VN: don't propagate values across protected regions

Contributes to #33

src/DistIL/Analysis/ProtectedRegionAnalysis.cs

@@ -95,4 +95,7 @@ public IEnumerable<BasicBlock> GetExitBlocks()
         }
         return null;
     }
+
+    /// <summary> If this is a handler region, returns the guard defining it. </summary>
+    public GuardInst? GetHandlerGuard() => StartBlock.Users().OfType<GuardInst>().FirstOrDefault();
 }

src/DistIL/CodeGen/Cil/LayoutedCFG.cs

@@ -65,7 +65,7 @@ AbsRange LayoutRegion(ProtectedRegion node)
             foreach (var child in node.Children) {
                 PlaceAntecessorBlocks(child.StartBlock);
                 var range = LayoutRegion(child);
-                var guard = child.StartBlock.Users().OfType<GuardInst>().FirstOrDefault();
+                var guard = child.GetHandlerGuard();
 
                 if (guard != null) {
                     clauses[clauseIndices[guard]].UpdateRanges(child.StartBlock, range);

src/DistIL/IR/Utils/IRVerifier.cs

@@ -107,7 +107,7 @@ private void CheckTerminator(BasicBlock block, ProtectedRegionAnalysis regionAna
                 if (region == regionAnalysis.Root) {
                     Error(block, "Cannot leave or resume from root region");
                 } else if (block.Last is ResumeInst resume) {
-                    var guard = region.StartBlock.Users().OfType<GuardInst>().FirstOrDefault();
+                    var guard = region.GetHandlerGuard();
 
                     if (guard == null) {
                         Error(block, "Missing guard for filter or finally region");

src/DistIL/Passes/ValueNumbering.cs

@@ -11,6 +11,7 @@ public MethodPassResult Run(MethodTransformContext ctx)
             DomTree = ctx.GetAnalysis<DominatorTree>(),
             AliasAnalysis = ctx.GetAnalysis<AliasAnalysis>(),
             FuncInfo = ctx.Compilation.GetAnalysis<GlobalFunctionEffects>(),
+            RegionAnalysis = ctx.GetAnalysis<ProtectedRegionAnalysis>(),
         };
         bool madeChanges = false;
 
@@ -36,6 +37,7 @@ class ValueTable
         public required DominatorTree DomTree;
         public required AliasAnalysis AliasAnalysis;
         public required GlobalFunctionEffects FuncInfo;
+        public required ProtectedRegionAnalysis RegionAnalysis;
 
         public bool Process(Instruction inst)
         {
@@ -70,6 +72,13 @@ public bool CheckAvail(Instruction def, Instruction user)
             // Trivial reject
             if (!DomTree.Dominates(def.Block, user.Block)) return false;
 
+            // Don't propagate values across different regions. #33
+            // This check is a bit conservative, there are cases where
+            // it is safe to propagate values into child regions.
+            var defRegion = RegionAnalysis.GetBlockRegion(def.Block);
+            var useRegion = RegionAnalysis.GetBlockRegion(user.Block);
+            if (defRegion != useRegion) return false;
+
             // Trivial accept instructions that don't access memory or are pure
             if (!(def is MemoryInst or CallInst)) return true;