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

1 files changed, 3 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);