From b20b664748671f8b62ce9d9fda45370047f8f447 Mon Sep 17 00:00:00 2001
From: Chris Pressey <cpressey@catseye.tc>
Date: Thu, 8 Feb 2018 14:04:51 +0000
Subject: [PATCH] Implement the "union rule for trashes" when analyzing `if`
 blocks.

---
 HISTORY.md                   |  1 +
 README.md                    |  5 -----
 src/sixtypical/analyzer.py   | 28 ++++++++++++++++++++--------
 tests/SixtyPical Analysis.md |  4 ++--
 4 files changed, 23 insertions(+), 15 deletions(-)

diff --git a/HISTORY.md b/HISTORY.md
index 0f2e050..8d00f67 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -6,6 +6,7 @@ History of SixtyPical
 
 *   `copy` is now understood to trash `a`, thus `copy ..., a` is not valid.
     Indirect addressing is supported in `ld`, as in `ld a, [ptr] + y`, to compensate.
+*   Implements the "union rule for trashes" when analyzing `if` blocks.
 *   Fixed bug where `trash` was not marking the location as being virtually altered.
 
 0.11
diff --git a/README.md b/README.md
index 756f013..ef9ea2c 100644
--- a/README.md
+++ b/README.md
@@ -67,11 +67,6 @@ that call this routine.
 These might be forced to specify an initial value so that they can always be
 assumed to be meaningful.
 
-### Union rule for trashes in `if`
-
-If one branch trashes {`a`} and the other branch trashes {`b`} then the whole
-`if` statement trashes {`a`, `b`}.
-
 ### Re-order routines and optimize tail-calls to fallthroughs
 
 Not because it saves 3 bytes, but because it's a neat trick.  Doing it optimally
diff --git a/src/sixtypical/analyzer.py b/src/sixtypical/analyzer.py
index 64083bc..c90c2e7 100644
--- a/src/sixtypical/analyzer.py
+++ b/src/sixtypical/analyzer.py
@@ -101,13 +101,6 @@ class Context(object):
         c._writeable = set(self._writeable)
         return c
 
-    def set_from(self, c):
-        assert c.routines == self.routines
-        assert c.routine == self.routine
-        self._touched = set(c._touched)
-        self._meaningful = set(c._meaningful)
-        self._writeable = set(c._writeable)
-
     def each_meaningful(self):
         for ref in self._meaningful:
             yield ref
@@ -330,25 +323,44 @@ class Analyzer(object):
                 context.set_touched(ref)
                 context.set_unmeaningful(ref)
         elif opcode == 'if':
+            incoming_meaningful = set(context.each_meaningful())
+
             context1 = context.clone()
             context2 = context.clone()
             self.analyze_block(instr.block1, context1)
             if instr.block2 is not None:
                 self.analyze_block(instr.block2, context2)
+
+            outgoing_meaningful = set(context1.each_meaningful()) & set(context2.each_meaningful())
+            outgoing_trashes = incoming_meaningful - outgoing_meaningful
+
             # TODO may we need to deal with touched separately here too?
             # probably not; if it wasn't meaningful in the first place, it
             # doesn't really matter if you modified it or not, coming out.
             for ref in context1.each_meaningful():
+                if ref in outgoing_trashes:
+                    continue
                 context2.assert_meaningful(
                     ref, exception_class=InconsistentInitializationError,
                     message='initialized in block 1 but not in block 2 of `if {}`'.format(src)
                 )
             for ref in context2.each_meaningful():
+                if ref in outgoing_trashes:
+                    continue
                 context1.assert_meaningful(
                     ref, exception_class=InconsistentInitializationError,
                     message='initialized in block 2 but not in block 1 of `if {}`'.format(src)
                 )
-            context.set_from(context1)
+
+            # merge the contexts.  this used to be a method called `set_from`
+            context._touched = set(context1._touched) | set(context2._touched)
+            context._meaningful = outgoing_meaningful
+            context._writeable = set(context1._writeable) | set(context2._writeable)
+
+            for ref in outgoing_trashes:
+                context.set_touched(ref)
+                context.set_unmeaningful(ref)
+
         elif opcode == 'repeat':
             # it will always be executed at least once, so analyze it having
             # been executed the first time.
diff --git a/tests/SixtyPical Analysis.md b/tests/SixtyPical Analysis.md
index 1105302..39c8603 100644
--- a/tests/SixtyPical Analysis.md	
+++ b/tests/SixtyPical Analysis.md	
@@ -1417,7 +1417,7 @@ trashes {`a`, `b`}.
     |         trash x
     |     }
     | }
-    ? UnmeaningfulOutputError: x in foo
+    ? ForbiddenWriteError: x in foo
 
     | routine foo
     |   inputs a, x, z
@@ -1429,7 +1429,7 @@ trashes {`a`, `b`}.
     |         trash x
     |     }
     | }
-    ? UnmeaningfulOutputError: a in foo
+    ? ForbiddenWriteError: a in foo
 
 ### repeat ###