mpich: Add CUDA variant (#29901)

CUDA device memory is supported with the ch4 device.

Update provides to include MPI-4.0 functionality

var/spack/repos/builtin/packages/mpich/package.py

@@ -10,7 +10,7 @@
 from spack import *
 
 
-class Mpich(AutotoolsPackage):
+class Mpich(AutotoolsPackage, CudaPackage):
     """MPICH is a high performance and widely portable implementation of
     the Message Passing Interface (MPI) standard."""
 
@@ -94,7 +94,13 @@ class Mpich(AutotoolsPackage):
 '-Wl,-flat_namespace'.'''
     )
 
-    provides('mpi@:3.1')
+    # Todo: cuda can be a conditional variant, but it does not seem to work when
+    # overriding the variant from CudaPackage.
+    conflicts('+cuda', when='@:3.3')
+    conflicts('+cuda', when='device=ch3')
+
+    provides('mpi@:4.0')
+    provides('mpi@:3.1', when='@:3.2')
     provides('mpi@:3.0', when='@:3.1')
     provides('mpi@:2.2', when='@:1.2')
     provides('mpi@:2.1', when='@:1.1')
@@ -433,7 +439,6 @@ def die_without_fortran(self):
     def configure_args(self):
         spec = self.spec
         config_args = [
-            '--without-cuda',
             '--disable-silent-rules',
             '--enable-shared',
             '--with-hwloc-prefix={0}'.format(
@@ -468,6 +473,8 @@ def configure_args(self):
         elif 'pmi=cray' in spec:
             config_args.append('--with-pmi=cray')
 
+        config_args += self.with_or_without('cuda', activation_value='prefix')
+
         # setup device configuration
         device_config = ''
         if 'device=ch4' in spec: