diff --git a/.github/workflows/linux.yml b/.github/workflows/linux.yml
index 9cbc8a79cd..062863fda4 100644
--- a/.github/workflows/linux.yml
+++ b/.github/workflows/linux.yml
@@ -12,9 +12,11 @@ jobs:
     container:
       image: fedora:40
     steps:
-    - uses: actions/checkout@v4
     - name: Install dependencies
-      run: dnf install -y ninja-build cmake gtest-devel re2c clang util-linux
+      run: dnf install -y ninja-build cmake gtest-devel re2c clang util-linux git
+    - uses: actions/checkout@v4
+      with:
+        submodules: recursive
     - name: Linting
       run: misc/ci.py
     - name: Configure with CMake
@@ -41,16 +43,18 @@ jobs:
     container:
       image: rockylinux:8
     steps:
-    - uses: actions/checkout@v4
-    - uses: codespell-project/actions-codespell@master
-      with:
-        ignore_words_list: fo,wee,addin,notin
     - name: Install dependencies
       run: |
         dnf install -y make gcc-c++ libasan clang-analyzer cmake dnf-plugins-core epel-release
         dnf config-manager --set-enabled powertools
-        dnf install -y gtest-devel p7zip p7zip-plugins
-
+        dnf install -y gtest-devel p7zip p7zip-plugins git
+    - uses: actions/checkout@v4
+      with:
+        submodules: recursive
+    - uses: codespell-project/actions-codespell@master
+      with:
+        ignore_words_list: fo,wee,addin,notin
+        skip: ./parallel-hashmap
     - name: Build debug ninja
       shell: bash
       env:
@@ -103,12 +107,14 @@ jobs:
     container:
       image: ubuntu:20.04
     steps:
-    - uses: actions/checkout@v4
     - name: Install dependencies
       run: |
         apt update
-        apt install -y python3-pytest ninja-build clang-tidy python3-pip clang libgtest-dev
+        apt install -y python3-pytest ninja-build clang-tidy python3-pip clang libgtest-dev git
         pip3 install cmake==3.17.*
+    - uses: actions/checkout@v4
+      with:
+        submodules: recursive
     - name: Configure (GCC)
       run: cmake -Bbuild-gcc -DCMAKE_BUILD_TYPE=Debug -G'Ninja Multi-Config'
 
@@ -159,11 +165,13 @@ jobs:
       matrix:
         image: ['ubuntu:20.04', 'ubuntu:22.04', 'ubuntu:24.04']
     steps:
-    - uses: actions/checkout@v4
     - name: Install dependencies
       run: |
         apt update
-        apt install -y g++ python3
+        apt install -y g++ python3 git
+    - uses: actions/checkout@v4
+      with:
+        submodules: recursive
     - name: ${{ matrix.image }}
       run: |
         python3 configure.py --bootstrap
@@ -176,6 +184,8 @@ jobs:
     runs-on: [ubuntu-latest]
     steps:
     - uses: actions/checkout@v4
+      with:
+        submodules: recursive
 
     - name: Build
       uses: uraimo/run-on-arch-action@v2
diff --git a/.github/workflows/macos.yml b/.github/workflows/macos.yml
index f4f26abf69..910ebad5a4 100644
--- a/.github/workflows/macos.yml
+++ b/.github/workflows/macos.yml
@@ -12,6 +12,8 @@ jobs:
 
     steps:
     - uses: actions/checkout@v4
+      with:
+        submodules: recursive
 
     - name: Install dependencies
       run: brew install re2c p7zip cmake
diff --git a/.github/workflows/windows.yml b/.github/workflows/windows.yml
index c20889d40d..9e6c68ef4c 100644
--- a/.github/workflows/windows.yml
+++ b/.github/workflows/windows.yml
@@ -21,6 +21,8 @@ jobs:
 
     steps:
     - uses: actions/checkout@v4
+      with:
+        submodules: recursive
 
     - name: Install dependencies
       run: choco install re2c
diff --git a/.gitmodules b/.gitmodules
new file mode 100644
index 0000000000..5659dfa757
--- /dev/null
+++ b/.gitmodules
@@ -0,0 +1,3 @@
+[submodule "parallel-hashmap"]
+	path = parallel-hashmap
+	url = https://github.com/greg7mdp/parallel-hashmap.git
diff --git a/CMakeLists.txt b/CMakeLists.txt
index b8fdee7d3a..1e446508e5 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -61,6 +61,8 @@ else()
 	endif()
 endif()
 
+include_directories(parallel-hashmap)
+
 # --- optional re2c
 set(RE2C_MAJOR_VERSION 0)
 find_program(RE2C re2c)
diff --git a/configure.py b/configure.py
index c88daad508..b2abdfd50a 100755
--- a/configure.py
+++ b/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']
diff --git a/misc/ci.py b/misc/ci.py
index 20a4415f8a..d2220f2bb5 100755
--- a/misc/ci.py
+++ b/misc/ci.py
@@ -7,6 +7,7 @@
 	'misc/afl-fuzz-tokens/',
 	'src/depfile_parser.cc',
 	'src/lexer.cc',
+	'parallel-hashmap/',
 ]
 
 error_count = 0
diff --git a/parallel-hashmap b/parallel-hashmap
new file mode 160000
index 0000000000..d88c5e1507
--- /dev/null
+++ b/parallel-hashmap
@@ -0,0 +1 @@
+Subproject commit d88c5e15079047777b418132ece5879e7c9aaa2b
diff --git a/src/hash_map.h b/src/hash_map.h
index 3f465338ac..5448a3fcef 100644
--- a/src/hash_map.h
+++ b/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_