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CommonBlock

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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.

Last edited on 2006-11-02 17:56:13 by MatthewFluet.