Re: [Tarantool-patches] [PATCH luajit 2/2] ARM: Make hard-float 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>
>
> Reported by Peter Cawley.
>
> (cherry picked from commit 32a683d226a3b57adbfeabff17d1ae0f7676545d)
>
> On the arm architecture, the `tobit()` function incorrectly truncates
s/arm/ARM/
> instead of rounding to the nearest integer. This leads to inconsistency
> between various VMs (comparing to x86/x64, for example) and between the
> arm 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.
>
> The test is the same as for the previous commit.
>
> Sergey Kaplun:
> * added the description for the problem
>
> Part of tarantool/tarantool#12880
> ---
>   src/vm_arm.dasc | 13 ++++++++++++-
>   1 file changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/src/vm_arm.dasc b/src/vm_arm.dasc
> index 7ed555f8..139f7fed 100644
> --- a/src/vm_arm.dasc
> +++ b/src/vm_arm.dasc
> @@ -1829,9 +1829,19 @@ static void build_subroutines(BuildCtx *ctx)
>     |
>     |//-- Bit library --------------------------------------------------------
>     |
> -  |// FP number to bit conversion for soft-float. Clobbers r0-r3.
>     |->vm_tobit_fb:
>     |  bhi ->fff_fallback
> +  |.if FPU
> +  |// FP number to bit conversion for hard-float. Clobbers r0, d0-d1.
> +  |  vldr d1, >9
> +  |  vmov d0, CARG1, CARG2
> +  |  vadd.f64 d0, d0, d1
> +  |  vmov CARG1, s0
> +  |  bx lr
> +  |9:
> +  |  .long 0, 0x43380000		// (double)(2^52 + 2^51).
> +  |.else
> +  |// FP number to bit conversion for soft-float. Clobbers r0-r3.
>     |->vm_tobit:
>     |  lsl RB, CARG2, #1
>     |  adds RB, RB, #0x00200000
> @@ -1856,6 +1866,7 @@ static void build_subroutines(BuildCtx *ctx)
>     |  orr CARG1, CARG4, CARG1, lsl RB
>     |   rsblt CARG1, CARG1, #0
>     |  bx lr
> +  |.endif
>     |
>     |.macro .ffunc_bit, name
>     |  .ffunc_1 bit_..name

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