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

[James Bottomley <jejb@linux.vnet.ibm.com>]

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On Sat, 2018-03-10 at 14:29 +0100, Stephen Kitt wrote:
> Hi Bart,
> 
> On Fri, 9 Mar 2018 22:47:12 +0000, Bart Van Assche <Bart.VanAssche@wd
> c.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)))))

Couldn't we do the less clever thing of making the array a static const
and moving it to a header?  That way the compiler should be able to
work it out at compile time.

James

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