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

[Scott Knight]

Great! This is exactly what I was looking for and works perfectly.

Thanks,
Scott Knight


On Thu, Apr 17, 2014 at 2:27 PM, Greg Clayton <gclayton at apple.com> wrote:

>
> SBValue
> SBTarget::FindFirstGlobalVariable (const char* name);
>
> This doesn't support the "GetValueForVariablePath()", so you will need to
> do:
>
> ruby_current_vm = lldb.target.FindFirstGlobalVariable('ruby_current_vm');
>
> heaps_used =
> ruby_current_vm.GetValueForExpressionPath('->objspace->heap_pages.used').GetValueAsUnsigned(0)
>
> You can also use a very handy wrapper utility class called lldb.value:
>
> ruby_current_vm =
> lldb.value(lldb.target.FindFirstGlobalVariable('ruby_current_vm'))
>
> Now "ruby_current_vm" behaves like a C structure would, except you can't
> use "->" to refer to a child of a pointer you need to use ".". So you
> should be able to do:
>
> heaps_used = ruby_current_vm.objspace.heap_pages.used
>
> for i in xrange(heaps_used):
>     page = ruby_current_vm.objspace.heap_pages.sorted[i]
>
> You had a derefernce on "page" before, but, you can use page is a
> lldb.value, so you can just do "page.foo.bar" if you need anything inside
> of it.
>
> Greg
>
> On Apr 17, 2014, at 11:09 AM, Scott Knight <knightsc at gmail.com> wrote:
>
> > Thanks for the information Greg. I have a quick followup. I'm using the
> version of lldb that comes with XCode 5.1.1
> >
> > Launching it like this
> >
> > Scotts-MacBook-Pro:~ scottknight$
> /Applications/Xcode.app/Contents/Developer/usr/bin/lldb -p 13892
> > Attaching to process with:
> >     process attach -p 13892
> > Process 13892 stopped
> > Executable module set to
> "/Users/scottknight/.rbenv/versions/2.1.1/bin/ruby".
> > Architecture set to: x86_64-apple-macosx.
> >
> > When I tried using GetValueForVariablePath I got 'No value' back. See
> the output below.
> >
> > (lldb) v
> > lldb-310.2.37
> > (lldb) script
> > Python Interactive Interpreter. To exit, type 'quit()', 'exit()' or
> Ctrl-D.
> > >>> print lldb.frame.EvaluateExpression('ruby_current_vm')
> > (rb_vm_t *) $1 = 0x00007f9a01003000
> > >>> print lldb.frame.GetValueForVariablePath('ruby_current_vm')
> > No value
> >
> > Since ruby_current_vm is a global variable is there something different
> I would need to do to access it?
> >
> > Thanks,
> > Scott Knight
> >
> >
> > On Thu, Apr 17, 2014 at 1:45 PM, Greg Clayton <gclayton at apple.com>
> wrote:
> > 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
> > >
> > > _______________________________________________
> > > lldb-dev mailing list
> > > lldb-dev at cs.uiuc.edu
> > > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
> >
> >
>
>
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