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Recent impressive performance improvements in computer architecture have not led to significant gains in ease of debugging. Software debugging often relies on inserting run-time software checks. In many cases, however, it is hard to find the root cause of a bug. Moreover, program execution typically slows down significantly, often by 10-100 times. To address this problem, this paper introduces the Intelligent Watcher (iWatcher), novel architectural support to monitor dynamic execution with minimal overhead, automatically, and flexibly. iWatcher associates program-specified monitoring functions with memory locations. When any such location is accessed, the monitoring function is automatically triggered with low overhead. To further reduce overhead and support rollback, iWatcher can leverage Thread-Level Speculation (TLS). To test iWatcher, we use applications with various bugs. Our results show that iWatcher detects many more software bugs than Valgrind, a well-known open-source bug detector. Moreover, iWatcher only induces a 4-80% execution overhead, which is orders of magnitude less than Valgrind. Even with 20% of the dynamic loads monitored in a program, iWatcher adds only 66-174% overhead. Finally, TLS is effective at reducing overheads for programs with substantial monitoring.