 'NaN check was not omitted')
> > +
> > +os.exit(test:check() and 0 or 1)
>
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