RFR: 8303526: Changing "arbitrary" Name.compareTo() ordering breaks the regression suite [v2]

[Archie L. Cobbs]

> This was discovered during investigation for [JDK-8268622](https://bugs.openjdk.org/browse/JDK-8268622).
> 
> The method `Name.compareTo()` looks like this:
> 
> /** An arbitrary but consistent complete order among all Names.
>  */
> public int compareTo(Name other) {
>     return other.getIndex() - this.getIndex();
> }
> 
> Note the "arbitrary but consistent" wording. Unfortunately, the ordering can be "arbitrary" only if you don't mind breaking the regression suite.
> 
> For example, if you apply this patch:
> 
> --- a/src/jdk.compiler/share/classes/com/sun/tools/javac/util/Name.java
> +++ b/src/jdk.compiler/share/classes/com/sun/tools/javac/util/Name.java
> @@ -105,7 +105,7 @@ public abstract class Name implements javax.lang.model.element.Name, PoolConstan
>      /** An arbitrary but consistent complete order among all Names.
>       */
>      public int compareTo(Name other) {
> -        return other.getIndex() - this.getIndex();
> +        return this.getIndex() - other.getIndex();
>      }
> 
>      /** Return true if this is the empty name.
> 
> then the following tests will fail:
> ```TEST: tools/javac/8203436/T8203436b.java
> TEST: tools/javac/generics/7034019/T7034019c.java
> TEST: tools/javac/generics/7034019/T7034019d.java
> TEST: tools/javac/generics/diamond/neg/Neg21.java
> TEST: tools/javac/generics/diamond/neg/Neg22.java
> TEST: tools/javac/generics/inference/EagerReturnTypeResolution/EagerReturnTypeResolutionTestb.java
> TEST: tools/javac/patterns/InferenceUnitTest.java
> TEST: tools/javac/T8187978/FilterOutCandidatesForDiagnosticsTest.java
> TEST: tools/javac/varargs/6806876/T6806876.java
> 
> ==============================
> Test summary
> ==============================
>    TEST                                              TOTAL  PASS  FAIL ERROR
>>> jtreg:test/langtools:langtools_javac               3718  3709     9     0 <<
> ==============================
> 
> The compiler behavior should be deterministic whenever possible, and the javac regression suite should be robust. So clearly "arbitrary" behavior by this method is not really supported.
> 
> It turns out there is only one user of `Name.compareTo()`, the method `TypeSymbol.precedes()`:
> 
> /**
>  * A partial ordering between type symbols that refines the
>  * class inheritance graph.
>  *
>  * Type variables always precede other kinds of symbols.
>  */
> public final boolean precedes(TypeSymbol that, Types types) {
>     if (this == that)
>         return false;
>     if (type.hasTag(that.type.getTag())) {
>         if (type.hasTag(CLASS)) {
>             return
>                 types.rank(that.type) < types.rank(this.type) ||
>                 types.rank(that.type) == types.rank(this.type) &&
>                 that.getQualifiedName().compareTo(this.getQualifiedName()) < 0;
>         } else if (type.hasTag(TYPEVAR)) {
>             return types.isSubtype(this.type, that.type);
>         }
>     }
>     return type.hasTag(TYPEVAR);
> }
> 
> If we want the above partial ordering to be stable, we need to specify deterministic behavior for `Name.compareTo()`.
> 
> An obvious choice would be to require that `Name.compareTo()` orders consistently with `String.compareTo()`, i.e., lexicographically on the Unicode characters in the name.
> 
> Fortunately, this comparison can be done efficiently, i.e., without actually converting the `Name` into a `String`, because UTF-8 is "lexicographically consistent" with the characters it encodes, except that Java's use of Modified UTF-8 means a special check for 0x0000 (which is encoded as two bytes) is required.
> 
> When you do this the tests are still broken... but notice that `TypeSymbol.precedes()` is actually using the _reverse_ of `Name.compareTo()`. So somehow, the current `Name.compareTo()` is ordering things "backwards", at least from the point of view of the expectation of the regression tests.
> 
> So replacing `Name.compareTo()` with a version that orders lexicographically, and also un-reversing its treatment in `TypeSymbol.precedes()`, makes things the tests succeed again.
> 
> That's what this patch does.

Archie L. Cobbs has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:

 - Merge branch 'master' into JDK-8303526
 - Require Name.compareTo() to produce a lexicographic ordering.

-------------

Changes: https://git.openjdk.org/jdk/pull/12843/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12843&range=01
  Stats: 36 lines in 2 files changed: 31 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/12843.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/12843/head:pull/12843

PR: https://git.openjdk.org/jdk/pull/12843