CommonBlock
===========

<: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 ==

* <!ViewGitFile(mlton,master,mlton/ssa/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.