Have a default PCH for framework / infrastructure headers

Author: ktf

Common/Core/CMakeLists.txt

@@ -24,6 +24,31 @@ o2physics_add_library(AnalysisCore
         FFitWeights.cxx
         PUBLIC_LINK_LIBRARIES O2::Framework O2::DataFormatsParameters ROOT::EG O2::CCDB ROOT::Physics O2::FT0Base O2::FV0Base O2::DataFormatsParamTOF)
 
+# Precompiled header shared by DPL analysis workflows. Parsing the framework
+# headers dominates the cost of a workflow translation unit, and there are
+# ~1500 of them; o2physics_add_dpl_workflow reuses this by default.
+#
+# Skipped under recc, which caches compilations remotely instead -- the two do
+# not combine, as a PCH is a local artefact of one compiler invocation.
+#
+# The link libraries have to match what the reusing targets compile with, so
+# this carries only what every workflow already links.
+add_library(AnalysisPCH OBJECT analysisPCH.cxx)
+target_link_libraries(AnalysisPCH PUBLIC O2::Framework O2Physics::AnalysisCore)
+if(NOT DEFINED ENV{USE_RECC})
+  target_precompile_headers(AnalysisPCH PRIVATE
+    <Framework/ASoA.h>
+    <Framework/AnalysisTask.h>
+    <Framework/ConfigContext.h>
+    <Framework/DataProcessorSpec.h>
+    <Framework/runDataProcessing.h>
+
+    <memory>
+    <string>
+    <vector>
+  )
+endif()
+
 o2physics_target_root_dictionary(AnalysisCore
         HEADERS TrackSelection.h
         TrackSelectionDefaults.h

Common/Core/analysisPCH.cxx

@@ -0,0 +1,14 @@
+// Copyright 2019-2026 CERN and copyright holders of ALICE O2.
+// See https://alice-o2.web.cern.ch/copyright for details of the copyright holders.
+// All rights not expressly granted are reserved.
+//
+// This software is distributed under the terms of the GNU General Public
+// License v3 (GPL Version 3), copied verbatim in the file "COPYING".
+//
+// In applying this license CERN does not waive the privileges and immunities
+// granted to it by virtue of its status as an Intergovernmental Organization
+// or submit itself to any jurisdiction.
+
+// Carrier for the precompiled header shared by DPL analysis workflows; see
+// AnalysisPCH in this directory's CMakeLists.txt. It exists only to own the
+// PCH, so it has no code of its own.

cmake/O2PhysicsAddWorkflow.cmake

@@ -47,8 +47,24 @@ function(o2physics_add_dpl_workflow baseTargetName)
   set_property(TARGET ${targetExeName} PROPERTY JOB_POOL_COMPILE analysis)
   set_property(TARGET ${targetExeName} PROPERTY JOB_POOL_LINK analysis)
 
-  if(A_REUSE_FROM AND NOT DEFINED ENV{USE_RECC})
-    target_precompile_headers(${targetExeName} REUSE_FROM ${A_REUSE_FROM})
+  # Reuse a precompiled header. Without an explicit REUSE_FROM, fall back to the
+  # shared AnalysisPCH (Common/Core): a workflow translation unit spends most of
+  # its time parsing the framework headers, and there are ~1500 of them, so the
+  # default is worth more than the handful of targets that name their own.
+  #
+  # Set O2PHYSICS_DEFAULT_PCH to an empty string to opt out globally, e.g. when
+  # bisecting a PCH-related build failure.
+  if(NOT DEFINED O2PHYSICS_DEFAULT_PCH)
+    set(O2PHYSICS_DEFAULT_PCH AnalysisPCH)
+  endif()
+  set(_pch "${A_REUSE_FROM}")
+  if(NOT _pch)
+    set(_pch "${O2PHYSICS_DEFAULT_PCH}")
+  endif()
+  # A target cannot reuse its own PCH, and the carrier is not built when recc
+  # is caching compilations remotely instead.
+  if(_pch AND NOT _pch STREQUAL targetExeName AND NOT DEFINED ENV{USE_RECC})
+    target_precompile_headers(${targetExeName} REUSE_FROM ${_pch})
   endif()
 
   set(jsonFile $<TARGET_FILE_BASE_NAME:${targetExeName}>.json)