(TPFUG/Visa) Support COMPATBILITY mode(s) to aid compiler migration

In order to migrate to a new compiler, all new warnings or errors in user code must be corrected and/or user defined compiler overrides must be applied to suppress warnings enabled by default for the new compiler. Some shops may require all code remediation is done before migrating to the new compiler to avoid issues with emergency loads etc. Remediation can take considerable effort, resources, and coordination.

We need support for one or more “compatibility” phases of the new compiler to expedite rollout by supporting a graduated approach to remediating the full suite of new compiler warnings. For example, TPFGCC_VERSION could be set to the current or new compiler versions which would apply the full set of compiler options in each case, but could also be set to COMPAT which would use the new compiler version but disable as many of the new compiler warnings as possible and revert any default compile options back to the current compiler C/C++ standard defaults wherever possible. Use of the COMPAT version would represent the minimum amount of change required in order to use the new compiler for production code builds. IBM would determine what options were required for COMPAT. Using the COMPAT option could then become phase 1 in a multi-phased approach to a compiler upgrade.

To support later phases, we would also like IBM to fully support one or more user named COMPATn values for TPFGCC_VERSION. These COMPATn compiler versions would represent interim phases (compiler option settings) between the minimalist IBM provided COMPAT and the new compiler version with no compiler warnings suppressed or C/C++ standard defaults back-levelled.

Today, users can override compiler options for all builds in user exit file maketpf.user_rules_setup1. This ability needs to be preserved so users can override settings in any supported TPFGCC_VERSION option, including compatibility modes.