// safely publish these objects and we won't need this whole discussion.
  // TODO(user,lukes): consider adding volatile to all these fields since in current known JVMs
  // that should resolve the issue. This comes at the cost of adding more write barriers to the
  // implementations.

  private Futures() {}

  /**

   * parameters used to build this multimap.
   */
  @Override
  public ImmutableList<V> get(K key) {
    // This cast is safe as its type is known in constructor.
    ImmutableList<V> list = (ImmutableList<V>) map.get(key);
    return (list == null) ? ImmutableList.<V>of() : list;
  }

        } else {
          // TODO(benyu): handle ? super T somehow.
          return type;
        }
      }
      throw new AssertionError("must have been one of the known types");
    }

    TypeVariable<?> captureAsTypeVariable(Type[] upperBounds) {
      String name =
          "capture#" + id.incrementAndGet() + "-of ? extends " + Joiner.on('&').join(upperBounds);

---
title: "Go, Backwards Compatibility, and GODEBUG"
layout: article
---

<!--
This document is kept in the Go repo, not x/website,
because it documents the full list of known GODEBUG settings,
which are tied to a specific release.
-->

## Introduction {#intro}

Go's emphasis on backwards compatibility is one of its key strengths.
There are, however, times when we cannot maintain complete compatibility.

     of liability) contained within the Source Code Form of the Covered
     Software, except that You may alter any license notices to the extent
     required to remedy known factual inaccuracies.

3.5. Application of Additional Terms

     You may choose to offer, and to charge a fee for, warranty, support,
     indemnity or liability obligations to one or more recipients of Covered

   * the lock acquisition graph.
   */
  private static class LockGraphNode {

    /**
     * The map tracking the locks that are known to be acquired before this lock, each associated
     * with an example stack trace. Locks are weakly keyed to allow proper garbage collection when
     * they are no longer referenced.
     */

   *     {@code toIndex > fromIndex}
   * @since 32.0.0
   */
  public static void rotate(int[] array, int distance, int fromIndex, int toIndex) {
    // There are several well-known algorithms for rotating part of an array (or, equivalently,
    // exchanging two blocks of memory). This classic text by Gries and Mills mentions several:
    // https://ecommons.cornell.edu/bitstream/handle/1813/6292/81-452.pdf.

            break;
          } else if (spliterator.getExactSizeIfKnown() == 0) {
            spliterator = prefix;
            break;
          }
        }

        // spliterator is known to be nonempty now
        spliterator.forEachRemaining(state::set);
        return java.util.Optional.of(state.get());
      }

      Spliterator<T> prefix = spliterator.trySplit();

   * parameters used to build this multimap.
   */
  @Override
  public ImmutableSet<V> get(K key) {
    // This cast is safe as its type is known in constructor.
    ImmutableSet<V> set = (ImmutableSet<V>) map.get(key);
    return MoreObjects.firstNonNull(set, emptySet);
  }

	instructions["BL"] = obj.ACALL
	// MCR differs from MRC by the way fields of the word are encoded.
	// (Details in arm.go). Here we add the instruction so parse will find
	// it, but give it an opcode number known only to us.
	instructions["MCR"] = aMCR

	return &Arch{
		LinkArch:       &arm.Linkarm,
		Instructions:   instructions,
		Register:       register,
		RegisterPrefix: registerPrefix,