Hi, I am the guy who implemented the (currently incomplete) vrp
for sdc.
I see vrp as a tool to avoid _unnecessary_ casts. _Not_ as means
to avoid _all_ casts.
void main(in string[] args) {
immutable size_t len = args.length % 10;
ubyte x = len;
ubyte[] a;
foreach (immutable i; 0u .. len)
a ~= i;
}
The problem with vrp for non-static immutable values is, that vrp
becomes a runtime-thing and I would like to avoid that!
Tracking the range of a Variable at runtime could cause
significant overhead!
Doing static analysis and tracking Variable-Ranges and
assignments at compile-time, is possible but difficult and there
are a number of reasons why not to do it.
1. Slows compilation down
2. implementation is not straight-forward and bugs could be hard
to find.
3. Code that relays on this may not be easy to read, and it may
be hard for the programmer to see why a particular assignment
does not need a cast!
For simple code like the one posted above it is easy to see why
it works.
anything mod 10 can only be in the range of [-9 .. 9]
But if we have much logic and control-flow between the assignment
and the definition of the assigning variable then not haveing an
explicit cast could cause a bit of puzzlement.
