[vm] Fix a bug in Utils::NBitMask.

The function was broken on arm64 due to incorrect
mask calculation (based on the target ISA).

Change-Id: Ib5c26bf4c7b901ae27f2a0580c4f88b2b7952212
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/121709
Commit-Queue: Vyacheslav Egorov <vegorov@google.com>
Reviewed-by: Aart Bik <ajcbik@google.com>

Author: Artem Serov

runtime/platform/utils.h

@@ -340,11 +340,9 @@ class Utils {
   static uword NBitMask(uint32_t n) {
     ASSERT(n <= kBitsPerWord);
     if (n == kBitsPerWord) {
-#if defined(TARGET_ARCH_X64)
-      return 0xffffffffffffffffll;
-#else
-      return 0xffffffff;
-#endif
+      static_assert((sizeof(uword) * kBitsPerByte) == kBitsPerWord,
+                            "Unexpected uword size");
+      return std::numeric_limits<uword>::max();
     }
     return (1ll << n) - 1;
   }

runtime/vm/utils_test.cc

@@ -323,4 +323,31 @@ VM_UNIT_TEST_CASE(DoublesBitEqual) {
   EXPECT(Utils::DoublesBitEqual(NAN, NAN));
 }
 
+VM_UNIT_TEST_CASE(NBitMask) {
+#if defined(ARCH_IS_64_BIT)
+  EXPECT_EQ(0ull, Utils::NBitMask(0));
+  EXPECT_EQ(0x1ull, Utils::NBitMask(1));
+  EXPECT_EQ(0x3ull, Utils::NBitMask(2));
+  EXPECT_EQ(0xfull, Utils::NBitMask(4));
+  EXPECT_EQ(0xffull, Utils::NBitMask(8));
+  EXPECT_EQ(0xffffull, Utils::NBitMask(16));
+  EXPECT_EQ(0x1ffffull, Utils::NBitMask(17));
+  EXPECT_EQ(0x7fffffffull, Utils::NBitMask(31));
+  EXPECT_EQ(0xffffffffull, Utils::NBitMask(32));
+  EXPECT_EQ(0x1ffffffffull, Utils::NBitMask(33));
+  EXPECT_EQ(0x7fffffffffffffffull, Utils::NBitMask(kBitsPerWord - 1));
+  EXPECT_EQ(0xffffffffffffffffull, Utils::NBitMask(kBitsPerWord));
+#else
+  EXPECT_EQ(0u, Utils::NBitMask(0));
+  EXPECT_EQ(0x1u, Utils::NBitMask(1));
+  EXPECT_EQ(0x3u, Utils::NBitMask(2));
+  EXPECT_EQ(0xfu, Utils::NBitMask(4));
+  EXPECT_EQ(0xffu, Utils::NBitMask(8));
+  EXPECT_EQ(0xffffu, Utils::NBitMask(16));
+  EXPECT_EQ(0x1ffffu, Utils::NBitMask(17));
+  EXPECT_EQ(0x7fffffffu, Utils::NBitMask(kBitsPerWord - 1));
+  EXPECT_EQ(0xffffffffu, Utils::NBitMask(kBitsPerWord));
+#endif
+}
+
 }  // namespace dart