[PATCH v2 1/2] Add TLS variable debug support for AIX [ 64-bit XCOFF files only]

From: Aditya Kamath <[email protected]>

This is a patch that adds TLS debug support for variables in AIX
for 64 bit XCOFF binaries only.

Currently in AIX, if we debug a TLS variable we get,
(gdb) print tls_counter
Cannot find thread-local variables on this target

This patch uses thread pointer in R13 plus adds the offset to get the TLS variable address.

Also adds the AIX specific gdbarch () methods.

After this patch we get,

gdb/gdb thread-local
GNU gdb (GDB) 18.0.50.20260728-git
Copyright (C) 2026 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "powerpc64-ibm-aix7.2.0.0".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<https://www.gnu.org/software/gdb/bugs/>.

+------------------------------------------------------------------------------+
| Find the GDB manual online at:                                               |
| http://www.gnu.org/software/gdb/documentation/.                              |
| For help, type "help".                                                       |
| Type "apropos word" to search for commands related to "word".                |
+------------------------------------------------------------------------------+

Reading symbols from thread-local...
(gdb) b 16
Breakpoint 1 at 0x10000894: file thread-local.c, line 16.
(gdb) r
Starting program: /home/aditya/binutils-gdb/thread-local
[New Thread 258 (tid 118292825) (id 2)]
[Switching to thread 2 (Thread 258 (tid 118292825))]

Thread 2 hit Breakpoint 1, thread_function (arg=0x1) at thread-local.c:16
16          sleep(1);
(gdb) p tls_counter
$1 = 100
(gdb) q
---
 gdb/rs6000-aix-tdep.c | 76 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 76 insertions(+)

diff --git a/gdb/rs6000-aix-tdep.c b/gdb/rs6000-aix-tdep.c
index d823fea5645..aa68654ff41 100644
--- a/gdb/rs6000-aix-tdep.c
+++ b/gdb/rs6000-aix-tdep.c
@@ -39,6 +39,7 @@
 #include "gdbsupport/xml-utils.h"
 #include "trad-frame.h"
 #include "frame-unwind.h"
+#include "inferior.h"
 
 /* If the kernel has to deliver a signal, it pushes a sigcontext
    structure on the stack and then calls the signal handler, passing
@@ -1355,6 +1356,73 @@ rs6000_aix_core_xfer_shared_libraries_aix (struct gdbarch *gdbarch,
 				    offset, len, 0);
 }
 
+/* For AIX, use the rs6000_aix_fetch_tls_load_module_address gdbarch method.
+
+   Thread-local variables accessed via a simple TP-relative offset (the Local
+   Exec and Initial Exec TLS models) are only valid for the main executable and
+   for shared libraries that were pulled in at program startup.  Variables
+   belonging to a library loaded later via dlopen() have their storage
+   allocated dynamically; their addresses must be resolved through the DTV
+   (Dynamic Thread Vector) and cannot be computed with a plain TP offset.
+
+   Until full DTV-based lookup is implemented, reject any objfile that is a
+   shared library (OBJF_SHARED) which has been dlopen'd and only
+   allow the main executable (OBJF_MAINLINE).  Returning 0 here causes
+   rs6000_aix_get_thread_local_address() to be called with lm_addr == 0,
+   which it treats as nothing for now.  */
+
+static CORE_ADDR
+rs6000_aix_fetch_tls_load_module_address (struct objfile *objfile)
+{
+  /* TLS variables from shared libraries cannot be directly fetched
+     via the thread pointer if they were loaded by dlopen().  */
+  if (objfile->flags & OBJF_SHARED)
+    throw_error (TLS_GENERIC_ERROR,
+		 _("TLS lookup via thread pointer is not supported for "
+		   "shared library \"%s\"; full DTV-based lookup is not "
+		   "yet implemented for AIX"),
+		 objfile_name (objfile));
+
+  return 0;
+}
+
+/* Use the AIX get_thread_local_address gdbarch function.
+
+   On 64-bit AIX the thread pointer is in R13.  For the Local Exec TLS model
+   (used by the main executable) the XCOFF symbol value is a signed
+   TP-relative offset, so the per-thread address is:
+
+     address = thread_pointer + (int64_t) offset
+
+   This only works correctly when the variable's storage is allocated
+   statically relative to the thread pointer, which is guaranteed for the
+   main executable. */
+
+static CORE_ADDR
+rs6000_aix_get_thread_local_address (struct gdbarch *gdbarch, ptid_t ptid,
+				     CORE_ADDR lm_addr, CORE_ADDR offset)
+{
+  ppc_gdbarch_tdep *tdep = gdbarch_tdep<ppc_gdbarch_tdep> (gdbarch);
+
+  /* 64-bit AIX: the thread pointer is in R13.  */
+  if (tdep->wordsize != 8)
+    throw_error (TLS_GENERIC_ERROR,
+		 _("TLS lookup not yet supported for 32-bit AIX"));
+
+  int tp_regnum = PPC_R0_REGNUM + 13;
+
+  regcache *regcache
+    = get_thread_arch_regcache (current_inferior (), ptid, gdbarch);
+  target_fetch_registers (regcache, tp_regnum);
+
+  ULONGEST tp;
+  if (regcache->cooked_read (tp_regnum, &tp) != REG_VALID)
+    throw_error (TLS_GENERIC_ERROR,
+		 _("Unable to fetch thread pointer for TLS lookup"));
+
+  return tp + (CORE_ADDR)(int64_t) offset;
+}
+
 static void
 rs6000_aix_init_osabi (struct gdbarch_info info, struct gdbarch *gdbarch)
 {
@@ -1411,6 +1479,14 @@ rs6000_aix_init_osabi (struct gdbarch_info info, struct gdbarch *gdbarch)
   set_gdbarch_auto_wide_charset (gdbarch, rs6000_aix_auto_wide_charset);
 
   set_gdbarch_make_solib_ops (gdbarch, make_aix_solib_ops);
+
+
+  /* TLS support for AIX.  */
+  set_gdbarch_fetch_tls_load_module_address
+    (gdbarch, rs6000_aix_fetch_tls_load_module_address);
+  set_gdbarch_get_thread_local_address
+    (gdbarch, rs6000_aix_get_thread_local_address);
+
   frame_unwind_append_unwinder (gdbarch, &aix_sighandle_frame_unwind);
 }
 
-- 
2.51.2