CMakePackage: added 'ipo' variant (#18374)

+ipo sets CMAKE_INTERPROCEDURAL_OPTIMIZATION=ON
The option is not supported for CMake < 3.9

lib/spack/spack/build_systems/cmake.py

@@ -12,7 +12,7 @@
 import spack.build_environment
 from llnl.util.filesystem import working_dir
 from spack.util.environment import filter_system_paths
-from spack.directives import depends_on, variant
+from spack.directives import depends_on, variant, conflicts
 from spack.package import PackageBase, InstallError, run_after
 
 # Regex to extract the primary generator from the CMake generator
@@ -94,6 +94,13 @@ class CMakePackage(PackageBase):
             description='CMake build type',
             values=('Debug', 'Release', 'RelWithDebInfo', 'MinSizeRel'))
 
+    # https://cmake.org/cmake/help/latest/variable/CMAKE_INTERPROCEDURAL_OPTIMIZATION.html
+    variant('ipo', default=False,
+            description='CMake interprocedural optimization')
+    # CMAKE_INTERPROCEDURAL_OPTIMIZATION only exists for CMake >= 3.9
+    conflicts('+ipo', when='^cmake@:3.8',
+              msg='+ipo is not supported by CMake < 3.9')
+
     depends_on('cmake', type='build')
 
     @property
@@ -147,6 +154,11 @@ def _std_args(pkg):
         except KeyError:
             build_type = 'RelWithDebInfo'
 
+        try:
+            ipo = pkg.spec.variants['ipo'].value
+        except KeyError:
+            ipo = False
+
         define = CMakePackage.define
         args = [
             '-G', generator,
@@ -154,6 +166,10 @@ def _std_args(pkg):
             define('CMAKE_BUILD_TYPE', build_type),
         ]
 
+        # CMAKE_INTERPROCEDURAL_OPTIMIZATION only exists for CMake >= 3.9
+        if pkg.spec.satisfies('^cmake@3.9:'):
+            args.append(define('CMAKE_INTERPROCEDURAL_OPTIMIZATION', ipo))
+
         if primary_generator == 'Unix Makefiles':
             args.append(define('CMAKE_VERBOSE_MAKEFILE', True))