[newlib-cygwin] Cygwin: obtain stack base on AArch64

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=e97c157432985faa5a26a900a8dfb7a388a74857

commit e97c157432985faa5a26a900a8dfb7a388a74857
Author: Radek Bartoň <[email protected]>
Date:   Wed Jun 4 13:38:10 2025 +0200

    Cygwin: obtain stack base on AArch64
    
    Signed-off-by: Radek Bartoň <[email protected]>

Diff:
---
 winsup/cygwin/include/cygwin/config.h  | 7 ++++++-
 winsup/testsuite/winsup.api/cygload.cc | 7 +++++++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/winsup/cygwin/include/cygwin/config.h b/winsup/cygwin/include/cygwin/config.h
index 2a7083278..d9f911d47 100644
--- a/winsup/cygwin/include/cygwin/config.h
+++ b/winsup/cygwin/include/cygwin/config.h
@@ -36,8 +36,13 @@ __attribute__((__gnu_inline__))
 extern inline struct _reent *__getreent (void)
 {
   register char *ret;
-#ifdef __x86_64__
+#if defined(__x86_64__)
   __asm __volatile__ ("movq %%gs:8,%0" : "=r" (ret));
+#elif defined(__aarch64__)
+  /* x18 register points to TEB, offset 0x8 points to stack base.
+     See _TEB structure definition in winsup\cygwin\local_includes\ntdll.h
+     for more details. */
+  __asm __volatile__ ("ldr %0, [x18, #0x8]" : "=r" (ret));
 #else
 #error unimplemented for this target
 #endif
diff --git a/winsup/testsuite/winsup.api/cygload.cc b/winsup/testsuite/winsup.api/cygload.cc
index afd3ee90f..08372a302 100644
--- a/winsup/testsuite/winsup.api/cygload.cc
+++ b/winsup/testsuite/winsup.api/cygload.cc
@@ -82,6 +82,13 @@ cygwin::padding::padding ()
     "movl %%fs:4, %0"
     :"=r"(stackbase)
     );
+# elif __aarch64__
+  // x18 register points to TEB. See _TEB structure definition in
+  // winsup\cygwin\local_includes\ntdll.h
+  __asm__ volatile (
+    "ldr %0, [x18, #0x8]"
+   :"=r" (stackbase)
+   );
 # else
 #  error Unknown architecture
 # endif