[PATCH v2] Implement native TLS support on Windows

[PATCH v2] Implement native TLS support on Windows

[Hannes Domani]

GCC 16 introduced native TLS variables on Windows, so this adds
debugger support for them.

The fetch_tls_load_module_address gdbarch method is used to get the
address of _tls_index of the OBJFILE, which is then forwarded as LM_ADDR
to windows_get_thread_local_address.

The TLS slot for a module can be found in
TIB->thread_local_storage[_tls_index].
---
Changes in v2:
- added NEWS entry
- more comments
- split up if()-conditions in windows_get_thread_local_address
---
 gdb/NEWS           |  2 ++
 gdb/windows-tdep.c | 73 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 75 insertions(+)

diff --git a/gdb/NEWS b/gdb/NEWS
index 480e1854002..ee1a21179b5 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -78,6 +78,8 @@
 * The Windows native target now supports non-stop mode.  This feature
   requires Windows 10 or later.
 
+* Support for native Thread Local Storage (TLS) variables on Windows.
+
 * New targets
 
 GNU/Linux/MicroBlaze (gdbserver) microblazeel-*linux*
diff --git a/gdb/windows-tdep.c b/gdb/windows-tdep.c
index 38ee1d7275e..78bf49b9c81 100644
--- a/gdb/windows-tdep.c
+++ b/gdb/windows-tdep.c
@@ -954,6 +954,73 @@ windows_init_abi_common (struct gdbarch_info info, struct gdbarch *gdbarch)
   set_gdbarch_get_siginfo_type (gdbarch, windows_get_siginfo_type);
 }
 
+/* Implement the fetch_tls_load_module_address gdbarch method.
+   Usually this method returns an address representing a load module, but this
+   doesn't exist on Windows.  Instead it returns the address of the
+   "_tls_index" variable of OBJFILE, which is then forwarded as LM_ADDR
+   in windows_get_thread_local_address below.  */
+
+static CORE_ADDR
+windows_tls_index_address (struct objfile *objfile)
+{
+  bound_minimal_symbol minsym
+    = lookup_minimal_symbol_linkage ("_tls_index", objfile, false);
+  if (minsym.minsym == nullptr)
+    throw_error (TLS_GENERIC_ERROR, _("Cannot find address of _tls_index"));
+
+  return minsym.value_address ();
+}
+
+/* Implement the get_thread_local_address gdbarch method.
+   Each module (with TLS variables) gets its own TLS slot represented by
+   _tls_index, which can be found in TIB->thread_local_storage[_tls_index].
+   The address of _tls_index is provided by the LM_ADDR argument.  */
+
+static CORE_ADDR
+windows_get_thread_local_address (struct gdbarch *gdbarch, ptid_t ptid,
+				  CORE_ADDR lm_addr, CORE_ADDR offset)
+{
+  int ptr_bytes;
+  int tls_offset; /* Offset of thread_local_storage in TIB.  */
+  if (gdbarch_ptr_bit (gdbarch) == 32)
+    {
+      ptr_bytes = 4;
+      tls_offset = 44;
+    }
+  else
+    {
+      ptr_bytes = 8;
+      tls_offset = 88;
+    }
+
+  gdb_byte buf[8];
+  if (target_read_memory (lm_addr, buf, 4))
+    throw_error (TLS_GENERIC_ERROR, _("Cannot read _tls_index"));
+
+  enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);
+  uint32_t tls_index = extract_unsigned_integer (buf, 4, byte_order);
+
+  CORE_ADDR tlb;
+  if (!target_get_tib_address (ptid, &tlb))
+    throw_error (TLS_GENERIC_ERROR, _("Cannot get tib address"));
+
+  if (target_read_memory (tlb + tls_offset, buf, ptr_bytes))
+    throw_error (TLS_GENERIC_ERROR, _("Cannot read thread_local_storage"));
+
+  CORE_ADDR tls_ptr = extract_unsigned_integer (buf, ptr_bytes, byte_order);
+  if (tls_ptr == 0)
+    throw_error (TLS_NOT_ALLOCATED_YET_ERROR, _("TLS not allocated yet"));
+
+  if (target_read_memory (tls_ptr + tls_index * ptr_bytes, buf, ptr_bytes))
+    throw_error (TLS_GENERIC_ERROR, _("Cannot read TLS slot"));
+
+  CORE_ADDR slot_ptr = extract_unsigned_integer (buf, ptr_bytes, byte_order);
+  if (slot_ptr == 0)
+    throw_error (TLS_NOT_ALLOCATED_YET_ERROR, _("TLS slot not allocated yet"));
+
+  return slot_ptr + offset;
+}
+
 /* See windows-tdep.h.  */
 void
 windows_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)
@@ -963,6 +1030,12 @@ windows_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)
 
