Hi Bart, On Fri, 9 Mar 2018 22:47:12 +0000, Bart Van Assche 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