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Re: Setting watchpoints in GDB

That's right, single stepping was MUCH worse, especially because the PPC processor won't single step over the class of instructions that are used to implement locks, so the program would just hang if it ever tried to take a lock. The kernel folks don't want to expose the DABR register (the one used for hardware watchpoints on the PPC) to user space, so page protection is the best we can do.

The way gdb does watchpoints is sometimes too clever for it's own good, since it watches all the components of the expression to see if ANY of them change. SO if you tell it to watch this->that->theOther, it will watch all three memory locations. That's sometimes very useful, but can mean you end up watching lots more than you expected. That's why you might want to resolve the address you want to watch first, then tell gdb to watch just that location.

Also, page protection watchpoints work VERY POORLY for watching anything on the stack, you really can't do that.

Jim

On Oct 25, 2004, at 12:59 AM, Jonas Maebe wrote:


On 25 okt 2004, at 00:59, Paul Sargent wrote:

Thing is..... OMG how slow is it? It says it's adding a hardware watchpoint. If that's hardware.....!!!!! I have a feeling what I was seeing before as a hang was just a very slow running program.

True hardware watchpoints are still not supported in the PPC version of gdb afaik (even though (most?) ppc's has this capability). I think it currently simply makes the page containing the watched data read-only, and then after each segmentation fault checks whether the watched data has changed.

If there are lots of other changes to the page containing the data you are watching, this is indeed very slow. The old way where gdb always used software watchpoints on ppc (single stepping through the program and checking whether the watched value changed after each instruction) was even worse though :)


Jonas

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Jim Ingham email@hidden
Developer Tools - gdb

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References: 
 >Setting watchpoints in GDB (From: Paul Sargent <email@hidden>)
 >Re: Setting watchpoints in GDB (From: Jonas Maebe <email@hidden>)
 >Re: Setting watchpoints in GDB (From: Paul Sargent <email@hidden>)
 >Re: Setting watchpoints in GDB (From: Jonas Maebe <email@hidden>)



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