[sanitizer] Correct 64-bit atomic_store on 32-bit "other" platforms

Summary:
I think there might be something to optimize in `atomic_store`.
Currently, if everything goes well (and we have a different new value), we
always iterate 3 times.
For example, `with a = 0`, `oldval = a`, `newval = 42`, we get:
```
oldval = 0, newval = 42, curval = 0  
oldval = 0, newval = 42, curval = 42 
oldval = 42, newval = 42, curval = 42
```
and then it breaks.

Unless I am not seeing something, I don't see a point to the third iteration.
If the current value is the one we want, we should just break.
This means that 2 iterations (with a different newval) should be sufficient to
achieve what we want.

Reviewers: dvyukov, alekseyshl

Reviewed By: dvyukov

Subscribers: kubamracek, delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D46597

llvm-svn: 331890

compiler-rt/lib/sanitizer_common/sanitizer_atomic_clang_other.h

@@ -86,7 +86,7 @@ INLINE void atomic_store(volatile T *a, typename T::Type v, memory_order mo) {
     typename T::Type cur;
     for (;;) {
       cur = __sync_val_compare_and_swap(&a->val_dont_use, cmp, v);
-      if (cmp == v)
+      if (cur == cmp || cur == v)
         break;
       cmp = cur;
     }