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Re: [Bug 1923629] [NEW] RISC-V Vector Instruction vssub.vv not saturatin


From: Alistair Francis
Subject: Re: [Bug 1923629] [NEW] RISC-V Vector Instruction vssub.vv not saturating
Date: Thu, 15 Apr 2021 13:57:06 +1000

+ LIU Zhiwei and Kito Cheng

Alistair

On Wed, Apr 14, 2021 at 1:31 AM Tony Cole <1923629@bugs.launchpad.net> wrote:
>
> Public bug reported:
>
> I noticed doing a negate ( 0 – 0x80000000 ) using vssub.vv produces an
> incorrect result of 0x80000000 (should saturate to 0x7FFFFFFF).
>
> Here is the bit of the code:
>
>                 vmv.v.i         v16, 0
>
> 8f040457        vssub.vv        v8,v16,v8
>
> I believe the instruction encoding is correct (vssub.vv with vd = v8,
> vs2 = v16, rs1 = v8), but the result does not saturate in QEMU.
>
> I’ve just tested with what I think is the latest branch (
> https://github.com/sifive/qemu/tree/rvv-1.0-upstream-v7 commit 26 Feb
> 2021: 1151361fa7d45cc90d69086ccf1a4d8397931811 ) and the problem still
> exists.
>
> ** Affects: qemu
>      Importance: Undecided
>          Status: New
>
>
> ** Tags: riscv vector
>
> --
> You received this bug notification because you are a member of qemu-
> devel-ml, which is subscribed to QEMU.
> https://bugs.launchpad.net/bugs/1923629
>
> Title:
>   RISC-V Vector Instruction vssub.vv not saturating
>
> Status in QEMU:
>   New
>
> Bug description:
>   I noticed doing a negate ( 0 – 0x80000000 ) using vssub.vv produces an
>   incorrect result of 0x80000000 (should saturate to 0x7FFFFFFF).
>
>   Here is the bit of the code:
>
>                 vmv.v.i         v16, 0
>
>   8f040457      vssub.vv        v8,v16,v8
>
>   I believe the instruction encoding is correct (vssub.vv with vd = v8,
>   vs2 = v16, rs1 = v8), but the result does not saturate in QEMU.
>
>   I’ve just tested with what I think is the latest branch (
>   https://github.com/sifive/qemu/tree/rvv-1.0-upstream-v7 commit 26 Feb
>   2021: 1151361fa7d45cc90d69086ccf1a4d8397931811 ) and the problem still
>   exists.
>
> To manage notifications about this bug go to:
> https://bugs.launchpad.net/qemu/+bug/1923629/+subscriptions
>



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