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:

quoted

On Fri, 2018-03-09 at 23:33 +0100, Stephen Kitt wrote:

quoted

+/*
+ * 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