[dict-proto] C++ implementation of SwissNameDictionary, pt. 2

This CL is part of a series that adds the C++ implementation of
SwissNameDictionary, a deterministic property backing store based on
Swiss Tables.

This CL adds swiss-hash-table-helpers.h, which contains helpers
that are mostly independent from a particular swiss table
implementation (like SwissNameDIctionary) and can therefore be re-used
by potential other Swiss Table implementations in the future. As a
consequence of that, those helpers are largely taken unchanged from
Abseil.

Bug: v8:11388
Change-Id: I27636731c2166cb10240b847a1d7df0412aa0a33
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2687752
Commit-Queue: Frank Emrich <emrich@google.com>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72734}

BUILD.gn

@@ -3270,6 +3270,7 @@ v8_source_set("v8_base_without_compiler") {
     "src/objects/string.h",
     "src/objects/struct-inl.h",
     "src/objects/struct.h",
+    "src/objects/swiss-hash-table-helpers.h",
     "src/objects/swiss-name-dictionary-inl.h",
     "src/objects/swiss-name-dictionary.cc",
     "src/objects/swiss-name-dictionary.h",

src/base/bits.h

@@ -107,6 +107,8 @@ inline constexpr unsigned CountLeadingZeros64(uint64_t value) {
 // CountTrailingZeros(value) returns the number of zero bits preceding the
 // least significant 1 bit in |value| if |value| is non-zero, otherwise it
 // returns {sizeof(T) * 8}.
+// See CountTrailingZerosNonZero for an optimized version for the case that
+// |value| is guaranteed to be non-zero.
 template <typename T, unsigned bits = sizeof(T) * 8>
 inline constexpr
     typename std::enable_if<std::is_integral<T>::value && sizeof(T) <= 8,
@@ -133,6 +135,24 @@ inline constexpr unsigned CountTrailingZeros64(uint64_t value) {
   return CountTrailingZeros(value);
 }
 
+// CountTrailingZerosNonZero(value) returns the number of zero bits preceding
+// the least significant 1 bit in |value| if |value| is non-zero, otherwise the
+// behavior is undefined.
+// See CountTrailingZeros for an alternative version that allows |value| == 0.
+template <typename T, unsigned bits = sizeof(T) * 8>
+inline constexpr
+    typename std::enable_if<std::is_integral<T>::value && sizeof(T) <= 8,
+                            unsigned>::type
+    CountTrailingZerosNonZero(T value) {
+  CONSTEXPR_DCHECK(value != 0);
+#if V8_HAS_BUILTIN_CTZ
+  return bits == 64 ? __builtin_ctzll(static_cast<uint64_t>(value))
+                    : __builtin_ctz(static_cast<uint32_t>(value));
+#else
+  return CountTrailingZeros<T, bits>(value);
+#endif
+}
+
 // Returns true iff |value| is a power of 2.
 template <typename T,
           typename = typename std::enable_if<std::is_integral<T>::value ||

src/objects/swiss-name-dictionary.h

@@ -5,11 +5,14 @@
 #ifndef V8_OBJECTS_SWISS_NAME_DICTIONARY_H_
 #define V8_OBJECTS_SWISS_NAME_DICTIONARY_H_
 
+#include <cstdint>
+
 #include "src/base/export-template.h"
 #include "src/common/globals.h"
 #include "src/objects/fixed-array.h"
 #include "src/objects/internal-index.h"
 #include "src/objects/js-objects.h"
+#include "src/objects/swiss-hash-table-helpers.h"
 #include "src/roots/roots.h"
 
 // Has to be the last include (doesn't have include guards):
@@ -68,6 +71,8 @@ namespace internal {
 //       corresponding bucket  hasn't been used before.
 class SwissNameDictionary : public HeapObject {
  public:
+  using Group = swiss_table::Group;
+
   inline int Capacity();
 
   inline static constexpr bool IsValidCapacity(int capacity);