[XLA:CPU] Re-use the same llvm::GlobalVariable for identical literals

This isn't necessary today, but it will be after an optimization change I'm
about to make.

LLVM has a constant merging pass too, but one of the motivations here is to
avoid the LLVM compile time overhead of having many large arrays in the IR.

PiperOrigin-RevId: 195032900

1 files changed, 18 insertions, 1 deletions

diff --git a/tensorflow/core/lib/hash/hash.h b/tensorflow/core/lib/hash/hash.h
index ca05e6346e..3f85303c0f 100644
--- a/tensorflow/core/lib/hash/hash.h
+++ b/tensorflow/core/lib/hash/hash.h
@@ -21,6 +21,7 @@ limitations under the License.
 #include <stddef.h>
 #include <stdint.h>
 
+#include <functional>
 #include <string>
 
 #include "tensorflow/core/lib/core/stringpiece.h"
@@ -49,12 +50,28 @@ inline uint64 Hash64Combine(uint64 a, uint64 b) {
 // In particular, tensorflow::hash is not the identity function for pointers.
 // This is important for power-of-two sized hashtables like FlatMap and FlatSet,
 // because otherwise they waste the majority of their hash buckets.
-template <typename T>
+//
+// The second type argument is only used for SFNIAE below.
+template <typename T, typename = void>
 struct hash {
   size_t operator()(const T& t) const { return std::hash<T>()(t); }
 };
 
 template <typename T>
+struct hash<T, typename std::enable_if<std::is_enum<T>::value>::type> {
+  size_t operator()(T value) const {
+    // This works around a defect in the std::hash C++ spec that isn't fixed in
+    // (at least) gcc 4.8.4:
+    // http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#2148
+    //
+    // We should be able to remove this and use the default
+    // tensorflow::hash<EnumTy>() once we stop building with GCC versions old
+    // enough to not have this defect fixed.
+    return std::hash<uint64>()(static_cast<uint64>(value));
+  }
+};
+
+template <typename T>
 struct hash<T*> {
   size_t operator()(const T* t) const {
     // Hash pointers as integers, but bring more entropy to the lower bits.