[flang] [llvm] [flang][runtime] Replace recursion with iterative work queue (PR #137727)

[Slava Zakharin via llvm-commits]

================
@@ -0,0 +1,153 @@
+//===-- lib/runtime/work-queue.cpp ------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "flang-rt/runtime/work-queue.h"
+#include "flang-rt/runtime/environment.h"
+#include "flang-rt/runtime/type-info.h"
+#include "flang/Common/visit.h"
+
+namespace Fortran::runtime {
+
+#if !defined(RT_DEVICE_COMPILATION)
+// FLANG_RT_DEBUG code is disabled when false.
+static constexpr bool enableDebugOutput{false};
+#endif
+
+RT_OFFLOAD_API_GROUP_BEGIN
+
+RT_API_ATTRS Componentwise::Componentwise(const typeInfo::DerivedType &derived)
+    : derived_{derived}, components_{derived_.component().Elements()} {
+  GetComponent();
+}
+
+RT_API_ATTRS void Componentwise::GetComponent() {
+  if (componentAt_ >= components_) {
+    component_ = nullptr;
+  } else {
+    const Descriptor &componentDesc{derived_.component()};
+    component_ = componentDesc.ZeroBasedIndexedElement<typeInfo::Component>(
+        componentAt_);
+  }
+}
+
+RT_API_ATTRS int Ticket::Continue(WorkQueue &workQueue) {
+  if (!begun) {
+    begun = true;
+    return common::visit(
+        [&workQueue](
+            auto &specificTicket) { return specificTicket.Begin(workQueue); },
+        u);
+  } else {
+    return common::visit(
+        [&workQueue](auto &specificTicket) {
+          return specificTicket.Continue(workQueue);
+        },
+        u);
+  }
+}
+
+RT_API_ATTRS WorkQueue::~WorkQueue() {
+  if (last_) {
+    if ((last_->next = firstFree_)) {
+      last_->next->previous = last_;
+    }
+    firstFree_ = first_;
+    first_ = last_ = nullptr;
+  }
+  while (firstFree_) {
+    TicketList *next{firstFree_->next};
+    if (!firstFree_->isStatic) {
+      delete firstFree_;
+    }
+    firstFree_ = next;
+  }
+}
+
+RT_API_ATTRS Ticket &WorkQueue::StartTicket() {
+  if (!firstFree_) {
+    firstFree_ = new TicketList;
+    firstFree_->isStatic = false;
+  }
+  TicketList *newTicket{firstFree_};
+  if ((firstFree_ = newTicket->next)) {
+    firstFree_->previous = nullptr;
+  }
+  TicketList *after{insertAfter_ ? insertAfter_->next : nullptr};
+  if ((newTicket->previous = insertAfter_ ? insertAfter_ : last_)) {
+    newTicket->previous->next = newTicket;
+  } else {
+    first_ = newTicket;
+  }
+  if ((newTicket->next = after)) {
+    after->previous = newTicket;
+  } else {
+    last_ = newTicket;
+  }
+  newTicket->ticket.begun = false;
+#if !defined(RT_DEVICE_COMPILATION)
+  if (enableDebugOutput && (executionEnvironment.internalDebugging & 1)) {
----------------
vzakhari wrote:

nit: could you turn literal `1` into a named constant so that it is easier to see which bits of `internalDebugging` are used for what (given that we may add more bits in future)?

https://github.com/llvm/llvm-project/pull/137727