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objfs C/C++ Roadmap to ccache Parity

Track progress toward ccache-level caching support.

Current Status

✅ Working:

  • Local CAS caching for .c/.cpp → .o
  • GCC and Clang support
  • CMake and Make integration
  • Basic flag handling
  • Content-addressed storage

❌ Missing (vs ccache):

  • Header dependency tracking
  • Comprehensive flag handling
  • LTO support
  • Precompiled headers
  • Link caching

Issue Tracking

Issues stored in git notes (refs/notes/issue-*).

Commands:

  • git-issue list - Show all issues
  • git-issue ready - Show ready to work on
  • git-issue deps - Show dependency graph
  • git-issue show <id> - View issue details
  • git-issue update <id> - Modify issue

Sync with remote:

  • git-issue setup-sync enable - Auto-sync notes on push/pull
  • Or manually: git push origin 'refs/notes/*'

High Priority (Correctness)

#f6021c5: Comprehensive compiler flag handling (no blockers)

  • Include all flags in cache key (-I, -D, -O, -std, etc)
  • Document which flags affect output
  • Test with all common flag combinations
  • Status: Ready to start

#1b51c7f: Header dependency tracking (blocked by #f6021c5)

  • Parse -MD output for .h dependencies
  • Include header hashes in cache key
  • Invalidate on header changes
  • Critical: Currently returns stale .o files
  • Status: Blocked until flags done

#86a021d: Remote execution support (no blockers)

  • Integrate with RE API v2
  • Upload sources to remote CAS
  • Execute on remote workers
  • Download results
  • Status: Ready to start

Medium Priority

#75ac80a: LTO flag support (relates to #f6021c5)

  • Handle -flto and variants
  • Cache GIMPLE bytecode correctly
  • Support thin LTO
  • Status: Can start anytime

Low Priority (Optimizations)

#7b7c653: Precompiled header caching (blocked by #1b51c7f)

  • Cache .pch files
  • Track transitive header deps
  • Status: Blocked

#d4f9b53: Link-time caching (no blockers)

  • Cache linking operations
  • Handle large executables
  • Platform-specific
  • Status: Can start anytime

Dependency Graph

f6021c5 (flags) ──blocks──> 1b51c7f (headers) ──blocks──> 7b7c653 (PCH)
                                ↑
                            relates_to
                                ↑
                          75ac80a (LTO)

86a021d (remote) ← independent
d4f9b53 (link)   ← independent

Next Steps

  1. Start with #f6021c5 (flags): Foundation for everything
  2. Then #1b51c7f (headers): Critical correctness issue
  3. Parallel #86a021d (remote): Independent, high value

After these three, objfs will match ccache for common use cases.

Milestones

Milestone 1: Correctness (flags + headers)

  • Complete flag handling
  • Header dependency tracking
  • No stale cache returns
  • Status: ccache-equivalent correctness

Milestone 2: Distribution (remote)

  • Remote execution working
  • Distributed cache sharing
  • Status: Beyond ccache capabilities

Milestone 3: Optimizations (LTO, PCH, link)

  • Advanced feature support
  • Full compiler flag coverage
  • Status: ccache parity + distribution

Testing Strategy

For each issue:

  1. Write tests with real C/C++ projects
  2. Compare behavior with ccache
  3. Verify cache hits/misses correct
  4. Benchmark performance

Test projects:

  • Simple: examples/cmake-example
  • Medium: Real OSS project (TBD)
  • Large: Major C++ codebase (TBD)

Success Criteria

ccache parity achieved when:

  • ✅ Header changes invalidate cache
  • ✅ All compiler flags handled correctly
  • ✅ No false cache hits (correctness)
  • ✅ Comparable hit rate to ccache
  • ✅ Distributed caching works
  • ✅ Remote execution works

Beyond ccache:

  • ✅ Distributed builds (not just cache)
  • ✅ Works for Rust + C/C++ in same project
  • ✅ Unified CAS for all languages