Add C11 atomic wrapper structs to prevent direct access and control memory ordering

[Copilot]

This PR implements a C11 atomic wrapper system that prevents direct access to atomic variables and provides explicit memory ordering control, addressing the overhead of sequential consistency.

Problem

OpenMPI currently uses C11 _Atomic types directly via typedefs, which allows users to accidentally perform direct assignments that default to sequential consistency:

// Current (problematic) - defaults to seq_cst
opal_atomic_int32_t counter = 0;  
counter = 42;  // Sequential consistency overhead

Solution

The implementation wraps all _Atomic types in structs to prevent direct access and provides accessor functions with explicit memory ordering control:

// New wrapper struct (prevents direct access)
typedef struct { _Atomic int32_t value; } opal_atomic_int32_t;

// Explicit memory ordering (defaults to relaxed)
opal_atomic_store_32_relaxed(&counter, 42);       // Relaxed ordering
opal_atomic_store_32(&counter, 42, memory_order_acquire);  // Explicit ordering

Key Changes

1. Wrapper Structs: All _Atomic types are now wrapped in structs with .value members
2. Accessor Functions: Provided load/store functions that require explicit memory ordering
3. Relaxed Defaults: Convenience functions default to memory_order_relaxed to avoid seq_cst overhead
4. Volatile Support: Added variants for volatile atomic variables used in lock-free data structures
5. API Migration: Updated core atomic operations and data structures to use the new API

Affected Components

• Core atomic infrastructure (opal_stdatomic.h, atomic_stdc.h)
• Object reference counting and memory management
• Lock-free data structures (LIFO, FIFO, lists)
• Threading and synchronization primitives
• BTL shared memory communication
• Memory pool and cache management

Verification

The wrapper successfully prevents compilation of direct atomic access:

opal_atomic_int32_t x = {.value = 0};
x = 5;  // ❌ Compilation error - direct assignment prevented
opal_atomic_store_32_relaxed(&x, 5);  // ✅ Required explicit API

This ensures all atomic operations use explicit memory ordering, eliminating accidental sequential consistency overhead while maintaining correctness.

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[github-actions]

Hello! The Git Commit Checker CI bot found a few problems with this PR:

d49a9f8: Complete C11 atomic wrapper implementation - worki...

• check_signed_off: does not contain a valid Signed-off-by line

f81e3ed: Fix additional atomic access issues and add volati...

• check_signed_off: does not contain a valid Signed-off-by line

3759195: Implement C11 atomic wrapper structs and fix direc...

• check_signed_off: does not contain a valid Signed-off-by line

0c0f801: Initial plan

• check_signed_off: does not contain a valid Signed-off-by line

Please fix these problems and, if necessary, force-push new commits back up to the PR branch. Thanks!

[github-actions]

Hello! The Git Commit Checker CI bot found a few problems with this PR:

d49a9f8: Complete C11 atomic wrapper implementation - worki...

• check_signed_off: does not contain a valid Signed-off-by line

f81e3ed: Fix additional atomic access issues and add volati...

• check_signed_off: does not contain a valid Signed-off-by line

3759195: Implement C11 atomic wrapper structs and fix direc...

• check_signed_off: does not contain a valid Signed-off-by line

0c0f801: Initial plan

• check_signed_off: does not contain a valid Signed-off-by line

Please fix these problems and, if necessary, force-push new commits back up to the PR branch. Thanks!

[github-actions]

Hello! The Git Commit Checker CI bot found a few problems with this PR:

d49a9f8: Complete C11 atomic wrapper implementation - worki...

• check_signed_off: does not contain a valid Signed-off-by line

f81e3ed: Fix additional atomic access issues and add volati...

• check_signed_off: does not contain a valid Signed-off-by line

3759195: Implement C11 atomic wrapper structs and fix direc...

• check_signed_off: does not contain a valid Signed-off-by line

0c0f801: Initial plan

• check_signed_off: does not contain a valid Signed-off-by line

Please fix these problems and, if necessary, force-push new commits back up to the PR branch. Thanks!

[bwbarrett]

I'm not sure I have objections to the overall idea, but the patches put all the context about why the change in the PR message, which means it's effectively lost once merged. THe commit messages should have the important context.

[bwbarrett]

@devreal what do you want to do with this PR?

[devreal]

Closing. I didn't expect Copilot to open a PR against the main repo.