[sanitizer] Use __atomic_load/store() built-ins for generic 32-bit targets

[glaubitz]

Summary

Simplifies the code and fixes the build on SPARC.

See discussion in: http://lists.llvm.org/pipermail/llvm-dev/2020-October/145937.html

[dvyukov]

Looks good with nits.
Please also upload with full file context, I wonder what's before that "} else {" on line 80.

compiler-rt/lib/sanitizer_common/sanitizer_atomic_clang_other.h

54

Also remove this comment.

82

Also remove this comment.

[glaubitz]

Addressed all comments and suggestions.

[glaubitz]

Replace tabs with spaces to fix indentation.

[jrtc27]

I suggested this initially as a quick fix, not the final solution. Really if we want to fix this properly we should just always use __atomic_store with an appropriately-translated memory order, as is done in libcxx (this is really just reimplementing a subset of libcxx as a standalone library).

[glaubitz]

This patch unbreaks the build on bi-arch systems which are not x86 or MIPS, so I think it's okay to pick up the change and rewrite the code in cleaner terms later.

[glaubitz]

Could someone with access commit this patch for me?

[dvyukov]

Merged as: https://github.com/llvm/llvm-project/commit/26c1ced41c262bb87619cfa8a03c1879c63fb5f7
Thanks!

compiler-rt/lib/sanitizer_common/sanitizer_atomic_clang_other.h

     // 64-bit load on 32-bit platform.
     // Gross, but simple and reliable.
     // Assume that it is not in read-only memory.
-    v = __sync_fetch_and_add(
-        const_cast<typename T::Type volatile *>(&a->val_dont_use), 0);
+    __atomic_load(const_cast<typename T::Type volatile *>(&a->val_dont_use), &v, __ATOMIC_SEQ_CST);
   }
   return v;
 }

   } else {
     // 64-bit store on 32-bit platform.
     // Gross, but simple and reliable.
-    typename T::Type cmp = a->val_dont_use;
-    typename T::Type cur;
-    for (;;) {
-      cur = __sync_val_compare_and_swap(&a->val_dont_use, cmp, v);
-      if (cur == cmp || cur == v)
-        break;
-      cmp = cur;
-    }
+    __atomic_store(&a->val_dont_use, &v, __ATOMIC_SEQ_CST);
   }
 }