Improve getFallthrough (#1643)

That method looks through tee_locals and other operations that receive a value and let it flow through them, like a block's final value, etc. It just handled a few such operations, with this PR all of them should be handled.

Also refactor it out of the OptimizeInstructions pass as I think it may be useful for propagating returned constants.

2 files changed, 38 insertions, 18 deletions

diff --git a/src/ir/properties.h b/src/ir/properties.h
index 7cbbe73f0..6848e9481 100644
--- a/src/ir/properties.h
+++ b/src/ir/properties.h
@@ -146,6 +146,42 @@ inline Index getZeroExtBits(Expression* curr) {
   return Bits::getMaskedBits(curr->cast<Binary>()->right->cast<Const>()->value.geti32());
 }
 
+// Returns a falling-through value, that is, it looks through a tee_local
+// and other operations that receive a value and let it flow through them.
+inline Expression* getFallthrough(Expression* curr) {
+  // If the current node is unreachable, there is no value
+  // falling through.
+  if (curr->type == unreachable) {
+    return curr;
+  }
+  if (auto* set = curr->dynCast<SetLocal>()) {
+    if (set->isTee()) {
+      return getFallthrough(set->value);
+    }
+  } else if (auto* block = curr->dynCast<Block>()) {
+    // if no name, we can't be broken to, and then can look at the fallthrough
+    if (!block->name.is() && block->list.size() > 0) {
+      return getFallthrough(block->list.back());
+    }
+  } else if (auto* loop = curr->dynCast<Loop>()) {
+    return getFallthrough(loop->body);
+  } else if (auto* iff = curr->dynCast<If>()) {
+    if (iff->ifFalse) {
+      // Perhaps just one of the two actually returns.
+      if (iff->ifTrue->type == unreachable) {
+        return getFallthrough(iff->ifFalse);
+      } else if (iff->ifFalse->type == unreachable) {
+        return getFallthrough(iff->ifTrue);
+      }
+    }
+  } else if (auto* br = curr->dynCast<Break>()) {
+    if (br->condition && br->value) {
+      return getFallthrough(br->value);
+    }
+  }
+  return curr;
+}
+
 } // Properties
 
 } // wasm
diff --git a/src/passes/OptimizeInstructions.cpp b/src/passes/OptimizeInstructions.cpp
index 55042d96f..73f78611b 100644
--- a/src/passes/OptimizeInstructions.cpp
+++ b/src/passes/OptimizeInstructions.cpp
@@ -258,22 +258,6 @@ Index getMaxBits(Expression* curr, LocalInfoProvider* localInfoProvider) {
   }
 }
 
-// looks through fallthrough operations, like tee_local, block fallthrough, etc.
-// too and block fallthroughs, etc.
-Expression* getFallthrough(Expression* curr) {
-  if (auto* set = curr->dynCast<SetLocal>()) {
-    if (set->isTee()) {
-      return getFallthrough(set->value);
-    }
-  } else if (auto* block = curr->dynCast<Block>()) {
-    // if no name, we can't be broken to, and then can look at the fallthrough
-    if (!block->name.is() && block->list.size() > 0) {
-      return getFallthrough(block->list.back());
-    }
-  }
-  return curr;
-}
-
 // Useful information about locals
 struct LocalInfo {
   static const Index kUnknown = Index(-1);
@@ -316,7 +300,7 @@ struct LocalScanner : PostWalker<LocalScanner> {
     auto type = getFunction()->getLocalType(curr->index);
     if (type != i32 && type != i64) return;
     // an integer var, worth processing
-    auto* value = getFallthrough(curr->value);
+    auto* value = Properties::getFallthrough(curr->value);
     auto& info = localInfo[curr->index];
     info.maxBits = std::max(info.maxBits, getMaxBits(value, this));
     auto signExtBits = LocalInfo::kUnknown;
@@ -421,7 +405,7 @@ struct OptimizeInstructions : public WalkerPass<PostWalker<OptimizeInstructions,
         Index extraShifts;
         auto bits = Properties::getAlmostSignExtBits(binary, extraShifts);
         if (extraShifts == 0) {
-          if (auto* load = getFallthrough(ext)->dynCast<Load>()) {
+          if (auto* load = Properties::getFallthrough(ext)->dynCast<Load>()) {
             // pattern match a load of 8 bits and a sign extend using a shl of 24 then shr_s of 24 as well, etc.
             if (LoadUtils::canBeSigned(load) &&
                 ((load->bytes == 1 && bits == 8) || (load->bytes == 2 && bits == 16))) {