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
diff --git a/absl/base/optimization.h b/absl/base/optimization.h
index 6fe03c4..db5cc09 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 83c7102..13779ed 100644
--- a/absl/container/flat_hash_map.h
+++ b/absl/container/flat_hash_map.h
@@ -76,6 +76,10 @@
// 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 0fb2ae6..304c2ab 100644
--- a/absl/container/flat_hash_set.h
+++ b/absl/container/flat_hash_set.h
@@ -72,6 +72,10 @@
// 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 ca1ed40..f2175d1 100644
--- a/absl/container/node_hash_map.h
+++ b/absl/container/node_hash_map.h
@@ -78,6 +78,10 @@
// 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 9421e11..a9ff7e1 100644
--- a/absl/container/node_hash_set.h
+++ b/absl/container/node_hash_set.h
@@ -74,6 +74,10 @@
// 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 f31fde4..74e2d7c 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).
//
