From: Sergey Bronnikov via Tarantool-patches <tarantool-patches@dev.tarantool.org> To: Sergey Kaplun <skaplun@tarantool.org>, Maxim Kokryashkin <m.kokryashkin@tarantool.org> Cc: tarantool-patches@dev.tarantool.org Subject: Re: [Tarantool-patches] [PATCH luajit 2/2] FFI: Fix 64 bit shift fold rules. Date: Tue, 8 Oct 2024 15:07:20 +0300 [thread overview] Message-ID: <b9f5309f-dae8-4862-bf10-b32b083916d2@tarantool.org> (raw) In-Reply-To: <7531b1a6a3f39f8f2d83a54befdc67af987cebaf.1727855711.git.skaplun@tarantool.org> [-- Attachment #1: Type: text/plain, Size: 4854 bytes --] Hello, Sergey, thanks for the patch! Please see my comments below. On 02.10.2024 11:09, Sergey Kaplun wrote: > From: Mike Pall <mike> > > Thanks to Peter Cawley. > > (cherry picked from commit 9e0437240f1fb4bfa7248f6ec8be0e3181016119) > > For `IR_BSHR`, `IR_BROL`, `IR_BROR` during `kfold_int64arith()` the left > argument is truncated down to 32 bits, which leads to incorrect results > if the right argument is >= 32. typo: is >= 2,147,483,647 > > Also, `IR_BSAR` does an unsigned shift rather than a signed shift, but > since this case branch is unreachable, it is harmless for now. > > This patch fixes all misbehaviours (including possible for `IR_BSAR`) to > preserve IR semantics. > > Sergey Kaplun: > * added the description and the test for the problem > > Part of tarantool/tarantool#10199 > --- > src/lj_opt_fold.c | 8 +- > .../lj-1079-fix-64-bitshift-folds.test.lua | 74 +++++++++++++++++++ > 2 files changed, 78 insertions(+), 4 deletions(-) > create mode 100644 test/tarantool-tests/lj-1079-fix-64-bitshift-folds.test.lua > > diff --git a/src/lj_opt_fold.c b/src/lj_opt_fold.c > index e2171e1b..2702f79f 100644 > --- a/src/lj_opt_fold.c > +++ b/src/lj_opt_fold.c > @@ -382,10 +382,10 @@ static uint64_t kfold_int64arith(jit_State *J, uint64_t k1, uint64_t k2, > case IR_BOR: k1 |= k2; break; > case IR_BXOR: k1 ^= k2; break; > case IR_BSHL: k1 <<= (k2 & 63); break; > - case IR_BSHR: k1 = (int32_t)((uint32_t)k1 >> (k2 & 63)); break; > - case IR_BSAR: k1 >>= (k2 & 63); break; > - case IR_BROL: k1 = (int32_t)lj_rol((uint32_t)k1, (k2 & 63)); break; > - case IR_BROR: k1 = (int32_t)lj_ror((uint32_t)k1, (k2 & 63)); break; > + case IR_BSHR: k1 >>= (k2 & 63); break; > + case IR_BSAR: k1 = (uint64_t)((int64_t)k1 >> (k2 & 63)); break; > + case IR_BROL: k1 = lj_rol(k1, (k2 & 63)); break; > + case IR_BROR: k1 = lj_ror(k1, (k2 & 63)); break; > default: lj_assertJ(0, "bad IR op %d", op); break; > } > #else > diff --git a/test/tarantool-tests/lj-1079-fix-64-bitshift-folds.test.lua b/test/tarantool-tests/lj-1079-fix-64-bitshift-folds.test.lua > new file mode 100644 > index 00000000..6cc0b319 > --- /dev/null > +++ b/test/tarantool-tests/lj-1079-fix-64-bitshift-folds.test.lua > @@ -0,0 +1,74 @@ > +local tap = require('tap') > + > +-- Test file to demonstrate LuaJIT misbehaviour on folding > +-- for bitshift operations. > +-- See also,https://github.com/LuaJIT/LuaJIT/issues/1079. > + > +local test = tap.test('lj-1079-fix-64-bitshift-folds'):skipcond({ > + ['Test requires JIT enabled'] = not jit.status(), > +}) > + > +local bit = require('bit') > + > +test:plan(4) > + > +-- Generic function for `bit.ror()`, `bit.rol()`. > +local function bitop_rotation(bitop) I would rename arg `bitop` to `bitop_func` to highlight the type of the value. > + local r = {} > + for i = 1, 4 do > + -- (i & k1) o k2 ==> (i o k2) & (k1 o k2) > + local int64 = bit.band(i, 7LL) > + r[i] = tonumber(bitop(int64, 32)) please add comments about magic constants here and below > + end > + return r > +end > + > +-- Similar function for `bit.rshift()`. > +local function bitop_rshift_signed() > + local r = {} > + for i = 1, 4 do > + -- (i & k1) o k2 ==> (i o k2) & (k1 o k2) > + -- XXX: Use `-i` instead of `i` to prevent other folding due > + -- to IR difference so the IRs don't match fold rule mask. > + -- (-i & 7LL) < 1 << 32 => result == 0. > + local int64 = bit.band(-i, 7LL) > + r[i] = tonumber(bit.rshift(int64, 32)) > + end > + return r > +end > + > +-- A little bit different example, which leads to the assertion > +-- failure due to the incorrect recording. > +local function bitop_rshift_huge() > + local r = {} > + for i = 1, 4 do > + -- (i & k1) o k2 ==> (i o k2) & (k1 o k2) > + -- XXX: Need to use cast to the int64_t via `+ 0LL`, see the > + -- documentation [1] for the details. > + -- [1]:https://bitop.luajit.org/semantics.html > + local int64 = bit.band(2 ^ 33 + i, 2 ^ 33 + 0LL) > + r[i] = tonumber(bit.rshift(int64, 32)) > + end > + return r > +end > + > +local function test_64bitness(subtest, payload_func, bitop) > + subtest:plan(1) > + > + jit.off() > + jit.flush() > + local results_joff = payload_func(bitop) > + jit.on() > + -- Reset hotcounters. > + jit.opt.start('hotloop=1') > + local results_jon = payload_func(bitop) > + subtest:is_deeply(results_jon, results_joff, > + 'same results for VM and JIT for ' .. subtest.name) > +end > + > +test:test('rol', test_64bitness, bitop_rotation, bit.rol) > +test:test('ror', test_64bitness, bitop_rotation, bit.ror) > +test:test('rshift signed', test_64bitness, bitop_rshift_signed) > +test:test('rshift huge', test_64bitness, bitop_rshift_huge) have you added additional whitespaces intentionally? > + > +test:done(true) [-- Attachment #2: Type: text/html, Size: 6196 bytes --]
next prev parent reply other threads:[~2024-10-08 12:07 UTC|newest] Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top 2024-10-02 8:09 [Tarantool-patches] [PATCH luajit 0/2] Fixes for 64 bit operands of the bit library Sergey Kaplun via Tarantool-patches 2024-10-02 8:09 ` [Tarantool-patches] [PATCH luajit 1/2] Fix bit op coercion in DUALNUM builds Sergey Kaplun via Tarantool-patches 2024-10-08 10:12 ` Sergey Bronnikov via Tarantool-patches 2024-10-11 19:08 ` Maxim Kokryashkin via Tarantool-patches 2024-10-02 8:09 ` [Tarantool-patches] [PATCH luajit 2/2] FFI: Fix 64 bit shift fold rules Sergey Kaplun via Tarantool-patches 2024-10-08 12:07 ` Sergey Bronnikov via Tarantool-patches [this message] 2024-10-08 14:24 ` Sergey Kaplun via Tarantool-patches 2024-10-09 14:29 ` Sergey Bronnikov via Tarantool-patches 2024-10-11 19:12 ` Maxim Kokryashkin via Tarantool-patches 2024-10-18 15:17 ` [Tarantool-patches] [PATCH luajit 0/2] Fixes for 64 bit operands of the bit library Sergey Kaplun via Tarantool-patches
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