   /* On Windows, "long"s are only 32bit.  */
   set_gdbarch_long_bit (gdbarch, 32);
+
+  /* Enable TLS support.  */
+  set_gdbarch_fetch_tls_load_module_address (gdbarch,
+					     windows_tls_index_address);
+  set_gdbarch_get_thread_local_address (gdbarch,
+					windows_get_thread_local_address);
 }
 
 /* See windows-tdep.h.  */
-- 
2.54.0

Re: [PATCH v2] Implement native TLS support on Windows

[Eli Zaretskii]

> From: Hannes Domani <ssbssa@yahoo.de>
> Date: Sat,  9 May 2026 15:24:29 +0200
> 
> GCC 16 introduced native TLS variables on Windows, so this adds
> debugger support for them.
> 
> The fetch_tls_load_module_address gdbarch method is used to get the
> address of _tls_index of the OBJFILE, which is then forwarded as LM_ADDR
> to windows_get_thread_local_address.
> 
> The TLS slot for a module can be found in
> TIB->thread_local_storage[_tls_index].
> ---
> Changes in v2:
> - added NEWS entry
> - more comments
> - split up if()-conditions in windows_get_thread_local_address
> ---
>  gdb/NEWS           |  2 ++
>  gdb/windows-tdep.c | 73 ++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 75 insertions(+)

Thanks, the NEWS part is okay.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>

Re: [PING] [PATCH v2] Implement native TLS support on Windows

[Hannes Domani]

Ping.

