[compiler-rt] r331890 - [sanitizer] Correct 64-bit atomic_store on 32-bit "other" platforms

[Kostya Kortchinsky via llvm-commits]

Author: cryptoad
Date: Wed May  9 09:20:52 2018
New Revision: 331890

URL: http://llvm.org/viewvc/llvm-project?rev=331890&view=rev
Log:
[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

Modified:
    compiler-rt/trunk/lib/sanitizer_common/sanitizer_atomic_clang_other.h

Modified: compiler-rt/trunk/lib/sanitizer_common/sanitizer_atomic_clang_other.h
URL: http://llvm.org/viewvc/llvm-project/compiler-rt/trunk/lib/sanitizer_common/sanitizer_atomic_clang_other.h?rev=331890&r1=331889&r2=331890&view=diff
==============================================================================
--- compiler-rt/trunk/lib/sanitizer_common/sanitizer_atomic_clang_other.h (original)
+++ compiler-rt/trunk/lib/sanitizer_common/sanitizer_atomic_clang_other.h Wed May  9 09:20:52 2018
@@ -86,7 +86,7 @@ INLINE void atomic_store(volatile T *a,
     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;
     }