linux-kernel - Re: [PATCH RFCv3 08/14] arm64: introduce aarch64_insn_gen

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Message-ID: <20140718084320.GB9548@arm.com>
Date:	Fri, 18 Jul 2014 09:43:20 +0100
From:	Will Deacon <will.deacon@....com>
To:	Z Lim <zlim.lnx@...il.com>
Cc:	Catalin Marinas <Catalin.Marinas@....com>,
	Jiang Liu <liuj97@...il.com>,
	AKASHI Takahiro <takahiro.akashi@...aro.org>,
	"David S. Miller" <davem@...emloft.net>,
	Daniel Borkmann <dborkman@...hat.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH RFCv3 08/14] arm64: introduce aarch64_insn_gen_movewide()

On Fri, Jul 18, 2014 at 06:47:22AM +0100, Z Lim wrote:
> (resending this email in case the first one got caught in your spam
> filter. sorry.)
> 
> On Thu, Jul 17, 2014 at 10:41:02AM +0100, Will Deacon wrote:
> > On Wed, Jul 16, 2014 at 11:04:22PM +0100, Zi Shen Lim wrote:
> > > On Wed, Jul 16, 2014 at 05:17:15PM +0100, Will Deacon wrote:
> > > > On Tue, Jul 15, 2014 at 07:25:06AM +0100, Zi Shen Lim wrote:
> > > > > Introduce function to generate move wide (immediate) instructions.
> [...]
> > > > > +       switch (variant) {
> > > > > +       case AARCH64_INSN_VARIANT_32BIT:
> > > > > +               BUG_ON(shift != 0 && shift != 16);
> > > > > +               break;
> > > > > +       case AARCH64_INSN_VARIANT_64BIT:
> > > > > +               insn |= BIT(31);
> > > > > +               BUG_ON(shift != 0 && shift != 16 && shift != 32 &&
> > > > > +                      shift != 48);
> > > >
> > > > Would be neater as a nested switch, perhaps? If you reorder the
> > > > outer-switch, you could probably fall-through too and combine the shift
> > > > checks.
> > >
> > > Not sure I picture what you had in mind... I couldn't come up with a
> > > neater version with the properties you described.
> > >
> > > The alternative I had was using masks instead of integer values, but
> > > one could argue that while neater, it could also be harder to read:
> > >
> > >     switch (variant) {
> > >     case AARCH64_INSN_VARIANT_32BIT:
> > >             BUG_ON(shift & ~BIT(4));
> > >             break;
> > >     case AARCH64_INSN_VARIANT_64BIT:
> > >             insn |= BIT(31);
> > >             BUG_ON(shift & ~GENMASK(5, 4));
> > >     ...
> >
> > I was thinking of using nested switches, but that doesn't fall out like I
> > hoped. How about:
> >
> >       switch (variant) {
> >       case AARCH64_INSN_VARIANT_64BIT:
> >               BUG_ON(shift != 32 && shift != 48);
> 
> Sorry this won't work. For example, on the valid case of shift==0,
> we'll barf right here - no fallthrough.
> 
> Shall we just leave the code as is? :)

Yeah, I'm an idiot ;)

Cheers,

Will
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