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

[Stephen Kitt <steve@sk2.org>]

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On Sat, 10 Mar 2018 12:49:17 -0800, James Bottomley <jejb@linux.vnet.ibm.com>
wrote:
> On Sat, 2018-03-10 at 14:29 +0100, Stephen Kitt wrote:
> > 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.

I hoped so too, but const-ifying a variable doesn’t make it a compile-time
constant (even though the optimiser can resolve it at build-time), so GCC
still considers uses such as

	u8 cdb[COMMAND_SIZE(MAINTENANCE_IN)];

as variable-length arrays, which C90 forbids (and which we’re trying to
eliminate here).

Regards,

Stephen

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