Erik Eckstein 9a124742b0 Optimizer: add the DeadAccessScopeElimination optimization pass ...

It eliminates dead access scopes if they are not conflicting with other scopes.

Removes:
```
  %2 = begin_access [modify] [dynamic] %1
  ...                                       // no uses of %2
  end_access %2
```

However, dead _conflicting_ access scopes are not removed.
If a conflicting scope becomes dead because an optimization e.g. removed a load, it is still important to get an access violation at runtime.
Even a propagated value of a redundant load from a conflicting scope is undefined.

```
  %2 = begin_access [modify] [dynamic] %1
  store %x to %2
  %3 = begin_access [read] [dynamic] %1    // conflicting with %2!
  %y = load %3
  end_access %3
  end_access %2
  use(%y)
```
After redundant-load-elimination:
```
  %2 = begin_access [modify] [dynamic] %1
  store %x to %2
  %3 = begin_access [read] [dynamic] %1    // now dead, but still conflicting with %2
  end_access %3
  end_access %2
  use(%x)                                  // propagated from the store, but undefined here!
```
In this case the scope `%3` is not removed because it's important to get an access violation error at runtime before the undefined value `%x` is used.

This pass considers potential conflicting access scopes in called functions.
But it does not consider potential conflicting access in callers (because it can't!).
However, optimizations, like redundant-load-elimination, can only do such transformations if the outer access scope is within the function, e.g.

```
bb0(%0 : $*T):     // an inout from a conflicting scope in the caller
  store %x to %0
  %3 = begin_access [read] [dynamic] %1
  %y = load %3     // cannot be propagated because it cannot be proved that %1 is the same address as %0
  end_access %3
```

All those checks are only done for dynamic access scopes, because they matter for runtime exclusivity checking.
Dead static scopes are removed unconditionally.

2025-11-24 14:49:45 +01:00

9.7 KiB

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