Small quality of life enhancement proposals

[rmiguelc]

1.1 - Constant conjunction

It would be interesting to be able to do the following:

#define HASH_INSTANCES 4
#define HASH_LENGTH 16 //in bits

#define TOTAL_OUTPUT_LENGTH ( HASH_INSTANCES * HASH_LENGTH )

The compiler accepts this, but if I write bit<TOTAL_OUTPUT_LENGTH> var it forbids it:

../../f/skeleton.p4(52):syntax error, unexpected (, expecting INTEGER
bit<(

1.2 - Logarithms and powers

And to use power or logarithm base 2 functions.

#define NUMBER_OF_SWITCH_PORTS 128
#define NUMBER_OF_PORTID_BITS ( log(NUMBER_OF_SWITCH_PORTS) + 1 )

typedef PortId bit<NUMBER_OF_PORTID_BITS>

2.1 - Shift operator

bit<5> b = 1;
bit<5> var_one = 1 << 1; //this will compile just fine
bit<5> var_two = 1 << b; //this will not compile

../../f/skeleton.p4(81): error: << left operand of shift must be a numeric type, not int
bit<2> var_two = 1 << b;

[mihaibudiu]

I would suggest filing separate issues for each problem; this makes it much simpler to solve them one by one. The first one is a duplicate of #47.

The second one is trickier, since you would have to define what the log is for non-powers of two and for negative numbers. In general you should define a macro for the log and compute the number of ports:

#define LOG_PORTS 7
#define PORTS (1 << LOG_PORTS)

For the third one the spec requires that the size of the shifted operand to be specified, unless both operands are infinite-precision. Remember that shift will discard bits, and it is not clear how wide is your 1. If you replace 1 with 5w1 it will work. In your example b evaluates to a constant, but in general you need to know how wide the 1 if you evaluate this at run-time. Another possibility is to declare b as const.

[mihaibudiu]

So I don't really know if there is anything to fix for this issue.
Maybe we can close it.

[rmiguelc]

Thank you for responding, Mihai :-)
Looking forward to seeing #47 resolved!