hurd: Only check for TLS initialization inside rtld or in static builds

When glibc is built as a shared library, TLS is always initialized by
the call of TLS_INIT_TP () macro made inside the dynamic loader, prior
to running the main program (see dl-call_tls_init_tp.h). We can take
advantage of this: we know for sure that __LIBC_NO_TLS () will evaluate
to 0 in all other cases, so let the compiler know that explicitly too.

Also, only define _hurd_tls_init () and TLS_INIT_TP () under the same
conditions (either !SHARED or inside rtld), to statically assert that
this is the case.

Other than a microoptimization, this also helps with avoiding awkward
sharing of the __libc_tls_initialized variable between ld.so and libc.so
that we would have to do otherwise -- we know for sure that no sharing
is required, simply because __libc_tls_initialized would always be set
to true inside libc.so.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
Message-Id: <20230319151017.531737-25-bugaevc@gmail.com>

2 files changed, 47 insertions, 17 deletions

diff --git a/sysdeps/mach/hurd/i386/dl-tls-initialized.c b/sysdeps/mach/hurd/i386/dl-tls-initialized.c
new file mode 100644
index 0000000..493ec23
--- /dev/null
+++ b/sysdeps/mach/hurd/i386/dl-tls-initialized.c
@@ -0,0 +1,21 @@
+/* Determine whether TLS is initialized, for i386/Hurd.
+   Copyright (C) 1995-2023 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library 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
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef SHARED
+unsigned short __init1_desc;
+#endif
diff --git a/sysdeps/mach/hurd/i386/tls.h b/sysdeps/mach/hurd/i386/tls.h
index 0f8dd24..ee7b800 100644
--- a/sysdeps/mach/hurd/i386/tls.h
+++ b/sysdeps/mach/hurd/i386/tls.h
@@ -69,18 +69,6 @@ _Static_assert (offsetof (tcbhead_t, __private_ss) == 0x30,
                   |  (desc->high_word & 0xff000000));			      \
   })
 
-/* Return 1 if TLS is not initialized yet.  */
-#ifndef SHARED
-extern unsigned short __init1_desc;
-#define __HURD_DESC_INITIAL(gs, ds) ((gs) == (ds) || (gs) == __init1_desc)
-#else
-#define __HURD_DESC_INITIAL(gs, ds) ((gs) == (ds))
-#endif
-
-#define __LIBC_NO_TLS()							      \
-  ({ unsigned short ds, gs;						      \
-     asm ("movw %%ds,%w0; movw %%gs,%w1" : "=q" (ds), "=q" (gs));	      \
-     __builtin_expect(__HURD_DESC_INITIAL(gs, ds), 0); })
 #endif
 
 /* The TCB can have any size and the memory following the address the
@@ -125,6 +113,28 @@ extern unsigned short __init1_desc;
 
 # define HURD_SEL_LDT(sel) (__builtin_expect ((sel) & 4, 0))
 
+#ifndef SHARED
+extern unsigned short __init1_desc;
+# define __HURD_DESC_INITIAL(gs, ds) ((gs) == (ds) || (gs) == __init1_desc)
+#else
+# define __HURD_DESC_INITIAL(gs, ds) ((gs) == (ds))
+#endif
+
+#if !defined (SHARED) || IS_IN (rtld)
+/* Return 1 if TLS is not initialized yet.  */
+extern inline bool __attribute__ ((unused))
+__LIBC_NO_TLS (void)
+{
+  unsigned short ds, gs;
+  asm ("movw %%ds, %w0\n"
+       "movw %%gs, %w1"
+       : "=q" (ds), "=q" (gs));
+  return __glibc_unlikely (__HURD_DESC_INITIAL (gs, ds));
+}
+
+/* Code to initially initialize the thread pointer.  This might need
+   special attention since 'errno' is not yet available and if the
+   operation can cause a failure 'errno' must not be touched.  */
 static inline bool __attribute__ ((unused))
 _hurd_tls_init (tcbhead_t *tcb)
 {
@@ -168,11 +178,10 @@ out:
   return success;
 }
 
-/* Code to initially initialize the thread pointer.  This might need
-   special attention since 'errno' is not yet available and if the
-   operation can cause a failure 'errno' must not be touched.  */
-# define TLS_INIT_TP(descr) \
-    _hurd_tls_init ((tcbhead_t *) (descr))
+# define TLS_INIT_TP(descr) _hurd_tls_init ((tcbhead_t *) (descr))
+#else /* defined (SHARED) && !IS_IN (rtld) */
+# define __LIBC_NO_TLS() 0
+#endif
 
 # if __GNUC_PREREQ (6, 0)