re PR go/89199 (libgo regression in implementation of CompareAndSwap functions…

re PR go/89199 (libgo regression in implementation of CompareAndSwap functions resulting in intermittent testcase failures on ppc64le power9 after r268458)

	PR go/89199
    sync/atomic: use strong form of atomic_compare_exchange_n
    
    In the recent change to use atomic_compare_exchange_n I thought we
    could use the weak form, which can spuriously fail. But that is not
    how it is implemented in the gc library, and it is not what the rest
    of the library expects.
    
    Thanks to Lynn Boger for identifying the problem.
    
    Fixes https://gcc.gnu.org/PR89199
    
    Reviewed-on: https://go-review.googlesource.com/c/161359

From-SVN: r268591

gcc/go/gofrontend/MERGE

-74ffeddbe6fef446129af65581b3a9094715bc22
+d89db31db68d09aa13a6137122cc096c1d92597b
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/sync/atomic/atomic.c

@@ -69,7 +69,7 @@ _Bool CompareAndSwapInt32 (int32_t *, int32_t, int32_t)
 _Bool
 CompareAndSwapInt32 (int32_t *val, int32_t old, int32_t new)
 {
-  return __atomic_compare_exchange_n (val, &old, new, true, __ATOMIC_SEQ_CST,
+  return __atomic_compare_exchange_n (val, &old, new, false, __ATOMIC_SEQ_CST,
 				      __ATOMIC_RELAXED);
 }
 
@@ -82,7 +82,7 @@ CompareAndSwapInt64 (int64_t *val, int64_t old, int64_t new)
 {
   if (((uintptr_t) val & 7) != 0)
     val = NULL;
-  return __atomic_compare_exchange_n (val, &old, new, true, __ATOMIC_SEQ_CST,
+  return __atomic_compare_exchange_n (val, &old, new, false, __ATOMIC_SEQ_CST,
 				      __ATOMIC_RELAXED);
 }
 
@@ -93,7 +93,7 @@ _Bool CompareAndSwapUint32 (uint32_t *, uint32_t, uint32_t)
 _Bool
 CompareAndSwapUint32 (uint32_t *val, uint32_t old, uint32_t new)
 {
-  return __atomic_compare_exchange_n (val, &old, new, true, __ATOMIC_SEQ_CST,
+  return __atomic_compare_exchange_n (val, &old, new, false, __ATOMIC_SEQ_CST,
 				      __ATOMIC_RELAXED);
 }
 
@@ -106,7 +106,7 @@ CompareAndSwapUint64 (uint64_t *val, uint64_t old, uint64_t new)
 {
   if (((uintptr_t) val & 7) != 0)
     val = NULL;
-  return __atomic_compare_exchange_n (val, &old, new, true, __ATOMIC_SEQ_CST,
+  return __atomic_compare_exchange_n (val, &old, new, false, __ATOMIC_SEQ_CST,
 				      __ATOMIC_RELAXED);
 }
 
@@ -117,7 +117,7 @@ _Bool CompareAndSwapUintptr (uintptr_t *, uintptr_t, uintptr_t)
 _Bool
 CompareAndSwapUintptr (uintptr_t *val, uintptr_t old, uintptr_t new)
 {
-  return __atomic_compare_exchange_n (val, &old, new, true, __ATOMIC_SEQ_CST,
+  return __atomic_compare_exchange_n (val, &old, new, false, __ATOMIC_SEQ_CST,
 				      __ATOMIC_RELAXED);
 }