Re: [Tarantool-patches] [PATCH luajit 1/2] ARM64: Make tobit conversions match JIT backend behavior.

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

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Hello,

thanks for the patch! LGTM with a minor comment.

Sergey

On 7/21/26 17:53, Sergey Kaplun wrote:
> From: Mike Pall <mike>
>
> 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

We usually add parentheses to the name of the function. Here and below.

Feel free to ignore.

> +-- 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)

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