optimize shifts of 0

Author: kripken

src/passes/OptimizeInstructions.cpp

@@ -517,6 +517,12 @@ struct OptimizeInstructions : public WalkerPass<PostWalker<OptimizeInstructions,
             }
           }
         }
+        // some operations have no effect TODO: many more
+        if (right->value == Literal(int32_t(0))) {
+          if (binary->op == ShlInt32 || binary->op == ShrUInt32 || binary->op == ShrSInt32) {
+            return binary->left;
+          }
+        }
         // the square of some operations can be merged
         if (auto* left = binary->left->dynCast<Binary>()) {
           if (left->op == binary->op) {

test/passes/optimize-instructions.txt

@@ -1965,31 +1965,32 @@
  (func $zero-shifts-is-not-sign-ext (type $1)
   (drop
    (i32.eq
-    (i32.shr_s
-     (i32.shl
-      (i32.load16_s align=1
-       (i32.const 790656516)
-      )
-      (i32.const 0)
-     )
-     (i32.const 0)
+    (i32.load16_s align=1
+     (i32.const 790656516)
     )
     (i32.const -5431187)
    )
   )
   (drop
    (i32.eq
-    (i32.shr_s
-     (i32.shl
-      (i32.load16_s align=1
-       (i32.const 790656516)
-      )
-      (i32.const 1)
+    (i32.shl
+     (i32.load16_s align=1
+      (i32.const 790656516)
      )
-     (i32.const 0)
+     (i32.const 1)
     )
     (i32.const -5431187)
    )
   )
  )
+ (func $zero-ops (type $2) (result i32)
+  (return
+   (i32.eq
+    (i32.load16_s align=1
+     (i32.const 790656516)
+    )
+    (i32.const -1337)
+   )
+  )
+ )
 )

test/passes/optimize-instructions.wast

@@ -2395,4 +2395,26 @@
     )
    )
   )
+  (func $zero-ops (result i32)
+   (return
+    (i32.eq
+     (i32.const -1337)
+     (i32.shr_u
+      (i32.add
+       (i32.const 0)
+       (i32.shr_s
+        (i32.shl
+         (i32.load16_s align=1
+          (i32.const 790656516)
+         )
+         (i32.const 0)
+        )
+        (i32.const 0)
+       )
+      )
+      (i32.const 0)
+     )
+    )
+   )
+  )
 )