refactor effective shift size computation

Author: kripken

src/ast/bits.h

@@ -39,6 +39,17 @@ struct Bits {
     // this is indeed a mask
     return 32 - CountLeadingZeroes(mask);
   }
+
+  // gets the number of effective shifts a shift operation does. In
+  // wasm, only 5 bits matter for 32-bit shifts, and 6 for 64.
+  static uint32_t getEffectiveShifts(Const* amount) {
+    if (amount->type == i32) {
+      return amount->value.geti32() & 31;
+    } else if (amount->type == i64) {
+      return amount->value.geti64() & 63;
+    }
+    WASM_UNREACHABLE();
+  }
 };
 
 } // namespace wasm

src/passes/OptimizeInstructions.cpp

@@ -188,14 +188,14 @@ Index getMaxBits(Expression* curr, LocalInfoProvider* localInfoProvider) {
       case OrInt32: case XorInt32: return std::max(getMaxBits(binary->left, localInfoProvider), getMaxBits(binary->right, localInfoProvider));
       case ShlInt32: {
         if (auto* shifts = binary->right->dynCast<Const>()) {
-          return std::min(Index(32), getMaxBits(binary->left, localInfoProvider) + (shifts->value.geti32() & 31));
+          return std::min(Index(32), getMaxBits(binary->left, localInfoProvider) + Bits::getEffectiveShifts(shifts));
         }
         return 32;
       }
       case ShrUInt32: {
         if (auto* shift = binary->right->dynCast<Const>()) {
           auto maxBits = getMaxBits(binary->left, localInfoProvider);
-          auto shifts = std::min(Index(shift->value.geti32() & 31), maxBits); // can ignore more shifts than zero us out
+          auto shifts = std::min(Index(Bits::getEffectiveShifts(shift)), 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() & 31), maxBits); // can ignore more shifts than zero us out
+          auto shifts = std::min(Index(Bits::getEffectiveShifts(shift)), maxBits); // can ignore more shifts than zero us out
           return std::max(Index(0), maxBits - shifts);
         }
         return 32;