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

[Kito Cheng]

Add Frank, he is the SiFive's qemu maintainer.

On Thu, Apr 15, 2021 at 11:57 AM Alistair Francis <alistair23@gmail.com> wrote:
>
> + 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
> >