Re: [PATCH 4/7] RISC-V: Recognize sign-extract + and cases for XVentanaCondOps

[Philipp Tomsich <philipp.tomsich@vrull.eu>]

On Fri, 18 Nov 2022 at 00:41, Jeff Law <jeffreyalaw@gmail.com> wrote:
>
>
> On 11/12/22 14:29, Philipp Tomsich wrote:
> > Users might use explicit arithmetic operations to create a mask and
> > then and it, in a sequence like
> >      cond = (bits >> SHIFT) & 1;
> >      mask = ~(cond - 1);
> >      val &= mask;
> > which will present as a single-bit sign-extract.
> >
> > Dependening on what combination of XVentanaCondOps and Zbs are
> > available, this will map to the following sequences:
> >   - bexti + vt.maskc, if both Zbs and XVentanaCondOps are present
> >   - andi + vt.maskc, if only XVentanaCondOps is available and the
> >                      sign-extract is operating on bits 10:0 (bit
> >                   11 can't be reached, as the immediate is
> >                   sign-extended)
> >   - slli + srli + and, otherwise.
> >
> > gcc/ChangeLog:
> >
> >       * config/riscv/xventanacondops.md: Recognize SIGN_EXTRACT
> >         of a single-bit followed by AND for XVentanaCondOps.
> >
> > Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
> > ---
> >
> >   gcc/config/riscv/xventanacondops.md | 46 +++++++++++++++++++++++++++++
> >   1 file changed, 46 insertions(+)
> >
> > diff --git a/gcc/config/riscv/xventanacondops.md b/gcc/config/riscv/xventanacondops.md
> > index 7930ef1d837..3e9d5833a4b 100644
> > --- a/gcc/config/riscv/xventanacondops.md
> > +++ b/gcc/config/riscv/xventanacondops.md
> > @@ -73,3 +73,49 @@
> >     "TARGET_XVENTANACONDOPS"
> >     [(set (match_dup 5) (match_dup 1))
> >      (set (match_dup 0) (and:X (neg:X (ne:X (match_dup 5) (const_int 0)))
> > +
> > +;; Users might use explicit arithmetic operations to create a mask and
> > +;; then and it, in a sequence like
>
> Nit.  Seems like a word is missing.  "make and then and it"??
>
>
> Do we really care about TARGET_XVENTANACONDOPS && ! TARGET_ZBS?

While Ventana might not plan to have this combination, nothing
prevents someone to implement only a single one of these — just as
users might choose to override the -march string.  Also note that (the
proposed) ZiCondOps will share most of its infrastructure with
XVentanaCondOps, we will have the same situation there.

> If there's a good reason to care about the !TARGET_ZBS case, then OK
> with the nit fixed.   If we agree that the !TARGET_ZBS case isn't all
> that important, then obviously OK with that pattern removed too.
>
> I'm about out of oomph today.  I may take a look at 7/7 tonight though.
> Given it hits target independent code we probably want to get resolution
> on that patch sooner rather than later.
>
> jeff
>