[PATCH] D46597: [sanitizer] Correct 64-bit atomic_store on 32-bit "other" platforms

[Kostya Kortchinsky via Phabricator via llvm-commits]

cryptoad created this revision.
cryptoad added reviewers: dvyukov, alekseyshl.
Herald added subscribers: Sanitizers, delcypher, kubamracek.

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.


Repository:
  rCRT Compiler Runtime

https://reviews.llvm.org/D46597

Files:
  lib/sanitizer_common/sanitizer_atomic_clang_other.h


Index: lib/sanitizer_common/sanitizer_atomic_clang_other.h
===================================================================
--- lib/sanitizer_common/sanitizer_atomic_clang_other.h
+++ lib/sanitizer_common/sanitizer_atomic_clang_other.h
@@ -86,7 +86,7 @@
     typename T::Type cur;
     for (;;) {
       cur = __sync_val_compare_and_swap(&a->val_dont_use, cmp, v);
-      if (cmp == v)
+      if (cur == v)
         break;
       cmp = cur;
     }


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