use parallel-hashmap

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "parallel-hashmap"]
+	path = parallel-hashmap
+	url = https://github.com/greg7mdp/parallel-hashmap.git

configure.py

@@ -345,6 +345,7 @@ def search_system_path(file_name: str) -> Optional[str]:  # type: ignore # Missi
               # Disable size_t -> int truncation warning.
               # We never have strings or arrays larger than 2**31.
               '/wd4267',
+              '/Iparallel-hashmap',
               '/DNOMINMAX', '/D_CRT_SECURE_NO_WARNINGS',
               '/D_HAS_EXCEPTIONS=0',
               '/DNINJA_PYTHON="%s"' % options.with_python]
@@ -363,6 +364,7 @@ def search_system_path(file_name: str) -> Optional[str]:  # type: ignore # Missi
               '-fno-exceptions',
               '-std=c++11',
               '-fvisibility=hidden', '-pipe',
+              '-Iparallel-hashmap',
               '-DNINJA_PYTHON="%s"' % options.with_python]
     if options.debug:
         cflags += ['-D_GLIBCXX_DEBUG', '-D_GLIBCXX_DEBUG_PEDANTIC']

src/hash_map.h

@@ -17,6 +17,8 @@
 
 #include <algorithm>
 #include <string.h>
+#include "parallel_hashmap/phmap.h"
+#include "parallel_hashmap/phmap_utils.h"
 #include "string_piece.h"
 #include "util.h"
 
@@ -53,8 +55,6 @@ unsigned int MurmurHash2(const void* key, size_t len) {
   return h;
 }
 
-#include <unordered_map>
-
 namespace std {
 template<>
 struct hash<StringPiece> {
@@ -73,7 +73,7 @@ struct hash<StringPiece> {
 /// mapping StringPiece => Foo*.
 template<typename V>
 struct ExternalStringHashMap {
-  typedef std::unordered_map<StringPiece, V> Type;
+  typedef phmap::flat_hash_map<StringPiece, V> Type;
 };
 
 #endif // NINJA_MAP_H_