Fix WP_MOK hardcoded to 8 bytes, use storage_size for portability

[sbryngelson]

Summary

Severity: HIGH — MPI file I/O reads from wrong offsets in single-precision builds.

File: src/post_process/m_data_input.f90, line 136 (and 2 other sites in the same file)

WP_MOK (the MPI offset for the working-precision real size) is hardcoded to int(8._wp, MPI_OFFSET_KIND), assuming 8-byte (double-precision) reals. In single-precision builds where wp corresponds to 4 bytes, this causes all MPI file read offsets to be wrong by a factor of 2.

Before

WP_MOK = int(8._wp, MPI_OFFSET_KIND)    ! assumes double precision

After

WP_MOK = int(storage_size(0._wp)/8, MPI_OFFSET_KIND)    ! actual byte size

All 3 occurrences in the file are fixed.

Why this went undetected

The codebase is predominantly used in double precision where wp = 8 bytes, so the hardcoded value happens to be correct.

Test plan

• Build in both single and double precision
• Verify MPI parallel I/O reads correct data in both modes

🤖 Generated with Claude Code

Fixes #1207

Summary by CodeRabbit

Bug Fixes

• Fixed MPI I/O offset calculations to dynamically determine floating-point word size instead of hard-coded values, improving data alignment and correctness in parallel file operations across data input, output, and simulation modules.

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📝 Walkthrough

Walkthrough

This PR fixes a critical bug where MPI I/O offset calculations hardcoded 8-byte assumptions for working precision real size, breaking single-precision builds. The fix replaces hardcoded values with dynamic calculations using storage_size(0._wp)/8 to derive the actual working precision byte width.

Changes

Cohort / File(s)	Summary
Post-Process MPI I/O src/post_process/m_data_input.f90	Replaced three hardcoded int(8._wp, MPI_OFFSET_KIND) calls with int(storage_size(0._wp)/8, MPI_OFFSET_KIND) in conservative variable and IB data MPI read paths.
Pre-Process MPI I/O src/pre_process/m_data_output.fpp, src/pre_process/m_start_up.fpp	Replaced two WP_MOK initializations in data output setup and one in MPI file read path from fixed 8-byte to dynamic storage-size calculation.
Simulation MPI I/O src/simulation/m_data_output.fpp, src/simulation/m_start_up.fpp	Replaced three hardcoded WP_MOK values in data output offset computation and one in MPI read metadata setup with storage-aware calculations.

Estimated Code Review Effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Suggested Labels

Review effort 3/5, size:M

Poem

🐰 Eight bytes assumed, but times have changed,
Single precision left deranged,
Dynamic sizing saves the day,
MPI offsets now find their way,
Real storage size, at last revealed! ✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: fixing hardcoded WP_MOK value from 8 bytes to dynamic storage_size-based computation for portability.
Description check	✅ Passed	The description covers the bug, severity, affected code, solution approach, and reasoning. It includes issue reference (#1207) and test plan checklist items.
Linked Issues check	✅ Passed	The PR addresses all 10 occurrences of hardcoded WP_MOK across five files identified in issue #1207, replacing each with storage_size(0._wp)/8 for precision-portable MPI offset computation.
Out of Scope Changes check	✅ Passed	All changes are narrowly scoped to replacing hardcoded WP_MOK calculations with storage_size-based equivalents, directly addressing the single-precision MPI I/O bug without introducing unrelated modifications.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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[codeant-ai]

Nitpicks 🔍

🔒 No security issues identified

⚡ Recommended areas for review storage_size semantics The code now uses storage_size(0._wp)/8 to compute bytes per working precision real. Verify that storage_size returns bits on all supported compilers and that dividing by 8 yields the expected bytes (e.g. 4 or 8). Confirm this produces an integer and that no unintended real-to-integer truncation/rounding can occur in target builds. Inconsistent integer kinds In s_read_ib_data_files WP_MOK (and related MPI-offset variables) are declared as default INTEGER, but the assignment uses INT(..., MPI_OFFSET_KIND). This can cause implicit kind conversions and subtle bugs when offsets are used with MPI calls. Ensure local declarations use KIND=MPI_OFFSET_KIND where appropriate.

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[Copilot]

Pull request overview

This PR fixes MPI file offset calculations that were hardcoded for 8-byte wp reals, making parallel I/O portable across single- and double-precision builds.

Changes:

• Replaced WP_MOK = int(8._wp, MPI_OFFSET_KIND) with a storage_size-based byte-size computation.
• Applied the fix across multiple MPI I/O read/setup paths to keep offsets consistent.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 44.05%. Comparing base (84c46e0) to head (adf2515).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1187   +/-   ##
=======================================
  Coverage   44.05%   44.05%           
=======================================
  Files          70       70           
  Lines       20498    20498           
  Branches     1990     1990           
=======================================
  Hits         9030     9030           
  Misses      10329    10329           
  Partials     1139     1139           

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