[clang] [CIR] Implement global array dtor support (PR #169070)

[Andy Kaylor via cfe-commits]

================
@@ -693,6 +696,101 @@ void LoweringPreparePass::lowerUnaryOp(cir::UnaryOp op) {
   op.erase();
 }
 
+cir::FuncOp LoweringPreparePass::getOrCreateDtorFunc(CIRBaseBuilderTy &builder,
+                                                     cir::GlobalOp op,
+                                                     mlir::Region &dtorRegion,
+                                                     cir::CallOp &dtorCall) {
+  assert(!cir::MissingFeatures::astVarDeclInterface());
+  assert(!cir::MissingFeatures::opGlobalThreadLocal());
+
+  cir::VoidType voidTy = builder.getVoidTy();
+  auto voidPtrTy = cir::PointerType::get(voidTy);
+
+  // Look for operations in dtorBlock
+  mlir::Block &dtorBlock = dtorRegion.front();
+
+  // The first operation should be a get_global to retrieve the address
+  // of the global variable we're destroying.
+  auto opIt = dtorBlock.getOperations().begin();
+  cir::GetGlobalOp ggop = mlir::cast<cir::GetGlobalOp>(*opIt);
+
+  // The simple case is just a call to a destructor, like this:
+  //
+  //   %0 = cir.get_global %globalS : !cir.ptr<!rec_S>
----------------
andykaylor wrote:

I agree that it's weird, but we need an `mlir::Value` to reference in subsequent operations. We could introduce a special operation that's only valid in GlobalOp ctor and dtor regions, like `cir.get_current_global` but there would still need to be an operation here. A special purpose op might be good because otherwise, it would be tricky to decide during lowering which `get_global` ops should refer to the current global and which should not in cases like this:

```
int someGlobal;
float anotherGlobal = (float)someGlobal;

```
which produces

```
cir.global external dso_local @someGlobal = #cir.int<0> : !s32i {alignment = 4 : i64}
cir.global external dso_local @anotherGlobal = ctor : !cir.float {
  %0 = cir.get_global @anotherGlobal : !cir.ptr<!cir.float>
  %1 = cir.get_global @someGlobal : !cir.ptr<!s32i>
  %2 = cir.load align(4) %1 : !cir.ptr<!s32i>, !s32i
  %3 = cir.cast int_to_float %2 : !s32i -> !cir.float
  cir.store align(4) %3, %0 : !cir.float, !cir.ptr<!cir.float>
}
```
Obviously, I'd want to defer that to a future change.

https://github.com/llvm/llvm-project/pull/169070