Re: [PATCH] scsi: resolve COMMAND_SIZE at compile time

[Stephen Kitt <steve@sk2.org>]

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Hi Bart,

On Fri, 9 Mar 2018 22:47:12 +0000, Bart Van Assche <Bart.VanAssche@wdc.com>
wrote:
> On Fri, 2018-03-09 at 23:33 +0100, Stephen Kitt wrote:
> > +/*
> > + * SCSI command sizes are as follows, in bytes, for fixed size commands,
> > per
> > + * group: 6, 10, 10, 12, 16, 12, 10, 10. The top three bits of an opcode
> > + * determine its group.
> > + * The size table is encoded into a 32-bit value by subtracting each
> > value
> > + * from 16, resulting in a value of 1715488362
> > + * (6 << 28 + 6 << 24 + 4 << 20 + 0 << 16 + 4 << 12 + 6 << 8 + 6 << 4 +
> > 10).
> > + * Command group 3 is reserved and should never be used.
> > + */
> > +#define COMMAND_SIZE(opcode) \
> > +	(16 - (15 & (1715488362 >> (4 * (((opcode) >> 5) & 7)))))  
> 
> To me this seems hard to read and hard to verify. Could this have been
> written as a combination of ternary expressions, e.g. using a gcc statement
> expression to ensure that opcode is evaluated once?

That’s what I’d tried initially, e.g.

#define COMMAND_SIZE(opcode) ({ \
int index = ((opcode) >> 5) & 7; \
index == 0 ? 6 : (index == 4 ? 16 : index == 3 || index == 5 ? 12 : 10); \
})

But gcc still reckons that results in a VLA, defeating the initial purpose of
the exercise.

Does it help if I make the magic value construction clearer?

#define SCSI_COMMAND_SIZE_TBL (	\
	   (16 -  6)		\
	+ ((16 - 10) <<  4)	\
	+ ((16 - 10) <<  8)	\
	+ ((16 - 12) << 12)	\
	+ ((16 - 16) << 16)	\
	+ ((16 - 12) << 20)	\
	+ ((16 - 10) << 24)	\
	+ ((16 - 10) << 28))

#define COMMAND_SIZE(opcode)						\
  (16 - (15 & (SCSI_COMMAND_SIZE_TBL >> (4 * (((opcode) >> 5) & 7)))))

Regards,

Stephen

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