Fix `String` ctor with `hibyte` for empty strings.

Mark empty strings as compressed for any value of `hibyte`.

Test: New test in 021-string2.
Test: testrunner.py --host --optimizing -t 021-string2
Bug: 250530521
Change-Id: Ie129fba599a24021eda0a4a81a39fd3fc3769597

2 files changed, 9 insertions, 1 deletions

diff --git a/runtime/mirror/string-alloc-inl.h b/runtime/mirror/string-alloc-inl.h
index 533053d934..925427a73b 100644
--- a/runtime/mirror/string-alloc-inl.h
+++ b/runtime/mirror/string-alloc-inl.h
@@ -216,7 +216,9 @@ inline ObjPtr<String> String::AllocFromByteArray(Thread* self,
   const uint8_t* const src = reinterpret_cast<uint8_t*>(array->GetData()) + offset;
   high_byte &= 0xff;  // Extract the relevant bits before determining `compressible`.
   const bool compressible =
-      kUseStringCompression && String::AllASCII<uint8_t>(src, byte_length) && (high_byte == 0);
+      kUseStringCompression &&
+      String::AllASCII<uint8_t>(src, byte_length) &&
+      (high_byte == 0 || byte_length == 0);
   const int32_t length_with_flag = String::GetFlaggedCount(byte_length, compressible);
   SetStringCountAndBytesVisitor visitor(length_with_flag, array, offset, high_byte << 8);
   return Alloc<kIsInstrumented>(self, length_with_flag, allocator_type, visitor);
diff --git a/test/021-string2/src/Main.java b/test/021-string2/src/Main.java
index 39595f3280..5da10ca722 100644
--- a/test/021-string2/src/Main.java
+++ b/test/021-string2/src/Main.java
@@ -119,6 +119,7 @@ public class Main {
         testEqualsConstString();
         testConstStringEquals();
         testStringConcat();
+        testEmptyWithHighByte();
 
         // Regression tests for String.setCharAt() breaking string compression invariants.
         Locale en_US = new Locale("en", "US");
@@ -760,6 +761,11 @@ public class Main {
         Assert.assertEquals("abc\u0440xyzw\u0440", "abc\u0440".concat("xyzw\u0440"));
     }
 
+    public static void testEmptyWithHighByte() {
+        String empty = new String(new byte[0], 1);
+        Assert.assertEquals("", empty);
+    }
+
     public static boolean $noinline$equalsConstString0(String s) {
         return s.equals("");
     }