[llvm] [DeadStoreElimination] Optimize tautological assignments (PR #75744)

[Shreyansh Chouhan via llvm-commits]

BK1603 wrote:

Just an update, still looking into this (I only get time for this during weekends.)

The crash is happening during the RegAllocGreedy pass. Specifically when we call `InterferenceCache::get` 
```
InterferenceCache::Entry *InterferenceCache::get(MCRegister PhysReg) {
  unsigned char E = PhysRegEntries[PhysReg.id()];
  if (E < CacheEntries && Entries[E].getPhysReg() == PhysReg) {
    if (!Entries[E].valid(LIUArray, TRI))
      Entries[E].revalidate(LIUArray, TRI);
    return &Entries[E];
  }
  // No valid entry exists, pick the next round-robin entry.
  E = RoundRobin;
  if (++RoundRobin == CacheEntries)
    RoundRobin = 0;
  for (unsigned i = 0; i != CacheEntries; ++i) {
    // Skip entries that are in use.
    if (Entries[E].hasRefs()) {
      if (++E == CacheEntries)
        E = 0;
      continue;
    }
    Entries[E].reset(PhysReg, LIUArray, TRI, MF);
    PhysRegEntries[PhysReg] = E;
    return &Entries[E];
  }
  llvm_unreachable("Ran out of interference cache entries.");
}
```
Instead of getting the actual pointer to the cache entry, we are getting the index of the entry. This later causes a segfault when we try to call `setEntry` on this returned index. (It in turn calls `Entry->addRef` and entry becomes an invalid address.) From what I can understand right now, this happens because
the condition `Entries[E].hasRefs()` ends up always being true. The variable `RefCount` is unsigned, and it underflows and wraps around to a value greater than 0 again, and we are never able to go to the `Entries[e].reset(PhysReg, LIUArray, TRI, MF)` line. 



https://github.com/llvm/llvm-project/pull/75744