On 5/9/26 15:24, Hannes Domani wrote:
> GCC 16 introduced native TLS variables on Windows, so this adds
> debugger support for them.
> 
> The fetch_tls_load_module_address gdbarch method is used to get the
> address of _tls_index of the OBJFILE, which is then forwarded as LM_ADDR
> to windows_get_thread_local_address.
> 
> The TLS slot for a module can be found in
> TIB->thread_local_storage[_tls_index].
> ---
> Changes in v2:
> - added NEWS entry
> - more comments
> - split up if()-conditions in windows_get_thread_local_address
> ---
>   gdb/NEWS           |  2 ++
>   gdb/windows-tdep.c | 73 ++++++++++++++++++++++++++++++++++++++++++++++
>   2 files changed, 75 insertions(+)
> 
> diff --git a/gdb/NEWS b/gdb/NEWS
> index 480e1854002..ee1a21179b5 100644
> --- a/gdb/NEWS
> +++ b/gdb/NEWS
> @@ -78,6 +78,8 @@
>   * The Windows native target now supports non-stop mode.  This feature
>     requires Windows 10 or later.
>   
> +* Support for native Thread Local Storage (TLS) variables on Windows.
> +
>   * New targets
>   
>   GNU/Linux/MicroBlaze (gdbserver) microblazeel-*linux*
> diff --git a/gdb/windows-tdep.c b/gdb/windows-tdep.c
> index 38ee1d7275e..78bf49b9c81 100644
> --- a/gdb/windows-tdep.c
> +++ b/gdb/windows-tdep.c
> @@ -954,6 +954,73 @@ windows_init_abi_common (struct gdbarch_info info, struct gdbarch *gdbarch)
>     set_gdbarch_get_siginfo_type (gdbarch, windows_get_siginfo_type);
>   }
>   
> +/* Implement the fetch_tls_load_module_address gdbarch method.
> +   Usually this method returns an address representing a load module, but this
> +   doesn't exist on Windows.  Instead it returns the address of the
> +   "_tls_index" variable of OBJFILE, which is then forwarded as LM_ADDR
> +   in windows_get_thread_local_address below.  */
> +
> +static CORE_ADDR
> +windows_tls_index_address (struct objfile *objfile)
> +{
> +  bound_minimal_symbol minsym
> +    = lookup_minimal_symbol_linkage ("_tls_index", objfile, false);
> +  if (minsym.minsym == nullptr)
> +    throw_error (TLS_GENERIC_ERROR, _("Cannot find address of _tls_index"));
> +
> +  return minsym.value_address ();
> +}
> +
> +/* Implement the get_thread_local_address gdbarch method.
> +   Each module (with TLS variables) gets its own TLS slot represented by
> +   _tls_index, which can be found in TIB->thread_local_storage[_tls_index].
> +   The address of _tls_index is provided by the LM_ADDR argument.  */
> +
> +static CORE_ADDR
> +windows_get_thread_local_address (struct gdbarch *gdbarch, ptid_t ptid,
> +				  CORE_ADDR lm_addr, CORE_ADDR offset)
> +{
> +  int ptr_bytes;
> +  int tls_offset; /* Offset of thread_local_storage in TIB.  */
> +  if (gdbarch_ptr_bit (gdbarch) == 32)
> +    {
> +      ptr_bytes = 4;
> +      tls_offset = 44;
> +    }
> +  else
> +    {
> +      ptr_bytes = 8;
> +      tls_offset = 88;
> +    }
> +
> +  gdb_byte buf[8];
> +  if (target_read_memory (lm_addr, buf, 4))
> +    throw_error (TLS_GENERIC_ERROR, _("Cannot read _tls_index"));
> +
> +  enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);
> +  uint32_t tls_index = extract_unsigned_integer (buf, 4, byte_order);
> +
> +  CORE_ADDR tlb;
> +  if (!target_get_tib_address (ptid, &tlb))
> +    throw_error (TLS_GENERIC_ERROR, _("Cannot get tib address"));
> +
> +  if (target_read_memory (tlb + tls_offset, buf, ptr_bytes))
> +    throw_error (TLS_GENERIC_ERROR, _("Cannot read thread_local_storage"));
> +
> +  CORE_ADDR tls_ptr = extract_unsigned_integer (buf, ptr_bytes, byte_order);
> +  if (tls_ptr == 0)
> +    throw_error (TLS_NOT_ALLOCATED_YET_ERROR, _("TLS not allocated yet"));
> +
> +  if (target_read_memory (tls_ptr + tls_index * ptr_bytes, buf, ptr_bytes))
> +    throw_error (TLS_GENERIC_ERROR, _("Cannot read TLS slot"));
> +
> +  CORE_ADDR slot_ptr = extract_unsigned_integer (buf, ptr_bytes, byte_order);
> +  if (slot_ptr == 0)
> +    throw_error (TLS_NOT_ALLOCATED_YET_ERROR, _("TLS slot not allocated yet"));
> +
> +  return slot_ptr + offset;
> +}
> +
>   /* See windows-tdep.h.  */
>   void
>   windows_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)
> @@ -963,6 +1030,12 @@ windows_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)
>   
>     /* On Windows, "long"s are only 32bit.  */
>     set_gdbarch_long_bit (gdbarch, 32);
> +
> +  /* Enable TLS support.  */
> +  set_gdbarch_fetch_tls_load_module_address (gdbarch,
> +					     windows_tls_index_address);
> +  set_gdbarch_get_thread_local_address (gdbarch,
> +					windows_get_thread_local_address);
>   }
>   
>   /* See windows-tdep.h.  */

Re: [PATCH v2] Implement native TLS support on Windows

[Tom Tromey]

>>>>> "Hannes" == Hannes Domani <ssbssa@yahoo.de> writes:

Hannes> GCC 16 introduced native TLS variables on Windows, so this adds
Hannes> debugger support for them.

Hannes> Changes in v2:
Hannes> - added NEWS entry
Hannes> - more comments
Hannes> - split up if()-conditions in windows_get_thread_local_address

Thank you.  I think this is ok.
Approved-By: Tom Tromey <tom@tromey.com>

Tom

Re: [PATCH v2] Implement native TLS support on Windows

[Hannes Domani]

 Am Freitag, 22. Mai 2026 um 15:49:31 MESZ hat Tom Tromey <tom@tromey.com> Folgendes geschrieben:

> >>>>> "Hannes" == Hannes Domani <ssbssa@yahoo.de> writes:
> 
> Hannes> GCC 16 introduced native TLS variables on Windows, so this adds
> Hannes> debugger support for them.
> 
> Hannes> Changes in v2:
> Hannes> - added NEWS entry
> Hannes> - more comments
> Hannes> - split up if()-conditions in windows_get_thread_local_address
> 
> Thank you.  I think this is ok.
> Approved-By: Tom Tromey <tom@tromey.com>

Pushed, thanks.


Hannes