Triggering Watchpoints

When a watchpoint triggers, the thread’s program counter (PC) points to the instruction following the instruction that caused the watchpoint to trigger. For example, this watchpoint on the variable arg_count triggered, placing the PC at the next instruction:

If the memory store instruction is the last instruction in a source statement, the PC points to the source line following the statement that triggered the watchpoint. (Breakpoints and watchpoints work differently. A breakpoint stops before an instruction executes. In contrast, a watchpoint stops after an instruction executes.)

Using Multiple Watchpoints

If a program modifies more than one byte with one program instruction or statement, which is normally the case when storing a word, TotalView triggers the watchpoint with the lowest memory location in the modified region. Although the program might be modifying locations monitored by other watchpoints, TotalView triggers the watchpoint only for the lowest memory location. This can occur when your watchpoints are monitoring adjacent memory locations and a single store instruction modifies these locations.

For example, suppose that you have two 1-byte watchpoints, one on location 0x10000 and the other on location 0x10001. Also suppose that your program uses a single instruction to store a 2-byte value at locations 0x10000 and 0x10001. If the 2-byte storage operation modifies both bytes, the watchpoint for location 0x10000 triggers. The watchpoint for location 0x10001 does not trigger.

Here’s a second example. Suppose that you have a 4-byte integer that uses storage locations 0x10000 through 0x10003, and you set a watchpoint on this integer. If a process modifies location 0x10002, TotalView triggers the watchpoint. Now suppose that you’re watching two adjacent 4-byte integers that are stored in locations 0x10000 through 0x10007. If a process writes to locations 0x10003 and 0x10004 (that is, one byte in each), TotalView triggers the watchpoint associated with location 0x10003. The watchpoint associated with location 0x10004 does not trigger.

Performance Impact of Copying Previous Data Values

TotalView keeps an internal copy of data in the watched memory locations for each process that shares the watchpoint. If you create watchpoints that cover a large area of memory or if your program has a large number of processes, you increase TotalView’s virtual memory requirements. Furthermore, TotalView refetches data for each memory location whenever it continues the process or thread. This can affect performance.