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MPI wrappers are recorded as compilers, not expanded

docs/rationale/mpi-wrappers-as-compilers.md

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MPI wrappers are recorded as compilers, not expanded

Context

MPI wrapper commands (mpicc, mpicxx, mpifort, ... from Open MPI and MPICH; mpiicc, mpiifort, ... from Intel MPI) wrap the real compiler, but they are a different kind of wrapper than ccache/distcc/ sccache: the real compiler is not an argument of the invocation. The underlying compiler and the extra -I/-L flags are baked into the wrapper at MPI-installation time and are discoverable only by running the wrapper itself (mpicc -show on MPICH, mpicc -showme on Open MPI). Bear's wrapper-unwrapping mechanism therefore cannot apply.

Two options were on the table:

  • (a) Treat the wrapper itself as the compiler and record the invocation verbatim, parsing its flags with the flag table of the compiler family it fronts.
  • (b) Expand at analysis time by running mpicc -show (or -showme) and recording the underlying compiler command with the baked-in flags inlined.

Option (b) executes tools during semantic analysis, which is slow (one fork+exec per invocation or at least per wrapper path), couples Bear to the presence and health of the MPI installation at analysis time, and differs per MPI vendor (-show vs -showme vs -compile_info). The expanded command is also not necessarily what clang tooling needs: clangd already solves exactly this problem on its side with --query-driver, which asks the wrapper for its implicit include paths.

Decision

Option (a): record the wrapper invocation verbatim. Flag parsing uses the GCC flag table (via table inheritance), because the wrappers pass their arguments through to a gcc-compatible driver. Intel MPI wrappers are the exception: they are recognized as the Intel compilers themselves (extra basenames on the existing Intel families), because mpiicc is icc/icx with MPI paths baked in and Intel's flag table has arity entries (e.g. -debug takes an argument) that the GCC table would mis-parse.

Consequences

  • No process execution during analysis; recognition stays pure and works even when the MPI toolchain is not installed on the machine that runs bear semantic.
  • Clang tooling users may need --query-driver (clangd) or an equivalent mechanism to learn the wrapper's baked-in include paths. Revisit expansion only if users ask for it.
  • Wrapper-info invocations (-showme, -show, -compile_info, -link_info) classify as info-and-exit, so probing the wrapper does not pollute the database.
  • In preload mode the wrapper's child compiler exec is intercepted too; the default duplicate filter (directory+file) collapses the pair and the wrapper entry wins because its event comes first.

References