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

common-block.sig common-block.fun

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.