[PATCH] libatomic: x86_64: Always try ifunc

[Xi Ruoyao <xry111@xry111.site>]

We used to skip ifunc check when CX16 is available.  But now we use
CX16+AVX+Intel/AMD for the "perfect" 16b load implementation, so CX16
alone is not a sufficient reason not to use ifunc (see PR104688).

This causes a subtle and annoying issue: when GCC is built with a
higher -march= setting in CFLAGS_FOR_TARGET, ifunc is disabled and
the worst (locked) implementation of __atomic_load_16 is always used.

There seems no good way to check if the CPU is Intel or AMD from
the built-in macros (maybe we can check every known model like __skylake,
__bdver2, ..., but it will be very error-prune and require an update
whenever we add the support for a new x86 model).  The best thing we can
do seems "always try ifunc" here.

Bootstrapped and tested on x86_64-linux-gnu.  Ok for trunk?

libatomic/ChangeLog:

	* configure.tgt: For x86_64, always set try_ifunc=yes.
---
 libatomic/configure.tgt | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/libatomic/configure.tgt b/libatomic/configure.tgt
index a92ae9e8309..39dd5686f2e 100644
--- a/libatomic/configure.tgt
+++ b/libatomic/configure.tgt
@@ -100,9 +100,7 @@ EOF
 	fi
 	cat > conftestx.c <<EOF
 #ifdef __x86_64__
-#ifndef __GCC_HAVE_SYNC_COMPARE_AND_SWAP_16
-#error need -mcx16
-#endif
+#error ifunc is always wanted for 16B atomic load
 #else
 #ifndef __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8
 #error need -march=i686
-- 
2.41.0