[PATCH] libatomic: Handle AVX+CX16 AMD like Intel for 16b atomics [PR104688]

[Jakub Jelinek <jakub@redhat.com>]

Hi!

Working virtually out of Baker Island.

We got a response from AMD in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104688#c10
so the following patch starts treating AMD with AVX and CMPXCHG16B
ISAs like Intel by using vmovdqa for atomic load/store in libatomic.

Ok for trunk if it passes bootstrap/regtest?

2022-11-13  Jakub Jelinek  <jakub@redhat.com>

	PR target/104688
	* config/x86/init.c (__libat_feat1_init): Revert 2022-03-17 change
	- on x86_64 no longer clear bit_AVX if CPU vendor is not Intel.

--- libatomic/config/x86/init.c.jj	2022-03-17 18:48:56.708723194 +0100
+++ libatomic/config/x86/init.c	2022-11-13 18:23:26.315440071 -1200
@@ -34,18 +34,6 @@ __libat_feat1_init (void)
   unsigned int eax, ebx, ecx, edx;
   FEAT1_REGISTER = 0;
   __get_cpuid (1, &eax, &ebx, &ecx, &edx);
-#ifdef __x86_64__
-  if ((FEAT1_REGISTER & (bit_AVX | bit_CMPXCHG16B))
-      == (bit_AVX | bit_CMPXCHG16B))
-    {
-      /* Intel SDM guarantees that 16-byte VMOVDQA on 16-byte aligned address
-	 is atomic, but so far we don't have this guarantee from AMD.  */
-      unsigned int ecx2 = 0;
-      __get_cpuid (0, &eax, &ebx, &ecx2, &edx);
-      if (ecx2 != signature_INTEL_ecx)
-	FEAT1_REGISTER &= ~bit_AVX;
-    }
-#endif
   /* See the load in load_feat1.  */
   __atomic_store_n (&__libat_feat1, FEAT1_REGISTER, __ATOMIC_RELAXED);
   return FEAT1_REGISTER;

	Jakub