cmake: gem5-related build system updates

The build system now allows incremental gem5 builds. Unfortunately,
the current solution always requires re-linking the executable.
Without the enforcement of re-linking, the fail code will be rebuilt
but not linked into gem5.

The number of cores for building gem5 is derived from /proc/cpuinfo.
As before, only the gem5.debug configuration is supported.

Change-Id: Ib13b15d1ecd62196eb251e0fd00953f4eb052feb

cmake/gem5.cmake

@@ -6,13 +6,29 @@ if(BUILD_GEM5)
   set(gem5_src_dir       ${PROJECT_SOURCE_DIR}/simulators/gem5)
   set(gem5_wrapper       ${PROJECT_SOURCE_DIR}/src/core/sal/gem5)
   set(gem5_build_config  build/ARM/gem5.debug)
-  set(core_count         9)
+  ## Determine the number of cores:
+  set(core_count 0) # unknown
+  # Linux:
+  set(cpuinfo_file "/proc/cpuinfo")
+  if(EXISTS "${cpuinfo_file}")
+    file(STRINGS "${cpuinfo_file}" procs REGEX "^processor.: [0-9]+$")
+    list(LENGTH procs core_count)
+  endif()
+  # Mac:
+  if(APPLE)
+    find_program(cmd_sys_pro "system_profiler")
+    if(cmd_sys_pro)
+      execute_process(COMMAND ${cmd_sys_pro} OUTPUT_VARIABLE info)
+      string(REGEX REPLACE "^.*Total Number Of Cores: ([0-9]+).*$" "\\1"
+        core_count "${info}")
+    endif()
+  endif()
+  MATH(EXPR VAR "${core_count}+1") # CPU++
+  MESSAGE(STATUS "Using ${core_count} core(s) for compiling with scons ...")
 
   # FIXMEs:
-  # - incremental builds working?
-  # - dependency for modified .cc files not correctly checked
-  # - core_count should be derived from the parent make -jX parameter
-  # - make gem5_build_config configurable in CMake
+  # - ${core_count} should be derived from the parent make -jX parameter
+  # - make ${gem5_build_config} configurable in CMake
   #   (alternative: gem5_build_config is set based on the CMake build
   #    config, e.g., "Debug" or "Release")
   # - Ideally, there is no additional "gem5-clean" target. Instead,
@@ -20,29 +36,65 @@ if(BUILD_GEM5)
   #   gem5 root dir. This seems easy for "make only" projects--things
   #   get shaky due to "scons".
 
-  # Enable ExternalProject CMake module
-  include(ExternalProject)
+  # Enable ExternalProject CMake module (located in $FAIL/cmake/)
+  include(ExternalProjectGem5Specific)
 
-  # Use cmake's external project feature to build gem5 (and link FailGem5)
-  ExternalProject_Add(
-    FailGem5_binary_external # the (unique) name of this custom target (= external project)
+  add_custom_target(gem5-force-re-linking
+    COMMAND @echo "Removing gem5 binary and CMake build stamp ..."
+    COMMAND rm "${gem5_src_dir}/${gem5_build_config}" -f
+    COMMAND rm "${PROJECT_BINARY_DIR}/FailGem5_binary_external-prefix/src/FailGem5_binary_external-stamp/FailGem5_binary_external-build" -f
+  )
+  # WARNING: This requires a known (and "fixed") implementation of
+  # ExternalProject_Add() because we enforce re-linking by deleting
+  # some internal CMake time-stamp files generated by the external
+  # project stuff. We therefore provide our own version
+  # ("ExternalProjectGem5Specific") using a different name.
+
+  # Generate gem5 headers using the scons build system (in our custom "dummy" mode):
+  # This effectivly calls the whole gem5 toolchain but replaces the compiler, the
+  # linker, ar and ranlib with usr/bin/true. It allows us to just generate the headers
+  # which, in turn, are required for fail compilation. This target has no dependencies.
+  ExternalProject_Add2_Gem5Specific(
+    FailGem5_generation # the (unique) name of this custom target (= external project)
     # Disable update, patch and configure step:
     UPDATE_COMMAND ""
     PATCH_COMMAND ""
     CONFIGURE_COMMAND ""
     SOURCE_DIR ${gem5_src_dir}
     BINARY_DIR ${gem5_src_dir}
+    BUILD_COMMAND scons ${gem5_build_config} --dummy
+    # Disable install step (for now)
+    INSTALL_COMMAND ""
+  )
+
+  # Use cmake's external project feature to build gem5 (and link FailGem5):
+  ExternalProject_Add2_Gem5Specific(
+    FailGem5_binary_external
+    DEPENDS FailGem5_generation
+    # Disable update, patch and configure step:
+    UPDATE_COMMAND ""
+    PATCH_COMMAND ""
+    SOURCE_DIR ${gem5_src_dir}
+    BINARY_DIR ${gem5_src_dir}
+    CONFIGURE_COMMAND ""
     # Build gem5 using scons build system:
     BUILD_COMMAND scons "CXX=${CMAKE_CXX_COMPILER} -p ${gem5_src_dir} --Xcompiler" EXTRAS=${gem5_wrapper} ${gem5_build_config} -j${core_count}
     # Disable install step (for now)
     INSTALL_COMMAND ""
   )
+
+  # Cleans up everything (gem5 and Fail related):
   add_custom_target(gem5-allclean
     COMMAND @echo "Cleaning Fail* and gem5 ..."
     COMMAND cd "${PROJECT_BINARY_DIR}/" && make clean
     COMMAND cd "${gem5_src_dir}/" && scons -c build/ARM
+    COMMAND cd "${gem5_src_dir}/" && rm -rf build/
   )
 
-  # Build "fail" library first (will be statically linked to gem5)
+  # Resolve header dependencies first (i.e., generate the header files):
+  add_dependencies(fail-sal FailGem5_generation)
+  # Always enforce re-linking (bugfix for cmake/scons):
+  add_dependencies(fail gem5-force-re-linking)
+  # Re-build/link gem5 if Fail code has changed:
   add_dependencies(FailGem5_binary_external fail)
 endif(BUILD_GEM5)