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.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[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!