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X-BeenThere: tarantool-patches@dev.tarantool.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: Tarantool development patches List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , From: Sergey Kaplun via Tarantool-patches Reply-To: Sergey Kaplun Cc: tarantool-patches@dev.tarantool.org Errors-To: tarantool-patches-bounces@dev.tarantool.org Sender: "Tarantool-patches" From: Mike Pall Thanks to Peter Cawley. (cherry picked from commit cdc2db3aeac442e139c3b1fda715fdfa78da8791) On the aarch64 architecture, the `tobit()` function incorrectly truncates instead of rounding to the nearest integer. This leads to inconsistency between various VMs (comparing to x86/x64, for example) and between the arm64 VM and JIT. This patch uses an addition of 2^52 + 2^51 constant to the given argument to convert it to the nearest integer value. After reading the lowest 32 bits of the register, we get the expected result. Sergey Kaplun: * added the description and the test for the problem Part of tarantool/tarantool#12880 --- src/vm_arm64.dasc | 20 ++++--------- .../lj-1253-tobit-conversion.test.lua | 28 +++++++++++++++++++ 2 files changed, 34 insertions(+), 14 deletions(-) create mode 100644 test/tarantool-tests/lj-1253-tobit-conversion.test.lua diff --git a/src/vm_arm64.dasc b/src/vm_arm64.dasc index 57131140..4ffd7378 100644 --- a/src/vm_arm64.dasc +++ b/src/vm_arm64.dasc @@ -1613,22 +1613,14 @@ static void build_subroutines(BuildCtx *ctx) | |//-- Bit library -------------------------------------------------------- | - |// FP number to bit conversion for soft-float. Clobbers CARG1-CARG3 + |// FP number to bit conversion. Clobbers CARG1-CARG2, FARG1-FARG2. |->vm_tobit_fb: | bls ->fff_fallback - | add CARG2, CARG1, CARG1 - | mov CARG3, #1076 - | sub CARG3, CARG3, CARG2, lsr #53 - | cmp CARG3, #53 - | bhi >1 - | and CARG2, CARG2, #U64x(001fffff,ffffffff) - | orr CARG2, CARG2, #U64x(00200000,00000000) - | cmp CARG1, #0 - | lsr CARG2, CARG2, CARG3 - | cneg CARG1w, CARG2w, mi - | br lr - |1: - | mov CARG1w, #0 + | fmov FARG1, CARG1 + | movz CARG2, #0x4338, lsl #48 + | fmov FARG2, CARG2 + | fadd FARG1, FARG1, FARG2 + | fmov CARG1w, s0 | br lr | |.macro .ffunc_bit, name diff --git a/test/tarantool-tests/lj-1253-tobit-conversion.test.lua b/test/tarantool-tests/lj-1253-tobit-conversion.test.lua new file mode 100644 index 00000000..31cf97ca --- /dev/null +++ b/test/tarantool-tests/lj-1253-tobit-conversion.test.lua @@ -0,0 +1,28 @@ +local tap = require('tap') + +-- Test file to demonstrate LuaJIT's incorrect bit.tobit +-- behaviour for arm64. +-- See also: https://github.com/LuaJIT/LuaJIT/issues/1253. + +local test = tap.test('lj-1253-tobit-conversion') + +test:plan(2) + +test:is(bit.tobit(1.7), 2, 'correct bit.tobit rounding') + +test:skipcond({ + ['Test requires JIT enabled'] = not jit.status(), +}) + +local results = {} + +jit.opt.start('hotloop=1') + +for i = 1, 4 do + -- Use constants on trace. + results[i] = bit.tobit(1.7) +end + +test:samevalues(results, 'consistent JIT and VM behaviour for bit.tobit') + +test:done(true) -- 2.55.0