x86: Use dl-symbol-redir-ifunc.h on cpu-tunables

The dl-symbol-redir-ifunc.h redirects compiler-generated libcalls to
arch-specific memory implementations to avoid ifunc calls where it is not
yet possible. The memcmp-isa-default-impl.h aims to fix the same issue
by calling the specific memset implementation directly.

Using the memcmp symbol directly allows the compiler to inline the memset
calls (especially because _dl_tunable_set_hwcaps uses constants values),
generating better code.

Checked on x86_64-linux-gnu.

Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>
Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

sysdeps/i386/i686/multiarch/dl-symbol-redir-ifunc.h

@@ -19,6 +19,11 @@
 #ifndef _DL_IFUNC_GENERIC_H
 #define _DL_IFUNC_GENERIC_H
 
+#ifndef SHARED
+
 asm ("memset = __memset_ia32");
+asm ("memcmp = __memcmp_ia32");
+
+#endif /* SHARED */
 
 #endif

sysdeps/x86/cpu-tunables.c

@@ -24,24 +24,11 @@
 #include <string.h>
 #include <cpu-features.h>
 #include <ldsodefs.h>
-
-/* We can't use IFUNC memcmp nor strlen in init_cpu_features from libc.a
-   since IFUNC must be set up by init_cpu_features.  */
-#if defined USE_MULTIARCH && !defined SHARED
-# ifdef __x86_64__
-/* DEFAULT_MEMCMP by sysdeps/x86_64/memcmp-isa-default-impl.h.  */
-#  include <sysdeps/x86_64/memcmp-isa-default-impl.h>
-# else
-#  define DEFAULT_MEMCMP	__memcmp_ia32
-# endif
-extern __typeof (memcmp) DEFAULT_MEMCMP;
-#else
-# define DEFAULT_MEMCMP	memcmp
-#endif
+#include <dl-symbol-redir-ifunc.h>
 
 #define CHECK_GLIBC_IFUNC_CPU_OFF(f, cpu_features, name, len)		\
   _Static_assert (sizeof (#name) - 1 == len, #name " != " #len);	\
-  if (!DEFAULT_MEMCMP (f, #name, len))					\
+  if (memcmp (f, #name, len) == 0)					\
     {									\
       CPU_FEATURE_UNSET (cpu_features, name)				\
       break;								\
@@ -51,7 +38,7 @@ extern __typeof (memcmp) DEFAULT_MEMCMP;
    which isn't available.  */
 #define CHECK_GLIBC_IFUNC_PREFERRED_OFF(f, cpu_features, name, len)	\
   _Static_assert (sizeof (#name) - 1 == len, #name " != " #len);	\
-  if (!DEFAULT_MEMCMP (f, #name, len))					\
+  if (memcmp (f, #name, len) == 0)					\
     {									\
       cpu_features->preferred[index_arch_##name]			\
 	&= ~bit_arch_##name;						\
@@ -62,7 +49,7 @@ extern __typeof (memcmp) DEFAULT_MEMCMP;
 #define CHECK_GLIBC_IFUNC_PREFERRED_BOTH(f, cpu_features, name,	\
 					  disable, len)			\
   _Static_assert (sizeof (#name) - 1 == len, #name " != " #len);	\
-  if (!DEFAULT_MEMCMP (f, #name, len))					\
+  if (memcmp (f, #name, len) == 0)					\
     {									\
       if (disable)							\
 	cpu_features->preferred[index_arch_##name] &= ~bit_arch_##name;	\
@@ -76,7 +63,7 @@ extern __typeof (memcmp) DEFAULT_MEMCMP;
 #define CHECK_GLIBC_IFUNC_PREFERRED_NEED_BOTH(f, cpu_features, name,	\
 					       need, disable, len)	\
   _Static_assert (sizeof (#name) - 1 == len, #name " != " #len);	\
-  if (!DEFAULT_MEMCMP (f, #name, len))					\
+  if (memcmp (f, #name, len) == 0)					\
     {									\
       if (disable)							\
 	cpu_features->preferred[index_arch_##name] &= ~bit_arch_##name;	\
@@ -177,7 +164,7 @@ TUNABLE_CALLBACK (set_hwcaps) (tunable_val_t *valp)
 	      CHECK_GLIBC_IFUNC_CPU_OFF (n, cpu_features, POPCNT, 6);
 	      CHECK_GLIBC_IFUNC_CPU_OFF (n, cpu_features, SSE4_1, 6);
 	      CHECK_GLIBC_IFUNC_CPU_OFF (n, cpu_features, SSE4_2, 6);
-	      if (!DEFAULT_MEMCMP (n, "XSAVEC", 6))
+	      if (memcmp (n, "XSAVEC", 6) == 0)
 		{
 		  /* Update xsave_state_size to XSAVE state size.  */
 		  cpu_features->xsave_state_size
@@ -290,12 +277,11 @@ attribute_hidden
 void
 TUNABLE_CALLBACK (set_x86_ibt) (tunable_val_t *valp)
 {
-  if (DEFAULT_MEMCMP (valp->strval, "on", sizeof ("on")) == 0)
+  if (memcmp (valp->strval, "on", sizeof ("on")) == 0)
     GL(dl_x86_feature_control).ibt = cet_always_on;
-  else if (DEFAULT_MEMCMP (valp->strval, "off", sizeof ("off")) == 0)
+  else if (memcmp (valp->strval, "off", sizeof ("off")) == 0)
     GL(dl_x86_feature_control).ibt = cet_always_off;
-  else if (DEFAULT_MEMCMP (valp->strval, "permissive",
-			   sizeof ("permissive")) == 0)
+  else if (memcmp (valp->strval, "permissive", sizeof ("permissive")) == 0)
     GL(dl_x86_feature_control).ibt = cet_permissive;
 }
 
