Export of internal Abseil changes

--
5dc885f2b62993bccf33a3f3b99f7e460c819c89 by Derek Mauro <dmauro@google.com>:

Remove the internal-only ABSL_INTERNAL_ASSUME now that ABSL_ASSUME
is available and ABSL_INTERNAL_ASSUME has no more users.

Improve the documentation to ABSL_ASSUME somewhat.

PiperOrigin-RevId: 434803125
Change-Id: I7c27418463ffc1c7e10ecd50e2d17f348f686af7

--
4aea19a0ef596228c9136a4c2446e6f25085f23c by Derek Mauro <dmauro@google.com>:

Update documentation to warn against using absl::Hash across
dynamically loaded libraries

Fixes #1128

PiperOrigin-RevId: 434723247
Change-Id: Ib0c7ba03b2cab98b42e19e85be6833192d4b4067
GitOrigin-RevId: 5dc885f2b62993bccf33a3f3b99f7e460c819c89

6 files changed, 49 insertions, 23 deletions

diff --git a/absl/base/optimization.h b/absl/base/optimization.h
index 6fe03c43..db5cc097 100644
--- a/absl/base/optimization.h
+++ b/absl/base/optimization.h
@@ -181,32 +181,21 @@
 #define ABSL_PREDICT_TRUE(x) (x)
 #endif
 
-// Platform and compilation mode dependent implementation of ABSL_ASSUME.
-#if !defined(NDEBUG)
-#define ABSL_INTERNAL_ASSUME(cond) assert(cond)
-#elif ABSL_HAVE_BUILTIN(__builtin_assume)
-#define ABSL_INTERNAL_ASSUME(cond) __builtin_assume(cond)
-#elif defined(__GNUC__) || ABSL_HAVE_BUILTIN(__builtin_unreachable)
-#define ABSL_INTERNAL_ASSUME(cond)        \
-  do {                                    \
-    if (!(cond)) __builtin_unreachable(); \
-  } while (0)
-#elif defined(_MSC_VER)
-#define ABSL_INTERNAL_ASSUME(cond) __assume(cond)
-#else
-#define ABSL_INTERNAL_ASSUME(cond)      \
-  do {                                  \
-    static_cast<void>(false && (cond)); \
-  } while (0)
-#endif
-
 // ABSL_ASSUME(cond)
+//
 // Informs the compiler that a condition is always true and that it can assume
-// it to be true for optimization purposes. The call has undefined behavior if
-// the condition is false.
+// it to be true for optimization purposes.
+//
+// WARNING: If the condition is false, the program can produce undefined and
+// potentially dangerous behavior.
+//
 // In !NDEBUG mode, the condition is checked with an assert().
+//
 // NOTE: The expression must not have side effects, as it may only be evaluated
-// in some compilation modes and not others.
+// in some compilation modes and not others. Some compilers may issue a warning
+// if the compiler cannot prove the expression has no side effects. For example,
+// the expression should not use a function call since the compiler cannot prove
+// that a function call does not have side effects.
 //
 // Example:
 //
@@ -216,7 +205,23 @@
 //   // assumption specified above.
 //   int y = x / 16;
 //
-#define ABSL_ASSUME(cond) ABSL_INTERNAL_ASSUME(cond)
+#if !defined(NDEBUG)
+#define ABSL_ASSUME(cond) assert(cond)
+#elif ABSL_HAVE_BUILTIN(__builtin_assume)
+#define ABSL_ASSUME(cond) __builtin_assume(cond)
+#elif defined(__GNUC__) || ABSL_HAVE_BUILTIN(__builtin_unreachable)
+#define ABSL_ASSUME(cond)                 \
+  do {                                    \
+    if (!(cond)) __builtin_unreachable(); \
+  } while (0)
+#elif defined(_MSC_VER)
+#define ABSL_ASSUME(cond) __assume(cond)
+#else
+#define ABSL_ASSUME(cond)               \
+  do {                                  \
+    static_cast<void>(false && (cond)); \
+  } while (0)
+#endif
 
