fix shift computation in getMaxBits - in wasm only the lower 5 bits matter for a 32-bit shift

Author: kripken

src/passes/OptimizeInstructions.cpp

@@ -195,7 +195,7 @@ Index getMaxBits(Expression* curr, LocalInfoProvider* localInfoProvider) {
       case ShrUInt32: {
         if (auto* shift = binary->right->dynCast<Const>()) {
           auto maxBits = getMaxBits(binary->left, localInfoProvider);
-          auto shifts = std::min(Index(shift->value.geti32()), maxBits); // can ignore more shifts than zero us out
+          auto shifts = std::min(Index(shift->value.geti32() & 31), maxBits); // can ignore more shifts than zero us out
           return std::max(Index(0), maxBits - shifts);
         }
         return 32;
@@ -204,7 +204,7 @@ Index getMaxBits(Expression* curr, LocalInfoProvider* localInfoProvider) {
         if (auto* shift = binary->right->dynCast<Const>()) {
           auto maxBits = getMaxBits(binary->left, localInfoProvider);
           if (maxBits == 32) return 32;
-          auto shifts = std::min(Index(shift->value.geti32()), maxBits); // can ignore more shifts than zero us out
+          auto shifts = std::min(Index(shift->value.geti32() & 31), maxBits); // can ignore more shifts than zero us out
           return std::max(Index(0), maxBits - shifts);
         }
         return 32;

test/passes/optimize-instructions.txt

@@ -805,7 +805,7 @@
      (i32.const -1)
      (i32.const 2147483647)
     )
-    (i32.const 32)
+    (i32.const 31)
    )
   )
   (drop
@@ -2006,4 +2006,13 @@
    (i32.const 1)
   )
  )
+ (func $neg-shifts-and-255 (type $2) (result i32)
+  (i32.and
+   (i32.shr_u
+    (i32.const -99)
+    (i32.const -32)
+   )
+   (i32.const 255)
+  )
+ )
 )

test/passes/optimize-instructions.wast

@@ -842,7 +842,7 @@
               (i32.const -1)
               (i32.const 2147483647)
             )
-            (i32.const 32)
+            (i32.const 31) ;; adjusted after we fixed shift computation to just look at lower 5 bits
           )
           (i32.const 24)
         )
@@ -2433,4 +2433,13 @@
     (i32.const 1)
    )
   )
+  (func $neg-shifts-and-255 (result i32)
+    (i32.and
+     (i32.shr_u
+      (i32.const -99)
+      (i32.const -32) ;; this shift does nothing
+     )
+     (i32.const 255)
+    )
+  )
 )