@@ -303,12 +289,11 @@ attribute_hidden
 void
 TUNABLE_CALLBACK (set_x86_shstk) (tunable_val_t *valp)
 {
-  if (DEFAULT_MEMCMP (valp->strval, "on", sizeof ("on")) == 0)
+  if (memcmp (valp->strval, "on", sizeof ("on")) == 0)
     GL(dl_x86_feature_control).shstk = cet_always_on;
-  else if (DEFAULT_MEMCMP (valp->strval, "off", sizeof ("off")) == 0)
+  else if (memcmp (valp->strval, "off", sizeof ("off")) == 0)
     GL(dl_x86_feature_control).shstk = cet_always_off;
-  else if (DEFAULT_MEMCMP (valp->strval, "permissive",
-			   sizeof ("permissive")) == 0)
+  else if (memcmp (valp->strval, "permissive", sizeof ("permissive")) == 0)
     GL(dl_x86_feature_control).shstk = cet_permissive;
 }
 #endif

sysdeps/x86_64/memcmp-isa-default-impl.h

@@ -1,28 +0,0 @@
-/* Set default memcmp impl based on ISA level.
-   Copyright (C) 2022-2023 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/>.  */
-
-#include <isa-level.h>
-#if MINIMUM_X86_ISA_LEVEL == 1 || MINIMUM_X86_ISA_LEVEL == 2
-# define DEFAULT_MEMCMP	__memcmp_sse2
-#elif MINIMUM_X86_ISA_LEVEL == 3
-# define DEFAULT_MEMCMP	__memcmp_avx2_movbe
-#elif MINIMUM_X86_ISA_LEVEL == 4
-# define DEFAULT_MEMCMP	__memcmp_evex_movbe
-#else
-# error "Unknown default memcmp implementation"
-#endif

sysdeps/x86_64/multiarch/dl-symbol-redir-ifunc.h

@@ -19,6 +19,8 @@
 #ifndef _DL_IFUNC_GENERIC_H
 #define _DL_IFUNC_GENERIC_H
 
+#ifndef SHARED
+
 #include <isa-level.h>
 
 #if MINIMUM_X86_ISA_LEVEL >= 4
@@ -31,4 +33,17 @@
 
 asm ("memset = " HAVE_MEMSET_IFUNC_GENERIC);
 
+
+#if MINIMUM_X86_ISA_LEVEL >= 4
+# define HAVE_MEMCMP_IFUNC_GENERIC "__memcmp_evex_movbe"
+#elif MINIMUM_X86_ISA_LEVEL == 3
+# define HAVE_MEMCMP_IFUNC_GENERIC "__memcmp_avx2_movbe"
+#else
+# define HAVE_MEMCMP_IFUNC_GENERIC "__memcmp_sse2"
+#endif
+
+asm ("memcmp = " HAVE_MEMCMP_IFUNC_GENERIC);
+
+#endif /* SHARED */
+
 #endif