 // ABSL_INTERNAL_UNIQUE_SMALL_NAME(cond)
 // This macro forces small unique name on a static file level symbols like
diff --git a/absl/container/flat_hash_map.h b/absl/container/flat_hash_map.h
index 83c71029..13779eda 100644
--- a/absl/container/flat_hash_map.h
+++ b/absl/container/flat_hash_map.h
@@ -76,6 +76,10 @@ struct FlatHashMapPolicy;
 // absl/hash/hash.h for information on extending Abseil hashing to user-defined
 // types.
 //
+// Using `absl::flat_hash_map` at interface boundries in dynamically loaded
+// libraries (e.g. .dll, .so) is unsupported due to way `absl::Hash` values may
+// be randomized across dynamically loaded libraries.
+//
 // NOTE: A `flat_hash_map` stores its value types directly inside its
 // implementation array to avoid memory indirection. Because a `flat_hash_map`
 // is designed to move data when rehashed, map values will not retain pointer
diff --git a/absl/container/flat_hash_set.h b/absl/container/flat_hash_set.h
index 0fb2ae6f..304c2abb 100644
--- a/absl/container/flat_hash_set.h
+++ b/absl/container/flat_hash_set.h
@@ -72,6 +72,10 @@ struct FlatHashSetPolicy;
 // absl/hash/hash.h for information on extending Abseil hashing to user-defined
 // types.
 //
+// Using `absl::flat_hash_set` at interface boundries in dynamically loaded
+// libraries (e.g. .dll, .so) is unsupported due to way `absl::Hash` values may
+// be randomized across dynamically loaded libraries.
+//
 // NOTE: A `flat_hash_set` stores its keys directly inside its implementation
 // array to avoid memory indirection. Because a `flat_hash_set` is designed to
 // move data when rehashed, set keys will not retain pointer stability. If you
diff --git a/absl/container/node_hash_map.h b/absl/container/node_hash_map.h
index ca1ed408..f2175d16 100644
--- a/absl/container/node_hash_map.h
+++ b/absl/container/node_hash_map.h
@@ -78,6 +78,10 @@ class NodeHashMapPolicy;
 // absl/hash/hash.h for information on extending Abseil hashing to user-defined
 // types.
 //
+// Using `absl::node_hash_map` at interface boundries in dynamically loaded
+// libraries (e.g. .dll, .so) is unsupported due to way `absl::Hash` values may
+// be randomized across dynamically loaded libraries.
+//
 // Example:
 //
 //   // Create a node hash map of three strings (that map to strings)
diff --git a/absl/container/node_hash_set.h b/absl/container/node_hash_set.h
index 9421e11e..a9ff7e1b 100644
--- a/absl/container/node_hash_set.h
+++ b/absl/container/node_hash_set.h
@@ -74,6 +74,10 @@ struct NodeHashSetPolicy;
 // absl/hash/hash.h for information on extending Abseil hashing to user-defined
 // types.
 //
+// Using `absl::node_hash_set` at interface boundries in dynamically loaded
+// libraries (e.g. .dll, .so) is unsupported due to way `absl::Hash` values may
+// be randomized across dynamically loaded libraries.
+//
 // Example:
 //
 //   // Create a node hash set of three strings
diff --git a/absl/hash/hash.h b/absl/hash/hash.h
index f31fde40..74e2d7c0 100644
--- a/absl/hash/hash.h
+++ b/absl/hash/hash.h
@@ -40,6 +40,11 @@
 // each process.  E.g., `absl::Hash<int>{}(9)` in one process and
 // `absl::Hash<int>{}(9)` in another process are likely to differ.
 //
+// `absl::Hash` may also produce different values from different dynamically
+// loaded libraries. For this reason, `absl::Hash` values must never cross
+// boundries in dynamically loaded libraries (including when used in types like
+// hash containers.)
+//
 // `absl::Hash` is intended to strongly mix input bits with a target of passing
 // an [Avalanche Test](https://en.wikipedia.org/wiki/Avalanche_effect).
 //