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From: Benjamin Barenblat <bbaren@google.com>
Subject: Remove endian-sensitivity from hash slow path
Forwarded: yes
Applied-Upstream: https://github.com/abseil/abseil-cpp/commit/9c6a50fdd80bb39fabd95faeda84f04062685ff3
Prior to this commit, the Abseil hash fast path was endian-agnostic, but
the slow path assumed a little-endian platform. Change the slow path to
be endian-correct, ensuring that values produced by the fast and slow
paths are equal even on big-endian systems.
The author works at Google. Upstream applied this patch as Piper revision
355424258 and exported it to GitHub; the Applied-Upstream URL above points to
the exported commit.
--- a/absl/hash/BUILD.bazel
+++ b/absl/hash/BUILD.bazel
@@ -37,6 +37,7 @@ cc_library(name = "hash",
linkopts = ABSL_DEFAULT_LINKOPTS,
deps = [
":city",
+ "//absl/base:config",
"//absl/base:core_headers",
"//absl/base:endian",
"//absl/container:fixed_array",
--- a/absl/hash/CMakeLists.txt
+++ b/absl/hash/CMakeLists.txt
@@ -25,6 +25,7 @@ absl_cc_library(NAME hash
COPTS
${ABSL_DEFAULT_COPTS}
DEPS
+ absl::config
absl::core_headers
absl::endian
absl::fixed_array
--- a/absl/hash/internal/hash.h
+++ b/absl/hash/internal/hash.h
@@ -38,7 +38,8 @@
#include <utility>
#include <vector>
-#include "absl/base/internal/endian.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/unaligned_access.h"
#include "absl/base/port.h"
#include "absl/container/fixed_array.h"
#include "absl/meta/type_traits.h"
@@ -804,26 +805,54 @@ class ABSL_DLL CityHashState
size_t len);
// Reads 9 to 16 bytes from p.
- // The first 8 bytes are in .first, the rest (zero padded) bytes are in
- // .second.
+ // The least significant 8 bytes are in .first, the rest (zero padded) bytes
+ // are in .second.
static std::pair<uint64_t, uint64_t> Read9To16(const unsigned char* p,
size_t len) {
- uint64_t high = little_endian::Load64(p + len - 8);
- return {little_endian::Load64(p), high >> (128 - len * 8)};
+ uint64_t low_mem = absl::base_internal::UnalignedLoad64(p);
+ uint64_t high_mem = absl::base_internal::UnalignedLoad64(p + len - 8);
+#ifdef ABSL_IS_LITTLE_ENDIAN
+ uint64_t most_significant = high_mem;
+ uint64_t least_significant = low_mem;
+#else
+ uint64_t most_significant = low_mem;
+ uint64_t least_significant = high_mem;
+#endif
+ return {least_significant, most_significant >> (128 - len * 8)};
}
// Reads 4 to 8 bytes from p. Zero pads to fill uint64_t.
static uint64_t Read4To8(const unsigned char* p, size_t len) {
- return (static_cast<uint64_t>(little_endian::Load32(p + len - 4))
- << (len - 4) * 8) |
- little_endian::Load32(p);
+ uint32_t low_mem = absl::base_internal::UnalignedLoad32(p);
+ uint32_t high_mem = absl::base_internal::UnalignedLoad32(p + len - 4);
+#ifdef ABSL_IS_LITTLE_ENDIAN
+ uint32_t most_significant = high_mem;
+ uint32_t least_significant = low_mem;
+#else
+ uint32_t most_significant = low_mem;
+ uint32_t least_significant = high_mem;
+#endif
+ return (static_cast<uint64_t>(most_significant) << (len - 4) * 8) |
+ least_significant;
}
// Reads 1 to 3 bytes from p. Zero pads to fill uint32_t.
static uint32_t Read1To3(const unsigned char* p, size_t len) {
- return static_cast<uint32_t>((p[0]) | //
- (p[len / 2] << (len / 2 * 8)) | //
- (p[len - 1] << ((len - 1) * 8)));
+ unsigned char mem0 = p[0];
+ unsigned char mem1 = p[len / 2];
+ unsigned char mem2 = p[len - 1];
+#ifdef ABSL_IS_LITTLE_ENDIAN
+ unsigned char significant2 = mem2;
+ unsigned char significant1 = mem1;
+ unsigned char significant0 = mem0;
+#else
+ unsigned char significant2 = mem0;
+ unsigned char significant1 = mem1;
+ unsigned char significant0 = mem2;
+#endif
+ return static_cast<uint32_t>(significant0 | //
+ (significant1 << (len / 2 * 8)) | //
+ (significant2 << ((len - 1) * 8)));
}
ABSL_ATTRIBUTE_ALWAYS_INLINE static uint64_t Mix(uint64_t state, uint64_t v) {
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