Fix AvoidReinterprets on reinterpreted loads of fewer than the full size (#2123)

* fix

* fix style

Author: kripken

src/passes/AvoidReinterprets.cpp

@@ -27,6 +27,14 @@
 
 namespace wasm {
 
+static bool canReplaceWithReinterpret(Load* load) {
+  // We can replace a full-size load with a valid pointer with
+  // a reinterpret of the same address. A partial load would see
+  // more bytes and possibly invalid data, and an unreachable
+  // pointer is just not interesting to handle.
+  return load->type != unreachable && load->bytes == getTypeSize(load->type);
+}
+
 static Load* getSingleLoad(LocalGraph* localGraph, GetLocal* get) {
   std::set<GetLocal*> seen;
   seen.insert(get);
@@ -104,7 +112,7 @@ struct AvoidReinterprets : public WalkerPass<PostWalker<AvoidReinterprets>> {
     for (auto& pair : infos) {
       auto* load = pair.first;
       auto& info = pair.second;
-      if (info.reinterpreted && load->type != unreachable) {
+      if (info.reinterpreted && canReplaceWithReinterpret(load)) {
         // We should use another load here, to avoid reinterprets.
         info.ptrLocal = Builder::addVar(func, i32);
         info.reinterpretedLocal =
@@ -131,8 +139,10 @@ struct AvoidReinterprets : public WalkerPass<PostWalker<AvoidReinterprets>> {
         if (isReinterpret(curr)) {
           auto* value = Properties::getFallthrough(curr->value);
           if (auto* load = value->dynCast<Load>()) {
-            // A reinterpret of a load - flip it right here.
-            replaceCurrent(makeReinterpretedLoad(load, load->ptr));
+            // A reinterpret of a load - flip it right here if we can.
+            if (canReplaceWithReinterpret(load)) {
+              replaceCurrent(makeReinterpretedLoad(load, load->ptr));
+            }
           } else if (auto* get = value->dynCast<GetLocal>()) {
             if (auto* load = getSingleLoad(localGraph, get)) {
               auto iter = infos.find(load);

test/passes/avoid-reinterprets.txt

@@ -1,5 +1,6 @@
 (module
  (type $0 (func))
+ (type $1 (func (result f32)))
  (memory $0 1)
  (func $simple (; 0 ;) (type $0)
   (drop
@@ -148,4 +149,18 @@
    (local.get $3)
   )
  )
+ (func $partial1 (; 6 ;) (type $1) (result f32)
+  (f32.reinterpret_i32
+   (i32.load16_u
+    (i32.const 3)
+   )
+  )
+ )
+ (func $partial2 (; 7 ;) (type $1) (result f32)
+  (f32.reinterpret_i32
+   (i32.load8_u
+    (i32.const 3)
+   )
+  )
+ )
 )

test/passes/avoid-reinterprets.wast

@@ -35,4 +35,18 @@
     (local.set $y (local.get $x))
     (drop (f32.reinterpret_i32 (local.get $y)))
   )
+  (func $partial1 (result f32)
+   (f32.reinterpret_i32
+    (i32.load16_u
+     (i32.const 3)
+    )
+   )
+  )
+  (func $partial2 (result f32)
+   (f32.reinterpret_i32
+    (i32.load8_u
+     (i32.const 3)
+    )
+   )
+  )
 )