Implement aarch64_get_thread_helper() for Windows AArch64

src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp

@@ -6695,13 +6695,14 @@ void MacroAssembler::java_round_float(Register dst, FloatRegister src,
 // by the call to JavaThread::aarch64_get_thread_helper() or, indeed,
 // the call setup code.
 //
-// On Linux, aarch64_get_thread_helper() clobbers only r0, r1, and flags.
+// On Linux and Windows, aarch64_get_thread_helper() is implemented in
+// assembly and clobbers only r0, r1, and flags.
 // On other systems, the helper is a usual C function.
 //
 void MacroAssembler::get_thread(Register dst) {
   RegSet saved_regs =
-    LINUX_ONLY(RegSet::range(r0, r1)  + lr - dst)
-    NOT_LINUX (RegSet::range(r0, r17) + lr - dst);
+    BSD_ONLY(RegSet::range(r0, r17) + lr - dst)
+    NOT_BSD (RegSet::range(r0, r1)  + lr - dst);
 
   protect_return_address();
   push(saved_regs, sp);

src/hotspot/os_cpu/windows_aarch64/javaThread_windows_aarch64.cpp

@@ -26,6 +26,15 @@
 #include "runtime/frame.inline.hpp"
 #include "runtime/javaThread.hpp"
 
+#include <windows.h>
+
+// CRT-provided TLS slot for this module (jvm.dll), set by the OS loader.
+extern "C" unsigned long _tls_index;
+
+// TLS offset read by the assembly code in `aarch64_get_thread_helper()`.  This
+// offset is initialized by `cache_global_variables()` before any JIT code.
+extern "C" int64_t _jvm_thr_current_tls_offset = 0;
+
 frame JavaThread::pd_last_frame() {
   assert(has_last_Java_frame(), "must have last_Java_sp() when suspended");
   vmassert(_anchor.last_Java_pc() != nullptr, "not walkable");
@@ -86,4 +95,21 @@ bool JavaThread::pd_get_top_frame(frame* fr_addr, void* ucontext, bool isInJava)
   return false;
 }
 
-void JavaThread::cache_global_variables() { }
+void JavaThread::cache_global_variables() {
+  // 0x58 is the offset of ThreadLocalStoragePointer within the TEB.  This is a
+  // stable Windows ABI constant but is not exposed in the SDK's minimal _TEB
+  // struct.
+  void** tls_array = *(void***)((char*)NtCurrentTeb() + 0x58);
+  char* tls_block = (char*)tls_array[_tls_index];
+
+  // Compute the offset of Thread::_thr_current within this module's TLS block.
+  // Unlike ELF, which provides `tlsdesc` relocations that let assembly resolve
+  // TLS variables symbolically at link/load time, Windows PE/COFF has no
+  // equivalent mechanism for armasm64.  So we compute the offset here in C++
+  // (where the compiler knows how to access __declspec(thread) variables) and
+  // store it in a plain global that the assembly can load directly.  In
+  // subsequent calls to `aarch64_get_thread_helper()`, the assembly will read
+  // the TEB to find the TLS block and then add this offset to find
+  // `Thread::_thr_current`.
+  _jvm_thr_current_tls_offset = (int64_t)((char*)&Thread::_thr_current - tls_block);
+}

src/hotspot/os_cpu/windows_aarch64/javaThread_windows_aarch64.hpp

@@ -46,8 +46,6 @@
   bool pd_get_top_frame(frame* fr_addr, void* ucontext, bool isInJava);
 public:
 
-  static Thread *aarch64_get_thread_helper() {
-     return Thread::current();
-  }
+  static Thread *aarch64_get_thread_helper();
 
 #endif // OS_CPU_WINDOWS_AARCH64_JAVATHREAD_WINDOWS_AARCH64_HPP

src/hotspot/os_cpu/windows_aarch64/threadLS_windows_aarch64.S

@@ -0,0 +1,64 @@
+;
+; Copyright (c) 2026, Oracle and/or its affiliates. All rights reserved.
+; DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+;
+; This code is free software; you can redistribute it and/or modify it
+; under the terms of the GNU General Public License version 2 only, as
+; published by the Free Software Foundation.
+;
+; This code is distributed in the hope that it will be useful, but WITHOUT
+; ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+; FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+; version 2 for more details (a copy is included in the LICENSE file that
+; accompanied this code).
+;
+; You should have received a copy of the GNU General Public License version
+; 2 along with this work; if not, write to the Free Software Foundation,
+; Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+;
+; Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+; or visit www.oracle.com if you need additional information or have any
+; questions.
+;
+
+    ; JavaThread::aarch64_get_thread_helper()
+    ;
+    ; Optimized TLS access to `Thread::_thr_current` on Windows AArch64.
+    ; Returns the current thread pointer in x0, clobbers x1, while all other
+    ; registers are preserved.
+
+    IMPORT _tls_index
+    IMPORT _jvm_thr_current_tls_offset
+
+    AREA threadls_text, CODE, READONLY
+    ALIGN 4
+
+    ; MSVC-decorated name for: static Thread* JavaThread::aarch64_get_thread_helper()
+    EXPORT |?aarch64_get_thread_helper@JavaThread@@SAPEAVThread@@XZ|
+
+|?aarch64_get_thread_helper@JavaThread@@SAPEAVThread@@XZ| PROC
+
+    ; x18 holds the TEB, 0x58 is a well-defined offset into the TEB on 64-bit
+    ; systems, so the following line loads the thread-local storage pointer
+    ; inside the TEB
+    ldr     x1, [x18, #0x58]
+
+    ; Load `_tls_index` and zero-extend it to 64 bits to occupy x0
+    ldr     x0, =_tls_index
+    ldr     w0, [x0]
+
+    ; `x0` holds the index, `x1` holds the array base address (each entry is 64
+    ; bits long), so in the following line, x1 = array_base[_tls_index]
+    ldr     x1, [x1, x0, lsl #3]
+
+    ; Load cached TLS offset of `Thread::_thr_current`
+    ldr     x0, =_jvm_thr_current_tls_offset
+    ldr     x0, [x0]
+
+    ; Load `Thread::_thr_current` value
+    ldr     x0, [x1, x0]
+
+    ret
+
+    ENDP
+    END

test/hotspot/gtest/aarch64/test_threadLS.cpp

@@ -0,0 +1,35 @@
+/*
+ * Copyright (c) 2026, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+#if defined(AARCH64) && !defined(ZERO)
+
+#include "runtime/javaThread.hpp"
+#include "unittest.hpp"
+
+TEST_VM(ThreadLS, get_thread_helper) {
+  Thread* expected = Thread::current();
+  Thread* actual = JavaThread::aarch64_get_thread_helper();
+  ASSERT_EQ(actual, expected);
+}
+
+#endif  // AARCH64 && !ZERO