[lldb-dev] lldb.frame.EvaluateExpression slows down when called a lot

[Greg Clayton]

Running expressions has all sorts of side effects like storing data in the inferior program and it also involves running the clang expression parser which can be expensive.

You can, from a frame, get a SBValue for a variable without using the expression parser:

    lldb::SBValue
    SBFrame.GetValueForVariablePath (const char *var_path);

So you can change your code to this:

heaps_used = lldb.frame.GetValueForVariablePath('ruby_current_vm->objspace->heap_pages.used').GetValueAsUnsigned(0)
   
for i in xrange(heaps_used):
    page = lldb.frame.GetValueForVariablePath('*ruby_current_vm->objspace->heap_pages.sorted[%i]' % i)

The GetValueForVariablePath() will find the variable and not create a temporary each time. It also doesn't use the expression parser at all so it won't call any code. The objects you access must be available in the hierarchy of the struct or class and the struct or class can't override the "->" operator. Other than that, the GetValueForVariablePath() knows how to access members ("ruby_current_vm->objspace->heap_pages.sorted"), dereference pointers using the array syntax ("my_ptr[12]"), deref a pointer ("*this->other_ptr"), and take the address of something ("&ruby_current_vm->objspace->heap_pages.sorted[12]").

So give the GetValueForVariablePath a try. The SBValue returned is something that represents the live variable value, not a const result like you get back from expression. SBValue you get back is tied to the frame from which you got it, so it will continue to evaluate correctly and its value will change if you step between calling functions with it. If the frame it came from goes away (step out), then it won't return any valid values again as it will detect the frame is gone and stop answering any questions. So you should always fetch a fresh value from the frame each time you want to use it.

Greg


On Apr 17, 2014, at 7:57 AM, Scott Knight <knightsc at gmail.com> wrote:

> I was recently using lldb to connect to a debug build of ruby to inspect the heap. In order to do this I was doing something like this
> 
> -----------
> heaps_used = lldb.frame.EvaluateExpression('ruby_current_vm->objspace->heap_pages.used').GetValueAsUnsigned(0)
>    
> for i in xrange(heaps_used):
>     page = lldb.frame.EvaluateExpression('*ruby_current_vm->objspace->heap_pages.sorted[%i]' % i)
> -----------
> 
> What I noticed was that for each EvaluateExpression a temporary $0, $1, $2, etc.. variable is created. If I ended up calling my python code multiple times more and more variables seemed to pile up and every EvaluateExpression call seemed to take longer and longer.
> 
> I tried calling EvaluateExpression how I would call expr from the lldb command line setting my own variable, so something like
> 
> lldb.frame.EvaluateExpression('int $test = 5')
> 
> But that seemed to error out. So is there some other way in the API that is better for accessing global variables that won't slow down. Is this something actually wrong with the debugger? I can create an actual test case similar to the test suite in lldb if that would be helpful.
> 
> Thanks,
> Scott Knight
> 
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