Add extern templates for common swisstable types.

Motivation: mitigate linker input size increase from swisstable optimizations.

Note: the changes in raw_hash_set.h are fixing build errors that happened when adding the explicit instantiations. The change in unchecked_deref is because set iterators have const reference access whereas map iterators have mutable reference access and the function is never actually called for sets (it's used in raw_hash_map) so it wasn't needed before. I'm not sure why the soo_slot/soo_iterator problems didn't cause compile errors earlier.
PiperOrigin-RevId: 615174043
Change-Id: Iac5eb2332a76e9b70021156fbb2b8def47a5391d

1 files changed, 41 insertions, 0 deletions

diff --git a/absl/container/flat_hash_map.h b/absl/container/flat_hash_map.h
index a33c794f..2f4457d6 100644
--- a/absl/container/flat_hash_map.h
+++ b/absl/container/flat_hash_map.h
@@ -31,15 +31,20 @@
 #define ABSL_CONTAINER_FLAT_HASH_MAP_H_
 
 #include <cstddef>
+#include <cstdint>
+#include <memory>
 #include <new>
+#include <string>
 #include <type_traits>
 #include <utility>
 
 #include "absl/algorithm/container.h"
+#include "absl/base/config.h"
 #include "absl/base/macros.h"
 #include "absl/container/internal/container_memory.h"
 #include "absl/container/internal/hash_function_defaults.h"  // IWYU pragma: export
 #include "absl/container/internal/raw_hash_map.h"  // IWYU pragma: export
+#include "absl/container/internal/raw_hash_set.h"  // IWYU pragma: export
 #include "absl/memory/memory.h"
 
 namespace absl {
@@ -632,6 +637,42 @@ struct IsUnorderedContainer<
 
 }  // namespace container_algorithm_internal
 
+// Explicit template instantiations for common map types in order to decrease
+// linker input size. Note that explicitly instantiating flat_hash_map itself
+// doesn't help because it has no non-alias members. If we need to decrease
+// linker input size more, we could potentially (a) add more key/value types,
+// e.g. string_view/Cord, (b) instantiate some template member functions, e.g.
+// operator[]/find. The EXTERN argument is `extern` for the declaration and
+// empty for the definition.
+#define ABSL_INTERNAL_TEMPLATE_FLAT_HASH_MAP(TEMPLATE, KEY, VALUE) \
+  TEMPLATE class absl::container_internal::raw_hash_map<           \
+      absl::container_internal::FlatHashMapPolicy<KEY, VALUE>,     \
+      absl::container_internal::hash_default_hash<KEY>,            \
+      absl::container_internal::hash_default_eq<KEY>,              \
+      std::allocator<std::pair<const KEY, VALUE>>>;                \
+  TEMPLATE class absl::container_internal::raw_hash_set<           \
+      absl::container_internal::FlatHashMapPolicy<KEY, VALUE>,     \
+      absl::container_internal::hash_default_hash<KEY>,            \
+      absl::container_internal::hash_default_eq<KEY>,              \
+      std::allocator<std::pair<const KEY, VALUE>>>;
+
+// We use exact-width integer types rather than `int`/`long`/`long long` because
+// these are the types recommended in the Google C++ style guide and which are
+// commonly used in Google code.
+ABSL_INTERNAL_TEMPLATE_FLAT_HASH_MAP(extern template, int32_t, int32_t);
+ABSL_INTERNAL_TEMPLATE_FLAT_HASH_MAP(extern template, std::string, int32_t);
+ABSL_INTERNAL_TEMPLATE_FLAT_HASH_MAP(extern template, int32_t, std::string);
+ABSL_INTERNAL_TEMPLATE_FLAT_HASH_MAP(extern template, int64_t, int64_t);
+ABSL_INTERNAL_TEMPLATE_FLAT_HASH_MAP(extern template, std::string, int64_t);
+ABSL_INTERNAL_TEMPLATE_FLAT_HASH_MAP(extern template, int64_t, std::string);
+ABSL_INTERNAL_TEMPLATE_FLAT_HASH_MAP(extern template, uint32_t, uint32_t);
+ABSL_INTERNAL_TEMPLATE_FLAT_HASH_MAP(extern template, std::string, uint32_t);
+ABSL_INTERNAL_TEMPLATE_FLAT_HASH_MAP(extern template, uint32_t, std::string);
+ABSL_INTERNAL_TEMPLATE_FLAT_HASH_MAP(extern template, uint64_t, uint64_t);
+ABSL_INTERNAL_TEMPLATE_FLAT_HASH_MAP(extern template, std::string, uint64_t);
+ABSL_INTERNAL_TEMPLATE_FLAT_HASH_MAP(extern template, uint64_t, std::string);
+ABSL_INTERNAL_TEMPLATE_FLAT_HASH_MAP(extern template, std::string, std::string);
+
 ABSL_NAMESPACE_END
 }  // namespace absl