Export of internal Abseil changes

--
3a9b4e8e5ecba532db5cc4ac12d12660307ce9fb by Derek Mauro <dmauro@google.com>:

Use the Bazel @platforms repository for platform constraints
Fixes #1000

PiperOrigin-RevId: 390644226

--
b34e4d2f8a86b54bd483ec4c9c3dd781ad2d8b68 by Abseil Team <absl-team@google.com>:

debugging: add some handling for RISC-V

The RISC-V architecture uses a downward growing stack and can host Linux using
ELF files.  Adjust a few sites accordingly to indicate how to handle the RISC-V
architecture.

PiperOrigin-RevId: 390631894

--
5fa3a0961bf3dd0799c048956a0128f7b8113f1e by Samuel Benzaquen <sbenza@google.com>:

Rename the buffer hash function to LowLevelHash.
Although it started as wyhash, it will depart from it so it does not make sense
to keep the name.

PiperOrigin-RevId: 390483506

--
2e7867a2301d58ad4cd5abcaa5fd6f0db973ae7b by Abseil Team <absl-team@google.com>:

This is an internal change.

PiperOrigin-RevId: 390349746
GitOrigin-RevId: 3a9b4e8e5ecba532db5cc4ac12d12660307ce9fb
Change-Id: I322c3762552a2107e6c6b108c25c01e5efa8aecd

1 files changed, 50 insertions, 0 deletions

diff --git a/absl/hash/internal/low_level_hash.h b/absl/hash/internal/low_level_hash.h
new file mode 100644
index 00000000..439968aa
--- /dev/null
+++ b/absl/hash/internal/low_level_hash.h
@@ -0,0 +1,50 @@
+// Copyright 2020 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// This file provides the Google-internal implementation of LowLevelHash.
+//
+// LowLevelHash is a fast hash function for hash tables, the fastest we've
+// currently (late 2020) found that passes the SMHasher tests. The algorithm
+// relies on intrinsic 128-bit multiplication for speed. This is not meant to be
+// secure - just fast.
+//
+// It is closely based on a version of wyhash, but does not maintain or
+// guarantee future compatibility with it.
+
+#ifndef ABSL_HASH_INTERNAL_LOW_LEVEL_HASH_H_
+#define ABSL_HASH_INTERNAL_LOW_LEVEL_HASH_H_
+
+#include <stdint.h>
+#include <stdlib.h>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace hash_internal {
+
+// Hash function for a byte array. A 64-bit seed and a set of five 64-bit
+// integers are hashed into the result.
+//
+// To allow all hashable types (including string_view and Span) to depend on
+// this algorithm, we keep the API low-level, with as few dependencies as
+// possible.
+uint64_t LowLevelHash(const void* data, size_t len, uint64_t seed,
+                      const uint64_t salt[5]);
+
+}  // namespace hash_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_HASH_INTERNAL_LOW_LEVEL_HASH_H_