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

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Message-ID: <20140716220422.GB18109@gup76>
Date:	Wed, 16 Jul 2014 15:04:22 -0700
From:	Zi Shen Lim <zlim.lnx@...il.com>
To:	Will Deacon <will.deacon@....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 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.
> 
> [...]
> 
> > +u32 aarch64_insn_gen_movewide(enum aarch64_insn_register dst,
> > +			      int imm, int shift,
> > +			      enum aarch64_insn_variant variant,
> > +			      enum aarch64_insn_movewide_type type)
> > +{
> > +	u32 insn;
> > +
> > +	switch (type) {
> > +	case AARCH64_INSN_MOVEWIDE_ZERO:
> > +		insn = aarch64_insn_get_movz_value();
> > +		break;
> > +	case AARCH64_INSN_MOVEWIDE_KEEP:
> > +		insn = aarch64_insn_get_movk_value();
> > +		break;
> > +	case AARCH64_INSN_MOVEWIDE_INVERSE:
> > +		insn = aarch64_insn_get_movn_value();
> > +		break;
> > +	default:
> > +		BUG_ON(1);
> > +	}
> > +
> > +	BUG_ON(imm < 0 || imm > 65535);
> 
> Do this check with masking instead?

Ok, if you prefer, I can change it to:

	BUG_ON(imm & ~GENMASK(15, 0));

> 
> > +
> > +	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));
	...

> 
> Will
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