x86: Set minimum x86-64 level marker [BZ #27318]

Since the full ISA set used in an ELF binary is unknown to compiler,
an x86-64 ISA level marker indicates the minimum, not maximum, ISA set
required to run such an ELF binary.  We never guarantee a library with
an x86-64 ISA level v3 marker doesn't contain other ISAs beyond x86-64
ISA level v3, like AVX VNNI.  We check the x86-64 ISA level marker for
the minimum ISA set.  Since -march=sandybridge enables only some ISAs
in x86-64 ISA level v3, we should set the needed ISA marker to v2.
Otherwise, libc is compiled with -march=sandybridge will fail to run on
Sandy Bridge:

$ ./elf/ld.so ./libc.so
./libc.so: (p) CPU ISA level is lower than required: needed: 7; got: 3

Set the minimum, instead of maximum, x86-64 ISA level marker should have
no impact on the glibc-hwcaps directory assignment logic in ldconfig nor
ld.so.

1 files changed, 6 insertions, 0 deletions

diff --git a/config.h.in b/config.h.in
index 06ee8ae..f21bf04 100644
--- a/config.h.in
+++ b/config.h.in
@@ -275,4 +275,10 @@
 /* Define if x86 ISA level should be included in shared libraries.  */
 #undef INCLUDE_X86_ISA_LEVEL
 
+/* Define if -msahf is enabled by default on x86.  */
+#undef HAVE_X86_LAHF_SAHF
+
+/* Define if -mmovbe is enabled by default on x86.  */
+#undef HAVE_X86_MOVBE
+
 #endif