Fix asm constraints in folly::MicroSpinLock::cas

[folly.git] / folly / SmallLocks.h

diff --git a/folly/SmallLocks.h b/folly/SmallLocks.h
index 01387aee724eae06da863f6f48f346db9be4cd9d..b092a91766b1ff6e91d41b36f81feecb3315ae0d 100644 (file)
--- a/folly/SmallLocks.h
+++ b/folly/SmallLocks.h
@@ -111,12 +111,13 @@ struct MicroSpinLock {
    */
   bool cas(uint8_t compare, uint8_t newVal) {
     bool out;
-    asm volatile("lock; cmpxchgb %2, (%3);"
-                 "setz %0;"
-                 : "=r" (out)
+    bool memVal; // only set if the cmpxchg fails
+    asm volatile("lock; cmpxchgb %[newVal], (%[lockPtr]);"
+                 "setz %[output];"
+                 : [output] "=r" (out), "=a" (memVal)
                  : "a" (compare), // cmpxchgb constrains this to be in %al
-                   "q" (newVal),  // Needs to be byte-accessible
-                   "r" (&lock_)
+                   [newVal] "q" (newVal),  // Needs to be byte-accessible
+                   [lockPtr] "r" (&lock_)
                  : "memory", "flags");
     return out;
   }