Add helper for lossless conversion of 31-bit pattern to Smi

R=sigurds@chromium.org

Change-Id: Ia6874ba5a705dda3a6f2b971cf50618a2229a40f
Reviewed-on: https://chromium-review.googlesource.com/c/1466502Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59525}

src/feedback-vector.cc

@@ -749,11 +749,7 @@ bool FeedbackNexus::ConfigureLexicalVarMode(int script_context_index,
                SlotIndexBits::encode(context_slot_index) |
                ImmutabilityBit::encode(immutable);
 
-  // Force {config} to be in Smi range by propagating the most significant Smi
-  // bit. This does not change any of the bitfield's bits.
-  config = (config << (32 - kSmiValueSize)) >> (32 - kSmiValueSize);
-
-  SetFeedback(Smi::FromInt(config));
+  SetFeedback(Smi::From31BitPattern(config));
   Isolate* isolate = GetIsolate();
   SetFeedbackExtra(*FeedbackVector::UninitializedSentinel(isolate),
                    SKIP_WRITE_BARRIER);

src/objects/smi.h

@@ -57,6 +57,13 @@ class Smi : public Object {
     return Smi((static_cast<Address>(value) << smi_shift_bits) | kSmiTag);
   }
 
+  // Given {value} in [0, 2^31-1], force it into Smi range by changing at most
+  // the MSB (leaving the lower 31 bit unchanged).
+  static inline Smi From31BitPattern(int value) {
+    return Smi::FromInt((value << (32 - kSmiValueSize)) >>
+                        (32 - kSmiValueSize));
+  }
+
   template <typename E,
             typename = typename std::enable_if<std::is_enum<E>::value>::type>
   static inline Smi FromEnum(E value) {