[llvm] [MemCpyOpt] Extend call slot optimization for non-dereferenceable destinations. (PR #217436)

[Nikita Popov via llvm-commits]

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@@ -924,8 +924,17 @@ bool MemCpyOptPass::performCallSlotOptzn(Instruction *cpyLoad,
                         ExplicitlyDereferenceableOnly) ||
       !isDereferenceablePointer(cpyDest, APInt(64, cpySize),
                                 SimplifyQuery(DL, DT, AC, C))) {
-    LLVM_DEBUG(dbgs() << "Call Slot: Dest pointer not dereferenceable\n");
-    return false;
+    // If the call is guaranteed to return normally (willreturn + nounwind),
+    // and there are no instructions between the call and the store that might
+    // trap or throw, execution will reach the store. Since the store would
+    // trap anyway if the pointer was not dereferenceable, we can forward the
+    // pointer to the call. Perform optimization only for non-memcpy/memset
+    // calls, as those are special cased later.
----------------
nikic wrote:

```suggestion
    // pointer to the call.
```

https://github.com/llvm/llvm-project/pull/217436