[flang] Make NEWUNIT= use a range suitable for INTEGER(KIND=1) and recycle unit numbers

[klausler]

Summary

Use a bit-set to manage runtime-generated I/O unit numbers, recycle
them after they're closed, and use a range of values that fits in
a minimal-sized integer.

flang/runtime/io-api.cpp

ExternalFileUnit &unit{
       ExternalFileUnit::LookUpOrCreate(unitNumber, terminator, wasExtant)};
   return &unit.BeginIoStatement<OpenStatementState>(
       unit, wasExtant, sourceFile, sourceLine);
 }
 
 Cookie IONAME(BeginOpenNewUnit)( // OPEN(NEWUNIT=j)
     const char *sourceFile, int sourceLine) {
   Terminator terminator{sourceFile, sourceLine};
-  ExternalFileUnit &unit{ExternalFileUnit::NewUnit(terminator)};
+  ExternalFileUnit &unit{
+      ExternalFileUnit::NewUnit(terminator, false /*not child I/O*/)};
   return &unit.BeginIoStatement<OpenStatementState>(
       unit, false /*was an existing file*/, sourceFile, sourceLine);
 }
 
 Cookie IONAME(BeginClose)(
     ExternalUnit unitNumber, const char *sourceFile, int sourceLine) {
   if (ExternalFileUnit * unit{ExternalFileUnit::LookUpForClose(unitNumber)}) {
     return &unit->BeginIoStatement<CloseStatementState>(

flang/runtime/unit-map.h

 //===-- runtime/unit-map.h --------------------------------------*- 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
 //
 //===----------------------------------------------------------------------===//
 
 // Maps Fortran unit numbers to their ExternalFileUnit instances.
 // A simple hash table with forward-linked chains per bucket.
 
 #ifndef FORTRAN_RUNTIME_UNIT_MAP_H_
 #define FORTRAN_RUNTIME_UNIT_MAP_H_
 
 #include "lock.h"
 #include "unit.h"
+#include "flang/Common/constexpr-bitset.h"
 #include "flang/Runtime/memory.h"
+#include <cstdint>
 #include <cstdlib>
 
 namespace Fortran::runtime::io {
 
 class UnitMap {
 public:
   ExternalFileUnit *LookUp(int n) {
     CriticalSection critical{lock_};
public:
   }
 
   // Unit look-up by name is needed for INQUIRE(FILE="...")
   ExternalFileUnit *LookUp(const char *path) {
     CriticalSection critical{lock_};
     return Find(path);
   }
 
-  ExternalFileUnit &NewUnit(const Terminator &terminator) {
-    CriticalSection critical{lock_};
-    return Create(nextNewUnit_--, terminator);
-  }
+  ExternalFileUnit &NewUnit(const Terminator &);
 
   // To prevent races, the unit is removed from the map if it exists,
   // and put on the closing_ list until DestroyClosed() is called.
   ExternalFileUnit *LookUpForClose(int);
 
   void DestroyClosed(ExternalFileUnit &);
   void CloseAll(IoErrorHandler &);
   void FlushAll(IoErrorHandler &);
ExternalFileUnit *Find(int n) {
     return nullptr;
   }
   ExternalFileUnit *Find(const char *path);
 
   ExternalFileUnit &Create(int, const Terminator &);
 
   Lock lock_;
   OwningPtr<Chain> bucket_[buckets_]{}; // all owned by *this
-  int nextNewUnit_{-1000}; // see 12.5.6.12 in Fortran 2018
+  common::BitSet<64> busyNewUnits_;
   OwningPtr<Chain> closing_{nullptr}; // units during CLOSE statement
 };
 } // namespace Fortran::runtime::io
 #endif // FORTRAN_RUNTIME_UNIT_MAP_H_

flang/runtime/unit-map.cpp

 //===-- runtime/unit-map.cpp ----------------------------------------------===//
 //
 // 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 "unit-map.h"
 
 namespace Fortran::runtime::io {
 
+// See 12.5.6.12 in Fortran 2018.  NEWUNIT= unit numbers are negative,
+// and not equal -1 (or ERROR_UNIT, if it were negative, which it isn't.)
+ExternalFileUnit &UnitMap::NewUnit(const Terminator &terminator) {
+  CriticalSection critical{lock_};
+  std::optional<std::size_t> n;
+  n = (~busyNewUnits_).LeastElement();
+  if (!n.has_value()) {
+    terminator.Crash(
+        "No available unit number for NEWUNIT= or internal child I/O");
+  }
+  busyNewUnits_.set(*n);
+  // bit position 0 <-> unit -2; kind=1 units are in [-65..-2]
+  return Create(static_cast<int>(-2 - *n), terminator);
+}
+
 ExternalFileUnit *UnitMap::LookUpForClose(int n) {
   CriticalSection critical{lock_};
   Chain *previous{nullptr};
   int hash{Hash(n)};
   for (Chain *p{bucket_[hash].get()}; p; previous = p, p = p->next.get()) {
     if (p->unit.unitNumber() == n) {
       if (previous) {
         previous->next.swap(p->next);

 
 void UnitMap::DestroyClosed(ExternalFileUnit &unit) {
   Chain *p{nullptr};
   {
     CriticalSection critical{lock_};
     Chain *previous{nullptr};
     for (p = closing_.get(); p; previous = p, p = p->next.get()) {
       if (&p->unit == &unit) {
+        int n{unit.unitNumber()};
+        if (n <= -2) {
+          busyNewUnits_.reset(static_cast<std::size_t>(-2 - n));
+        }
         if (previous) {
           previous->next.swap(p->next);
         } else {
           closing_.swap(p->next);
         }
         break;
       }
     }

flang/runtime/unit.h

public:
   static ExternalFileUnit &LookUpOrCrash(int unit, const Terminator &);
   static ExternalFileUnit &LookUpOrCreate(
       int unit, const Terminator &, bool &wasExtant);
   static ExternalFileUnit &LookUpOrCreateAnonymous(int unit, Direction,
       std::optional<bool> isUnformatted, const Terminator &);
   static ExternalFileUnit *LookUp(const char *path);
   static ExternalFileUnit &CreateNew(int unit, const Terminator &);
   static ExternalFileUnit *LookUpForClose(int unit);
-  static ExternalFileUnit &NewUnit(const Terminator &, bool forChildIo = false);
+  static ExternalFileUnit &NewUnit(const Terminator &, bool forChildIo);
   static void CloseAll(IoErrorHandler &);
   static void FlushAll(IoErrorHandler &);
 
   void OpenUnit(std::optional<OpenStatus>, std::optional<Action>, Position,
       OwningPtr<char> &&path, std::size_t pathLength, Convert,
       IoErrorHandler &);
   void OpenAnonymousUnit(std::optional<OpenStatus>, std::optional<Action>,
       Position, Convert, IoErrorHandler &);