linearScanEnd pragma

The linearScanEnd pragma can be used to tell the compiler how to compile a Nim case statement. Syntactically it has to be used as a statement:

case myInt
of 0:
  echo "most common case"
of 1:
  {.linearScanEnd.}
  echo "second most common case"
of 2: echo "unlikely: use branch table"
else: echo "unlikely too: use branch table for ", myInt

In the example, the case branches 0 and 1 are much more common than the other cases. Therefore the generated assembler code should test for these values first, so that the CPU’s branch predictor has a good chance to succeed (avoiding an expensive CPU pipeline stall). The other cases might be put into a jump table for O(1) overhead, but at the cost of a (very likely) pipeline stall.

The linearScanEnd pragma should be put into the last branch that should be tested against via linear scanning. If put into the last branch of the whole case statement, the whole case statement uses linear scanning.