x86: Add support to build strcmp/strlen/strchr with explicit ISA level

1. Add default ISA level selection in non-multiarch/rtld
   implementations.

2. Add ISA level build guards to different implementations.
    - I.e strcmp-avx2.S which is ISA level 3 will only build if
      compiled ISA level <= 3. Otherwise there is no reason to
      include it as we will always use one of the ISA level 4
      implementations (strcmp-evex.S).

3. Refactor the ifunc selector and ifunc implementation list to use
   the ISA level aware wrapper macros that allow functions below the
   compiled ISA level (with a guranteed replacement) to be skipped.

Tested with and without multiarch on x86_64 for ISA levels:
{generic, x86-64-v2, x86-64-v3, x86-64-v4}

And m32 with and without multiarch.

1 files changed, 28 insertions, 1 deletions

diff --git a/sysdeps/x86_64/strncmp.S b/sysdeps/x86_64/strncmp.S
index 13d9e82ee2..afb251d9fe 100644
--- a/sysdeps/x86_64/strncmp.S
+++ b/sysdeps/x86_64/strncmp.S
@@ -1,4 +1,31 @@
+/* strncmp dispatch for RTLD and non-multiarch build
+   Copyright (C) 1999-2022 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/>.  */
+
 /* Symbol = strncmp.  */
 
-#include "multiarch/strncmp-sse2.S"
+#define DEFAULT_IMPL_V1	"multiarch/strncmp-sse2.S"
+/* This may cause regressions on some processors that heavily prefer
+   aligned loads or have slow a implementation of the `pcmpstri`
+   instruction.  */
+#define DEFAULT_IMPL_V2	"multiarch/strncmp-sse4_2.S"
+#define DEFAULT_IMPL_V3	"multiarch/strncmp-avx2.S"
+#define DEFAULT_IMPL_V4	"multiarch/strncmp-evex.S"
+
+#include "isa-default-impl.h"
+
 libc_hidden_builtin_def (strncmp)