• Type: Correctness / MPI data-type mismatch
• Severity: High
• Component: Box communication helper
• Location:
  • Src/Base/AMReX_Box.cpp:166
  • Src/Base/AMReX_Box.cpp:176

Problem

AllGatherBoxes computes the total number of boxes in Long count_tot, then broadcasts it via

MPI_Bcast(&count_tot, 1, MPI_INT, root, comm);

On LP64 platforms (Long = 64-bit), this is a type-size mismatch (MPI_INT = 32-bit).

Impact

• Non-root ranks receive only the low 32 bits of count_tot.
• High bits are lost/undefined, which can lead to incorrect buffer sizing and undefined behavior for large totals.
• The subsequent overflow guard runs on corrupted data on non-root ranks.

Suggested patch

Use the AMReX MPI typemap for Long (or MPI_LONG if guaranteed compatible).

--- a/Src/Base/AMReX_Box.cpp
+++ b/Src/Base/AMReX_Box.cpp
@@
-    MPI_Bcast(&count_tot, 1, MPI_INT, root, comm);
+    MPI_Bcast(&count_tot, 1,
+              ParallelDescriptor::Mpi_typemap<Long>::type(),
+              root, comm);

Prepared by Codex