Fix some GC churn issues when dealing with Labels:

- reduce use of #substring(), which must copy the sub-region of the string
- specialize hashCode() implementation so that we don't need to push objects into an array.

--
MOS_MIGRATED_REVID=125798978

2 files changed, 18 insertions, 5 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/cmdline/Label.java b/src/main/java/com/google/devtools/build/lib/cmdline/Label.java
index 73c7e5ecdb..75d4677365 100644
--- a/src/main/java/com/google/devtools/build/lib/cmdline/Label.java
+++ b/src/main/java/com/google/devtools/build/lib/cmdline/Label.java
@@ -32,7 +32,6 @@ import java.io.IOException;
 import java.io.InvalidObjectException;
 import java.io.ObjectInputStream;
 import java.io.Serializable;
-import java.util.Objects;
 
 /**
  * A class to identify a BUILD target. All targets belong to exactly one package.
@@ -274,7 +273,15 @@ public final class Label implements Comparable<Label>, Serializable, SkylarkPrin
       }
       throw e;
     }
-    this.hashCode = Objects.hash(this.name, this.packageIdentifier);
+    this.hashCode = hashCode(this.name, this.packageIdentifier);
+  }
+
+  /**
+   * A specialization of Arrays.HashCode() that does not require constructing a 2-element array.
+   */
+  private static final int hashCode(Object obj1, Object obj2) {
+    int result = 31 + (obj1 == null ? 0 : obj1.hashCode());
+    return 31 * result + (obj2 == null ? 0 : obj2.hashCode());
   }
 
   private Object writeReplace() {
diff --git a/src/main/java/com/google/devtools/build/lib/cmdline/LabelValidator.java b/src/main/java/com/google/devtools/build/lib/cmdline/LabelValidator.java
index 52c7e0e1fd..42510c7f56 100644
--- a/src/main/java/com/google/devtools/build/lib/cmdline/LabelValidator.java
+++ b/src/main/java/com/google/devtools/build/lib/cmdline/LabelValidator.java
@@ -165,11 +165,11 @@ public final class LabelValidator {
         continue;
       }
       if (c == '/') {
-        if (targetName.substring(ii).startsWith("/../")) {
+        if (stringRegionMatch(targetName, "/../", ii)) {
           return "target names may not contain up-level references '..'";
-        } else if (targetName.substring(ii).startsWith("/./")) {
+        } else if (stringRegionMatch(targetName, "/./", ii)) {
           return "target names may not contain '.' as a path segment";
-        } else if (targetName.substring(ii).startsWith("//")) {
+        } else if (stringRegionMatch(targetName, "//", ii)) {
           return "target names may not contain '//' path separators";
         }
         continue;
@@ -194,6 +194,12 @@ public final class LabelValidator {
     return null; // ok
   }
 
+  // Prefer this implementation over calls to String#subString(), as the latter implies copying
+  // the subregion.
+  private static boolean stringRegionMatch(String fullString, String possibleMatch, int offset) {
+    return fullString.regionMatches(offset, possibleMatch, 0, possibleMatch.length());
+  }
+
   /**
    * Validate the label and parse it into a pair of package name and target name. If the label is
    * not valid, it throws an {@link BadLabelException}.