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, 13 insertions, 5 deletions

diff --git a/sysdeps/x86_64/multiarch/strcmp-sse2-unaligned.S b/sysdeps/x86_64/multiarch/strcmp-sse2-unaligned.S
index 0d691b78a8..33c18a28e8 100644
--- a/sysdeps/x86_64/multiarch/strcmp-sse2-unaligned.S
+++ b/sysdeps/x86_64/multiarch/strcmp-sse2-unaligned.S
@@ -16,11 +16,20 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
-#if IS_IN (libc)
+#include <isa-level.h>
 
-#include "sysdep.h"
+/* Continue building as ISA level 2.  We use this as ISA V2 default
+   because strcmp-sse42 uses pcmpstri (slow on some SSE4.2
+   processors) and this implementation is potenially faster than
+   strcmp-sse42 (aside from the slower page cross case).  */
+#if ISA_SHOULD_BUILD (2)
 
-ENTRY ( __strcmp_sse2_unaligned)
+# define STRCMP_ISA	_sse2_unaligned
+# include "strcmp-naming.h"
+
+# include "sysdep.h"
+
+ENTRY (STRCMP)
 	movl	%edi, %eax
 	xorl	%edx, %edx
 	pxor	%xmm7, %xmm7
@@ -208,6 +217,5 @@ L(cross_page):
 L(different):
 	subl	%ecx, %eax
 	ret
-END (__strcmp_sse2_unaligned)
-
+END (STRCMP)
 #endif