[pptt] in CODEGEN, undo my two recent changes, i.e. essentially go back to upstream/master (i.e. do not undo PR madgraph5#754, this is needed, keep also madgraph5#764)

** NB bug madgraph5#872 xsec mismatch will show up again in the CI and in manual tests **

Revert "[pptt] in CODEGEN for CPPProcess.cc, add back the static_assert expecting nprocesses == 1 (part of the fix for madgraph5#872 in PR madgraph5#935; undo changes from PR madgraph5#754 and madgraph5#764)"
This reverts commit b5576ad.

Revert "[pptt] in CODEGEN model_handling.py, disable mirror processing again (undo PR madgraph5#754) to fix madgraph5#872"
This reverts commit b28e04b.

Author: valassi

epochX/cudacpp/CODEGEN/PLUGIN/CUDACPP_SA_OUTPUT/madgraph/iolibs/template_files/gpu/process_function_definitions.inc

@@ -448,16 +448,12 @@ namespace mg5amcCpu
   {
     mgDebugInitialise();
 
-    // SANITY CHECKS for cudacpp code generation (see issues #272 and #343 and PRs #619, #626, #360, #396, #754 and #764)
-    // NB (AV July 2024): cudacpp is presently unsable to handle mirror processes, so they should be disabled also in Fortran (see issue #872 and PR #935)
+    // SANITY CHECKS for cudacpp code generation (see issues #272 and #343 and PRs #619, #626, #360, #396 and #754)
     // These variable are not used anywhere else in the code and their scope is limited to this sanity check
     {
-      // SANITY CHECKS FOR NPROCESSES
       // nprocesses == 2 may happen for "mirror processes" such as P0_uux_ttx within pp_tt012j (see PR #754)
       constexpr int nprocesses = %(nproc)i;
-      //static_assert( nprocesses == 1 || nprocesses == 2, "Assume nprocesses == 1 or 2" ); // AV: THIS IS WRONG AND LEADS TO BUG #872 (was introduced in PR #764 after PR #754)
-      static_assert( nprocesses == 1, "Assume nprocesses == 1" ); // AV: SANITY CHECK AGAINST BUG #872 (fortran/cudacpp xsec mismatch in pp_tt012j)
-      // SANITY CHECKS FOR PROCESS_ID
+      static_assert( nprocesses == 1 || nprocesses == 2, "Assume nprocesses == 1 or 2" );
       constexpr int process_id = %(proc_id)i; // code generation source: %(proc_id_source)s
       static_assert( process_id == 1, "Assume process_id == 1" );
     }

epochX/cudacpp/CODEGEN/PLUGIN/CUDACPP_SA_OUTPUT/model_handling.py

@@ -1419,14 +1419,14 @@ def generate_process_files(self):
         """Generate mgOnGpuConfig.h, CPPProcess.cc, CPPProcess.h, check_sa.cc, gXXX.cu links"""
         ###misc.sprint('Entering PLUGIN_OneProcessExporter.generate_process_files')
         ###if self.include_multi_channel:
-        ###    misc.sprint('self.include_multi_channel is already defined: this is madevent+second_exporter mode')
-        ###else:
-        ###    misc.sprint('self.include_multi_channel is not yet defined: this is standalone_cudacpp mode') # see issue #473
-        ###if not self.include_multi_channel: # OM this condition is likely wrong and need to be removed (added in PR #754; removed in PR #935 to fix #872)
-        if True: # AV fix fortran/cudacpp xsec mismatch in pp processes (added back in PR #935 to fix #872); also needed for standalone cudacpp in any case
-           if self.matrix_elements[0].get('has_mirror_process'):
-              self.matrix_elements[0].set('has_mirror_process', False)
-              self.nprocesses/=2
+        ###    misc.sprint('self.include_multi_channel is already defined: this is madevent+second_exporter mode') # FIXME? use self.in_madevent_mode instead?
+        if not self.include_multi_channel:
+            ###misc.sprint('self.include_multi_channel is not yet defined: this is standalone_cudacpp mode') # see issue #473
+            # AV: needed for (moved to?) standalone_cudacpp mode (but do we need those lines at all???)
+            # OM: this condition is likely wrong and need to be removed
+            if self.matrix_elements[0].get('has_mirror_process'):
+                self.matrix_elements[0].set('has_mirror_process', False)
+                self.nprocesses/=2
         super(PLUGIN_export_cpp.OneProcessExporterGPU, self).generate_process_files()
         self.edit_CMakeLists()
         self.edit_check_sa()