Export of internal Abseil changes

--
fcedaa5714efab8738446fa21620b827a40a3458 by Derek Mauro <dmauro@google.com>:

Uses Wyhash in the implementation of absl::Hash for hashing sequences
of more than 16 bytes on some platforms.

Due to the per-process randomization of the seed used by absl::Hash, users
should not notice this change, other than possibly getting better performance.

This change only affects platforms where absl::uint128 is implemented
with an intrinsic (and where sizeof(size_t)==8) since Wyhash relies on
fast 128-bit multiplication for speed.

PiperOrigin-RevId: 343956735

--
085e108c760084f19caa21dbeb2118de2be3f8f0 by Abseil Team <absl-team@google.com>:

Internal change for cord ring

PiperOrigin-RevId: 343919274

--
4c333278ad14d6692f203074b902506008ad624a by Jorg Brown <jorg@google.com>:

Minimize strings_internal::StringConstant further, by removing the need for the compiler to instantiate a compile-time class function.

PiperOrigin-RevId: 343878568

--
71c3c8c7b7821b67997e3d5345aaec67f93f266f by Abseil Team <absl-team@google.com>:

Internal change

PiperOrigin-RevId: 343838259
GitOrigin-RevId: fcedaa5714efab8738446fa21620b827a40a3458
Change-Id: Ifb91895a82d11e743acd42fe97ab7fb70712b7df

3 files changed, 26 insertions, 13 deletions

diff --git a/absl/strings/cord.cc b/absl/strings/cord.cc
index 9efd1357..ec1e9709 100644
--- a/absl/strings/cord.cc
+++ b/absl/strings/cord.cc
@@ -53,6 +53,7 @@ using ::absl::cord_internal::CordRepSubstring;
 using ::absl::cord_internal::CONCAT;
 using ::absl::cord_internal::EXTERNAL;
 using ::absl::cord_internal::FLAT;
+using ::absl::cord_internal::MAX_FLAT_TAG;
 using ::absl::cord_internal::SUBSTRING;
 
 namespace cord_internal {
@@ -95,9 +96,21 @@ static const size_t kFlatOverhead = offsetof(CordRep, data);
 // Flat allocation size is stored in tag, which currently can encode sizes up
 // to 4K, encoded as multiple of either 8 or 32 bytes.
 // If we allow for larger sizes, we need to change this to 8/64, 16/128, etc.
+// kMinFlatSize is bounded by tag needing to be at least FLAT * 8 bytes, and
+// ideally a 'nice' size aligning with allocation and cacheline sizes like 32.
+// kMaxFlatSize is bounded by the size resulting in a computed tag no greater
+// than MAX_FLAT_TAG. MAX_FLAT_TAG provides for additional 'high' tag values.
+static constexpr size_t kMinFlatSize = 32;
 static constexpr size_t kMaxFlatSize = 4096;
 static constexpr size_t kMaxFlatLength = kMaxFlatSize - kFlatOverhead;
-static constexpr size_t kMinFlatLength = 32 - kFlatOverhead;
+static constexpr size_t kMinFlatLength = kMinFlatSize - kFlatOverhead;
+
+static constexpr size_t AllocatedSizeToTagUnchecked(size_t size) {
+  return (size <= 1024) ? size / 8 : 128 + size / 32 - 1024 / 32;
+}
+
+static_assert(kMinFlatSize / 8 >= FLAT, "");
+static_assert(AllocatedSizeToTagUnchecked(kMaxFlatSize) <= MAX_FLAT_TAG, "");
 
 // Prefer copying blocks of at most this size, otherwise reference count.
 static const size_t kMaxBytesToCopy = 511;
@@ -117,7 +130,7 @@ static size_t RoundUpForTag(size_t size) {
 // undefined if the size exceeds the maximum size that can be encoded in
 // a tag, i.e., if size is larger than TagToAllocatedSize(<max tag>).
 static uint8_t AllocatedSizeToTag(size_t size) {
-  const size_t tag = (size <= 1024) ? size / 8 : 128 + size / 32 - 1024 / 32;
+  const size_t tag = AllocatedSizeToTagUnchecked(size);
   assert(tag <= std::numeric_limits<uint8_t>::max());
   return tag;
 }
diff --git a/absl/strings/internal/cord_internal.h b/absl/strings/internal/cord_internal.h
index aa91a691..195a7988 100644
--- a/absl/strings/internal/cord_internal.h
+++ b/absl/strings/internal/cord_internal.h
@@ -116,10 +116,16 @@ enum CordRepKind {
   CONCAT        = 0,
   EXTERNAL      = 1,
   SUBSTRING     = 2,
+  RING          = 3,
 
   // We have different tags for different sized flat arrays,
-  // starting with FLAT
-  FLAT          = 3,
+  // starting with FLAT, and limited to MAX_FLAT_TAG. The 224 value is based on
+  // the current 'size to tag' encoding of 8 / 32 bytes. If a new tag is needed
+  // in the future, then 'FLAT' and 'MAX_FLAT_TAG' should be adjusted as well
+  // as the Tag <---> Size logic so that FLAT stil represents the minimum flat
+  // allocation size. (32 bytes as of now).
+  FLAT = 4,
+  MAX_FLAT_TAG = 224,
 };
 
 struct CordRep {
diff --git a/absl/strings/internal/string_constant.h b/absl/strings/internal/string_constant.h
index b15f1d9b..a11336b7 100644
--- a/absl/strings/internal/string_constant.h
+++ b/absl/strings/internal/string_constant.h
@@ -35,18 +35,12 @@ namespace strings_internal {
 // below.
 template <typename T>
 struct StringConstant {
- private:
-  // Returns true if `view` points to constant data.
-  // Otherwise, it can't be constant evaluated.
-  static constexpr bool ValidateConstant(absl::string_view view) {
-    return view.empty() || 2 * view[0] != 1;
-  }
-
- public:
   static constexpr absl::string_view value = T{}();
   constexpr absl::string_view operator()() const { return value; }
 
-  static_assert(ValidateConstant(value),
+  // Check to be sure `view` points to constant data.
+  // Otherwise, it can't be constant evaluated.
+  static_assert(value.empty() || 2 * value[0] != 1,
                 "The input string_view must point to constant data.");
 };