[llvm] [DSE] Apply initializes attribute to DSE (PR #107282)

[Arthur Eubanks via llvm-commits]

================
@@ -2181,6 +2263,75 @@ struct DSEState {
   bool eliminateDeadDefs(const MemoryDefWrapper &KillingDefWrapper);
 };
 
+SmallVector<MemoryLocation, 1>
+DSEState::getInitializesArgMemLoc(const Instruction *I) {
+  const CallBase *CB = dyn_cast<CallBase>(I);
+  if (!CB)
+    return {};
+
+  // Collect aliasing arguments and their initializes ranges.
+  SmallMapVector<Value *, SmallVector<ArgumentInitInfo, 2>, 2> Arguments;
+  for (unsigned Idx = 0, Count = CB->arg_size(); Idx < Count; ++Idx) {
+    ConstantRangeList Inits;
+    Attribute InitializesAttr = CB->getParamAttr(Idx, Attribute::Initializes);
+    if (InitializesAttr.isValid())
+      Inits = InitializesAttr.getValueAsConstantRangeList();
+
+    Value *CurArg = CB->getArgOperand(Idx);
+    // We don't perform incorrect DSE on unwind edges in the current function,
+    // and use the "initializes" attribute to kill dead stores if:
+    // - The call does not throw exceptions, "CB->doesNotThrow()".
+    // - Or the callee parameter has "dead_on_unwind" attribute.
+    // - Or the argument is invisible to caller on unwind, and CB isa<CallInst>
+    // which means no unwind edges from this call in the current function.
----------------
aeubanks wrote:

I haven't heard the term "pure function call" refer to a call that doesn't unwind, I prefer something closer to the current version.

```
Or the argument is invisible to caller on unwind, and there are no unwind edges from this call in the current function (e.g. `CallInst`).
```

There are probably (?) more cases than just `CallInst` where we do this optimization so I use `e.g.` instead of `i.e.`.

https://github.com/llvm/llvm-project/pull/107282