[clang] [CIR] Upstream StackSave and StackRestoreOp (PR #136426)

[Henrich Lauko via cfe-commits]

================
@@ -1419,6 +1419,48 @@ def CallOp : CIR_CallOpBase<"call", [NoRegionArguments]> {
     }]>];
 }
 
+//===----------------------------------------------------------------------===//
+// StackSave & StackRestoreOp
+//===----------------------------------------------------------------------===//
+
+def StackSaveOp : CIR_Op<"stack_save"> {
+  let summary = "remembers the current state of the function stack";
+  let description = [{
+    Remembers the current state of the function stack. Returns a pointer
+    that later can be passed into cir.stack_restore.
+    Useful for implementing language features like variable length arrays.
+
+    ```mlir
+    %0 = cir.stack_save : <!u8i>
+    ```
+  }];
+
+  let results = (outs CIR_PointerType:$result);
+  let assemblyFormat = "attr-dict `:` qualified(type($result))";
+}
+
+def StackRestoreOp : CIR_Op<"stack_restore"> {
+  let summary = "restores the state of the function stack";
+  let description = [{
+    Restore the state of the function stack to the state it was
+    in when the corresponding cir.stack_save executed.
+    Useful for implementing language features like variable length arrays.
+
+    ```mlir
+    %0 = cir.alloca !cir.ptr<!u8i>, !cir.ptr<!cir.ptr<!u8i>>, ["saved_stack"] {alignment = 8 : i64}
+    %1 = cir.stack_save : <!u8i>
+    cir.store %1, %0 : !cir.ptr<!u8i>, !cir.ptr<!cir.ptr<!u8i>>
+    %2 = cir.load %0 : !cir.ptr<!cir.ptr<!u8i>>, !cir.ptr<!u8i>
+    cir.stack_restore %2 : !cir.ptr<!u8i>
+    ```
+  }];
+
+  let arguments = (ins CIR_PointerType:$ptr);
+  let assemblyFormat = "$ptr attr-dict `:` qualified(type($ptr))";
+
+  let llvmOp = "StackRestoreOp";
----------------
xlauko wrote:

Why is this here, since it is not used? Also it is only in one of these operations.

If used, I would expect not to have explicit `CIRToLLVMStackRestoreOpLowering` and `CIRToLLVMStackSaveOpLowering` patterns, but use the generated one.


https://github.com/llvm/llvm-project/pull/136426