[flang] Restructure runtime to avoid recursion

Recursion, both direct and indirect, prevents accurate stack size
calculation at link time for GPU device code. Restructure these
recursive (often mutually so) routines in the Fortran runtime with new
implementations based on an iterative work queue with
suspendable/resumable work tickets: Assign, Initialize, initializeClone,
Finalize, and Destroy.

Default derived type I/O is also recursive, but already disabled. It can
be added to this new framework later if the overall approach succeeds.

Note that derived type FINAL subroutine calls, defined assignments, and
defined I/O procedures all perform callbacks into user code, which may
well reenter the runtime library. This kind of recursion is not handled
by this change, although it may be possible to do so in the future using
thread-local work queues.

(Relanding this patch after reverting initial attempt due to some test
failures that needed some time to analyze and fix.)

Fixes #142481.

Author: klausler

flang-rt/include/flang-rt/runtime/environment.h

@@ -64,6 +64,9 @@ struct ExecutionEnvironment {
   bool defaultUTF8{false}; // DEFAULT_UTF8
   bool checkPointerDeallocation{true}; // FORT_CHECK_POINTER_DEALLOCATION
 
+  enum InternalDebugging { WorkQueue = 1 };
+  int internalDebugging{0}; // FLANG_RT_DEBUG
+
   // CUDA related variables
   std::size_t cudaStackLimit{0}; // ACC_OFFLOAD_STACK_SIZE
   bool cudaDeviceIsManaged{false}; // NV_CUDAFOR_DEVICE_IS_MANAGED

flang-rt/include/flang-rt/runtime/stat.h

@@ -24,7 +24,7 @@ class Terminator;
 enum Stat {
   StatOk = 0, // required to be zero by Fortran
 
-  // Interoperable STAT= codes
+  // Interoperable STAT= codes (>= 11)
   StatBaseNull = CFI_ERROR_BASE_ADDR_NULL,
   StatBaseNotNull = CFI_ERROR_BASE_ADDR_NOT_NULL,
   StatInvalidElemLen = CFI_INVALID_ELEM_LEN,
@@ -36,7 +36,7 @@ enum Stat {
   StatMemAllocation = CFI_ERROR_MEM_ALLOCATION,
   StatOutOfBounds = CFI_ERROR_OUT_OF_BOUNDS,
 
-  // Standard STAT= values
+  // Standard STAT= values (>= 101)
   StatFailedImage = FORTRAN_RUNTIME_STAT_FAILED_IMAGE,
   StatLocked = FORTRAN_RUNTIME_STAT_LOCKED,
   StatLockedOtherImage = FORTRAN_RUNTIME_STAT_LOCKED_OTHER_IMAGE,
@@ -49,10 +49,14 @@ enum Stat {
   // Additional "processor-defined" STAT= values
   StatInvalidArgumentNumber = FORTRAN_RUNTIME_STAT_INVALID_ARG_NUMBER,
   StatMissingArgument = FORTRAN_RUNTIME_STAT_MISSING_ARG,
-  StatValueTooShort = FORTRAN_RUNTIME_STAT_VALUE_TOO_SHORT,
+  StatValueTooShort = FORTRAN_RUNTIME_STAT_VALUE_TOO_SHORT, // -1
   StatMoveAllocSameAllocatable =
       FORTRAN_RUNTIME_STAT_MOVE_ALLOC_SAME_ALLOCATABLE,
   StatBadPointerDeallocation = FORTRAN_RUNTIME_STAT_BAD_POINTER_DEALLOCATION,
+
+  // Dummy status for work queue continuation, declared here to perhaps
+  // avoid collisions
+  StatContinue = 201
 };
 
 RT_API_ATTRS const char *StatErrorString(int);

flang-rt/include/flang-rt/runtime/type-info.h

@@ -154,12 +154,17 @@ class SpecialBinding {
   RT_API_ATTRS bool IsArgDescriptor(int zeroBasedArg) const {
     return (isArgDescriptorSet_ >> zeroBasedArg) & 1;
   }
-  RT_API_ATTRS bool isTypeBound() const { return isTypeBound_; }
+  RT_API_ATTRS bool IsTypeBound() const { return isTypeBound_ != 0; }
   RT_API_ATTRS bool IsArgContiguous(int zeroBasedArg) const {
     return (isArgContiguousSet_ >> zeroBasedArg) & 1;
   }
-  template <typename PROC> RT_API_ATTRS PROC GetProc() const {
-    return reinterpret_cast<PROC>(proc_);
+  template <typename PROC>
+  RT_API_ATTRS PROC GetProc(const Binding *bindings = nullptr) const {
+    if (bindings && isTypeBound_ > 0) {
+      return reinterpret_cast<PROC>(bindings[isTypeBound_ - 1].proc);
+    } else {
+      return reinterpret_cast<PROC>(proc_);
+    }
   }
 
   FILE *Dump(FILE *) const;
@@ -193,6 +198,8 @@ class SpecialBinding {
   //     When false, the defined I/O subroutine must have been
   //     called via a generic interface, not a generic TBP.
   std::uint8_t isArgDescriptorSet_{0};
+  // When a special binding is type-bound, this is its binding's index (plus 1,
+  // so that 0 signifies that it's not type-bound).
   std::uint8_t isTypeBound_{0};
   // True when a FINAL subroutine has a dummy argument that is an array that
   // is CONTIGUOUS or neither assumed-rank nor assumed-shape.
@@ -240,6 +247,7 @@ class DerivedType {
   RT_API_ATTRS bool noFinalizationNeeded() const {
     return noFinalizationNeeded_;
   }
+  RT_API_ATTRS bool noDefinedAssignment() const { return noDefinedAssignment_; }
 
   RT_API_ATTRS std::size_t LenParameters() const {
     return lenParameterKind().Elements();
@@ -322,6 +330,7 @@ class DerivedType {
   bool noInitializationNeeded_{false};
   bool noDestructionNeeded_{false};
   bool noFinalizationNeeded_{false};
+  bool noDefinedAssignment_{false};
 };
 
 } // namespace Fortran::runtime::typeInfo