Add ImpactX_UNITY_BUILD (#861)

* Add `ImpactX_UNITY_BUILD`

Unity builds combine all `.cpp` files into a single one
(a single translation unit, TU). This is useful for
performance optimization, computer science experiments,
or differentiable programming.

* CI: Add a Unity Build

* Fix Ambiguous `diagnostics` namespace

.github/workflows/ubuntu.yml

@@ -49,6 +49,7 @@ jobs:
           -DCMAKE_VERBOSE_MAKEFILE=ON  \
           -DImpactX_FFT=ON             \
           -DImpactX_PYTHON=ON          \
+          -DImpactX_UNITY_BUILD=ON     \
           -DMPIEXEC_POSTFLAGS="--use-hwthread-cpus"
         cmake --build build -j 4
 

CMakeLists.txt

@@ -116,6 +116,10 @@ option(ImpactX_PYTHON_IPO
     ${_ImpactX_PYTHON_IPO_DEFAULT}
 )
 
+# Unity builds combine all .cpp files into a single one (a single translation
+# unit, TU).
+option(ImpactX_UNITY_BUILD          "ImpactX library as unity build" OFF)
+
 if(ImpactX_FFT)
     set(ABLASTR_FFT ON CACHE STRING "FFT-based solvers" FORCE)
 endif()
@@ -165,6 +169,16 @@ set_target_properties(lib PROPERTIES
     WINDOWS_EXPORT_ALL_SYMBOLS ON
 )
 
+# Optional: build only a single TU
+if(ImpactX_UNITY_BUILD)
+    set_target_properties(lib PROPERTIES
+        UNITY_BUILD ON
+        UNITY_BUILD_MODE BATCH
+        # Number must be more than the number of .cpp files in ImpactX
+        UNITY_BUILD_BATCH_SIZE 10000
+    )
+endif()
+
 # executable application
 #   note: we currently avoid a dependency on a core library
 #         for simpler usage, but could make this an option

cmake/ImpactXFunctions.cmake

@@ -395,6 +395,9 @@ function(impactx_print_summary)
     else()
         set(LIB_TYPE "static")
     endif()
+    if(ImpactX_UNITY_BUILD)
+        set(LIB_TYPE "${LIB_TYPE} (unity build)")
+    endif()
     #message("  Testing: ${BUILD_TESTING}")
     message("  Build options:")
     message("    APP: ${ImpactX_APP}")

docs/source/install/cmake.rst

@@ -126,6 +126,7 @@ CMake Option                  Default & Values                               Des
 ============================= ============================================== ===========================================================
 ``BUILD_SHARED_LIBS``         ON/**OFF**                                     Build shared libraries for dependencies
 ``ImpactX_CCACHE``            **ON**/OFF                                     Search and use CCache to speed up rebuilds.
+``ImpactX_UNITY_BUILD``       ON/**OFF**                                     ImpactX library as unity build (single TU)
 ``ImpactX_ablastr_src``       *None*                                         Path to ABLASTR source directory (preferred if set)
 ``ImpactX_ablastr_repo``      ``https://github.com/ECP-WarpX/WarpX.git``     Repository URI to pull and build ABLASTR from
 ``ImpactX_ablastr_branch``    *we set and maintain a compatible commit*      Repository branch for ``ImpactX_ablastr_repo``

src/initialization/InitElement.cpp

@@ -502,7 +502,8 @@ namespace detail
                 pp_element.queryAddWithParser("cn", cn);
             }
 
-            m_lattice.emplace_back(diagnostics::BeamMonitor(openpmd_name, openpmd_backend, openpmd_encoding, period_sample_intervals));
+            using impactx::elements::diagnostics::BeamMonitor;
+            m_lattice.emplace_back(BeamMonitor(openpmd_name, openpmd_backend, openpmd_encoding, period_sample_intervals));
         } else if (element_type == "source")
         {
             std::string distribution, openpmd_path;