Sped up traversal over mutable maps by a factor...

Sped up traversal over mutable maps by a factor of two.

There was this comment in HashTable explaining why foreach was
implemented in terms of iterator. /*

   * We should implement this as a primitive operation over the
   * underlying array, but it can cause a behaviour change in edge cases
   * where:
   * - Someone modifies a map during iteration
   * - The insertion point is close to the iteration point.
   */

Item 1: foreach and iterator didn't behave the same if the map was
mutated in the midst of the traversal anyway. Item 2: protecting some
particular undefinition of inherently undefined behavior is a pretty
unconvincing reason to impose a 2x penalty on foreach.

Here are the before/after times for traversing the keys with foreach
vs. with iterator. Same impact on values and on the map itself. The
benchmark code is included in this commit.

  before: foreach 143700900   iterator 143848900
   after: foreach 67024400    iterator 144890300

Respecting the fact that this might be causing some behavior somewhere
to change, even though it would be pretty sick to be relying upon it,

** ATTENTION! POSSIBLE BEHAVIOR CHANGE! **

Review by dragos.

0 files changed, 0 insertions, 0 deletions