[PATCH] D104349: [compiler-rt][atomics][RFC] Enable __atomic_exchange handling case when `val == old`

[Kai Luo via Phabricator via llvm-commits]

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Looks the behavior is not documented when `val == old` in `__atomic_exchange`, so I added code to handle this case.
Or shall we document it as undefined behavior if `val == old`, since when `val == old` and lock code is called, content in `ptr` won't be exchanged finally.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D104349

Files:
  compiler-rt/lib/builtins/atomic.c


Index: compiler-rt/lib/builtins/atomic.c
===================================================================
--- compiler-rt/lib/builtins/atomic.c
+++ compiler-rt/lib/builtins/atomic.c
@@ -233,10 +233,19 @@
   return;
   LOCK_FREE_CASES(ptr);
 #undef LOCK_FREE_ACTION
+  int i;
   Lock *l = lock_for_pointer(ptr);
   lock(l);
-  memcpy(old, ptr, size);
-  memcpy(ptr, val, size);
+  if (val != old) {
+    memcpy(old, ptr, size);
+    memcpy(ptr, val, size);
+  } else {
+    for (i = 0; i < size; ++i) {
+      *((char *)ptr + i) ^= *((char *)old + i);
+      *((char *)old + i) ^= *((char *)ptr + i);
+      *((char *)ptr + i) ^= *((char *)old + i);
+    }
+  }
   unlock(l);
 }
 


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