ecbuild_compiler_flags

Set compiler specific default compilation flags for a given language.

ecbuild_compiler_flags( <lang> )

The procedure is as follows:

1. ecBuild does not set CMAKE_<lang>_FLAGS i.e. the user can set these via -D or the CMake cache and these will be the “base” flags.

2. ecBuild overwrites CMAKE_<lang>_FLAGS_<btype> in the CMake cache for all build types with compiler specific defaults for the currently loaded compiler i.e. any value set by the user via -D or the CMake cache has no effect.

3. Any value the user provides via ECBUILD_<lang>_FLAGS or ECBUILD_<lang>_FLAGS_<btype> overrides the corresponding CMAKE_<lang>_FLAGS or CMAKE_<lang>_FLAGS_<btype> without being written to the CMake cache.

Using custom compilation flags

If compilation flags need to be controlled on a per source file basis, ecBuild supports defining custom rules in a CMake or JSON file.

When using this approach, default compilation flags are NOT loaded!

Overriding compilation flags on a per source file basis using CMake rules

Compiler flags can be overridden on a per source file basis by setting the CMake variable ECBUILD_COMPILE_FLAGS to the full path of a CMake file defining the override rules. If set, <PNAME>_ECBUILD_COMPILE_FLAGS takes precendence and ECBUILD_COMPILE_FLAGS is ignored, allowing for rules that only apply to a subproject (e.g. in a bundle).

Flags can be overridden in 3 different ways:

1. By defining project specific flags for a language and (optionally) build type e.g.

   set(<PNAME>_Fortran_FLAGS "...") # common flags for all build types
   set(<PNAME>_Fortran_FLAGS_DEBUG "...") # only for DEBUG build type
   

2. By defining source file specific flags which are combined with the project and target specific flags

   set_source_files_properties(<source>
     PROPERTIES COMPILE_FLAGS "..."  # common flags for all build types
                COMPILE_FLAGS_DEBUG "...") # only for DEBUG build type
   

3. By defining source file specific flags which override the project and target specific flags

   set_source_files_properties(<source>
     PROPERTIES OVERRIDE_COMPILE_FLAGS "..."
                OVERRIDE_COMPILE_FLAGS_DEBUG "...")
   

See examples/override-compile-flags in the ecBuild source tree for a complete example using this technique.

Overriding compilation flags on a per source file basis using JSON rules

Compiler flags can be overridden on a per source file basis by setting the CMake variable ECBUILD_SOURCE_FLAGS to the full path of a JSON file defining the override rules. If set, <PNAME>_ECBUILD_SOURCE_FLAGS takes precendence and ECBUILD_SOURCE_FLAGS is ignored, allowing for rules that only apply to a subproject (e.g. in a bundle).

The JSON file lists shell glob patterns and the rule to apply to each source file matching the pattern, defined as an array [op, flag1, ...] containing an operator followed by one or more flags. Valid operators are:

+

Add the flags to the default compilation flags for matching files

=

Set the flags for matching files, disregarding default compilation flags

/

Remove the flags from the default compilation flags for matching files

Rules can be nested to e.g. only apply to a subdirectory by setting the rule to a dictionary, which will only apply to source files matching its pattern.

An example JSON file demonstrating different rule types is given below:

{
  "*"       : [ "+", "-g0" ],
  "*.cxx"   : [ "+", "-cxx11" ],
  "*.f90"   : [ "+", "-pipe" ],
  "foo.c"   : [ "+", "-O0" ],
  "foo.cc"  : [ "+", "-O2", "-pipe" ],
  "bar/*": {
    "*.f90" : [ "=", "-O1" ]
  },
  "baz/*": {
    "*.f90" : [ "/", "-pipe" ],
    "*.f90" : [ "/", "-O2" ],
    "*.f90" : [ "+", "-O3" ]
  }
}

See examples/override-compile-flags in the ecBuild source tree for a complete example using this technique.