CommonBlock is an optimization pass for the SSA IntermediateLanguage, invoked from SSASimplify.

Description

It eliminates equivalent blocks in a SSA function. The equivalence criteria requires blocks to have no arguments or statements and transfer via Raise, Return, or Goto of a single global variable.

Implementation

Details and Notes

  • Rewrites 

    L_X ()
  raise (global_Y)

    to

    L_X ()
  L_Y' ()

    and adds

    L_Y' ()
  raise (global_Y)

    to the SSA function.

  • Rewrites 

    L_X ()
  return (global_Y)

    to

    L_X ()
  L_Y' ()

    and adds

    L_Y' ()
  return (global_Y)

    to the SSA function.

  • Rewrites 

    L_X ()
  L_Z (global_Y)

    to

    L_X ()
  L_Y' ()

    and adds

    L_Y' ()
  L_Z (global_Y)

    to the SSA function.

The Shrink pass rewrites all uses of L_X to L_Y' and drops L_X.

For example, all uncaught Overflow exceptions in a SSA function share the same raising block.