[Lldb-commits] [lldb] Avoid expression evaluation in libStdC++ std::vector<bool> synthetic children provider (PR #108414)

[via lldb-commits]

jimingham wrote:

The goal here is excellent!  Not only is this a performance problem but if you call the version of GetValueFromExpression that doesn't take an expression options - as the code you are replacing does - you get the default value for "TryAllThreads" - which is `true`.  So if you are unlucky and the expression takes a little bit of time (maybe your machine is heavily loaded or something) then printing this summary could cause other threads to make progress out from under you.  In a GUI, that can result in the not helpful behavior where you stop at some point in Thread A, switch to Thread B - at which point the GUI loads the variables and runs the formatter for you and in doing so Thread A gets a chance to run.  Then you switch back to Thread A and it wasn't where you left it.  Bad debugger!

We should not use expressions in data formatters because they make them slow as you have seen.  But we for sure should never call an expression with `TryAllThreads` set to true in a data formatter.  If you are going to use expressions at all you should do it on only one thread and make sure that along the path of the function you call nobody takes locks that might deadlock the expression against other threads...  But we should try hard not to make that necessary, and shouldn't do so in ones we ship.

Another general rule is that if you are adding a significant algorithm to an SB API implementation, you should really make an API in the underlying lldb_private API and then call that.  We used to have all sorts of SB API specific implementations, which leads to code duplication and subtle differences in behavior depending on whether you find your way to the SB or private implementation.  We try not to do that anymore.

Can you do this instead with a ValueObject::CreateBooleanValue, and then just wrap that in the SBValue::CreateBooleanValue call?

https://github.com/llvm/llvm-project/pull/108414