[SignExt] OptimizeInstructions: Remove signexts of already-extended v…

…alues (WebAssembly#7072)

src/passes/OptimizeInstructions.cpp

@@ -80,8 +80,8 @@ static bool isSignedOp(BinaryOp op) {
 struct LocalInfo {
   static const Index kUnknown = Index(-1);
 
-  Index maxBits;
-  Index signExtedBits;
+  Index maxBits = -1;
+  Index signExtBits = 0;
 };
 
 struct LocalScanner : PostWalker<LocalScanner> {
@@ -99,18 +99,18 @@ struct LocalScanner : PostWalker<LocalScanner> {
       auto& info = localInfo[i];
       if (func->isParam(i)) {
         info.maxBits = getBitsForType(func->getLocalType(i)); // worst-case
-        info.signExtedBits = LocalInfo::kUnknown; // we will never know anything
+        info.signExtBits = LocalInfo::kUnknown; // we will never know anything
       } else {
-        info.maxBits = info.signExtedBits = 0; // we are open to learning
+        info.maxBits = info.signExtBits = 0; // we are open to learning
       }
     }
     // walk
     PostWalker<LocalScanner>::doWalkFunction(func);
     // finalize
     for (Index i = 0; i < func->getNumLocals(); i++) {
       auto& info = localInfo[i];
-      if (info.signExtedBits == LocalInfo::kUnknown) {
-        info.signExtedBits = 0;
+      if (info.signExtBits == LocalInfo::kUnknown) {
+        info.signExtBits = 0;
       }
     }
   }
@@ -137,11 +137,11 @@ struct LocalScanner : PostWalker<LocalScanner> {
         signExtBits = load->bytes * 8;
       }
     }
-    if (info.signExtedBits == 0) {
-      info.signExtedBits = signExtBits; // first info we see
-    } else if (info.signExtedBits != signExtBits) {
+    if (info.signExtBits == 0) {
+      info.signExtBits = signExtBits; // first info we see
+    } else if (info.signExtBits != signExtBits) {
       // contradictory information, give up
-      info.signExtedBits = LocalInfo::kUnknown;
+      info.signExtBits = LocalInfo::kUnknown;
     }
   }
 
@@ -1006,6 +1006,22 @@ struct OptimizeInstructions
       }
     }
 
+    // Simple sign extends can be removed if the value is already sign-extended.
+    auto signExtBits = getSignExtBits(curr->value);
+    if (signExtBits > 0) {
+      // Note that we can handle the case of |curr| having a larger sign-extend:
+      // if we have an 8-bit value in 32-bit, then there are 24 sign bits, and
+      // doing a sign-extend to 16 will only affect 16 of those 24, and the
+      // effect is to leave them as they are.
+      if ((curr->op == ExtendS8Int32 && signExtBits <= 8) ||
+          (curr->op == ExtendS16Int32 && signExtBits <= 16) ||
+          (curr->op == ExtendS8Int64 && signExtBits <= 8) ||
+          (curr->op == ExtendS16Int64 && signExtBits <= 16) ||
+          (curr->op == ExtendS32Int64 && signExtBits <= 32)) {
+        return replaceCurrent(curr->value);
+      }
+    }
+
     if (Abstract::hasAnyReinterpret(curr->op)) {
       // i32.reinterpret_f32(f32.reinterpret_i32(x))  =>  x
       // i64.reinterpret_f64(f64.reinterpret_i64(x))  =>  x
@@ -3611,16 +3627,22 @@ struct OptimizeInstructions
     return inner;
   }
 
-  // check if an expression is already sign-extended
+  // Check if an expression is already sign-extended to an exact number of bits.
   bool isSignExted(Expression* curr, Index bits) {
+    return getSignExtBits(curr) == bits;
+  }
+
+  // Returns the number of bits an expression is sign-extended (or 0 if it is
+  // not).
+  Index getSignExtBits(Expression* curr) {
     if (Properties::getSignExtValue(curr)) {
-      return Properties::getSignExtBits(curr) == bits;
+      return Properties::getSignExtBits(curr);
     }
     if (auto* get = curr->dynCast<LocalGet>()) {
-      // check what we know about the local
-      return localInfo[get->index].signExtedBits == bits;
+      // Check what we know about the local.
+      return localInfo[get->index].signExtBits;
     }
-    return false;
+    return 0;
   }
 
   // optimize trivial math operations, given that the right side of a binary