[llvm-commits] Final SwitchInst for PR1255: case-ranges.

[Nuno Lopes]

> (3) storing values.  If there is no default case, then the union of the case
> sets covers every value.  As Chris pointed out (well, maybe this is not what
> he pointed out), that means that the end of one range must be the start of
> another range, so there is no point in storing [Lo, Hi) pairs for each range,
> you can just store the start point of each range.  In the example from (1),
> rather than storing <-2, 0> for destA and <1, 2> for destB you can store
> <-2, 1, 2> and have destA, destB store indices into this.  The complete set
> of APInt values you would store for the above example is:
>    -2, 1, 3, 4, 5
>
> Of course you would want to store them ordered.  I suggest ordering them as
> signed numbers.  This isn't the same as considered switch cases to be signed,
> it is just an internal convenience and ensures that two switch instructions
> with the same ranges will have exactly the same list of APInt values stored
> in them.

This is an interesting representation. Of course, we can only make a  
final decision after performing a lot of statistics.
BTW, for this particular scheme, I believe it would be better to order  
numbers as *unsigned*, since then the creation of ConstantRanges out  
of a number sequence is trivial.


> Maybe the best thing to do is to only provide users with methods that are
> agnostic about internal storage.

Yep!
The usage cases we have right now are, at lest, the following:
  - Jump threading.  Means we need to replace a destination BB with another BB.
  - Remove unreachable cases.  Since we proved that a certain range is  
not reachable, we can make removeCase() join the removed range with a  
consecutive one. E.g. removing the case [4,6) in a list with <1, 4, 6,  
10> can either yield <1, 6, 10> or <1, 4, 10>  (using Duncan's above  
representation).
  - Replace switches with a single destination BB with a jump.  We  
just need to be able to iterate over destination BBs.
  - Iterate over case ranges + their destination BBs. E.g., for bitcode writer.

And I cannot remember of anything else right now..

Nuno