[lldb-dev] Printing non-truncated stdlib collections?
Enrico Granata
egranata at apple.com
Mon Nov 4 16:00:20 PST 2013
This one is not an issue. It's by design.
Solution: try
(lldb) frame variable vec[0]
Explanation: the expression command is using the compiler, clang, to generate code that runs in your inferior process. This requires the operator[] defined by your implementation of the STL to be around. If that is nowhere suitable to be find in the debug info (e.g. You never used it in your code) that expression will not be compilable since essentially it is as-if no vector::operator[] existed at all.
The way in which children in a vector are generated is totally unrelated to this. LLDB knows enough about the internals of the std::vector class to generate child variables as-if they were being accessed by indexing, but all without running code in your process. They are called "synthetic children" and are a debugger artifact essentially. Which is why sometimes we get it wrong: since we are not relying on anything but introspecting memory, we are more easily "fooled".
Our expression parser explicitly disallows such synthetic children for kicking in. Imagine if they did. Now your operator[] would not be invoked because the synthetic children feature would kick in and return a result. But what if you said
(lldb) expr vec[0] = 1
The synthetic children are a snapshot of memory so they don't know how to assign back to themselves. Hence why you don't want expressions to mix with synthetic children. It's a tricky business to get right.
Sent from the iPhone of
Enrico Granata <egranata@🍎.com>
> On Nov 4, 2013, at 3:47 PM, Dun Peal <dunpealer at gmail.com> wrote:
>
> OK, I posted the original question of this thread as the following bug: http://llvm.org/bugs/show_bug.cgi?id=17805
>
> Another issue I just stumbled across:
>
> (lldb) p vec[0]
> error: call to a function 'std::vector<std::vector<std::pair<int, int>, std::allocator<std::pair<int, int> > >, std::allocator<std::vector<std::pair<int, int>, std::allocator<std::pair<int, int> > > > >::operator[](unsigned long)' ('_ZNSt6vectorIS_ISt4pairIiiESaIS1_EESaIS3_EEixEm') that is not present in the target
> error: The expression could not be prepared to run in the target
>
>
>> On Mon, Nov 4, 2013 at 2:59 PM, Enrico Granata <egranata at apple.com> wrote:
>> The latter one I think is an expression parser issue.
>> It should be fixed in ToT already, but I CC’ed on this email Sean Callanan who works on this part of LLDB and might have more insights for you
>>
>> I tried to reproduce your issue on OSX Mavericks, but in spite of me trying to print ~11.000 pairs (I raised your 300 to 900 and put 12 pairs in each sub-vectors instead of 4), it took about 5 seconds to print everything
>>
>> If you do file a bug, which you totally should, more details on your environment might be interesting: OS, compiler, standard library, revision of LLDB, ..
>>
>> Enrico Granata
>> 📩 egranata@.com
>> ☎️ 27683
>>
>>> On Nov 4, 2013, at 2:36 PM, Dun Peal <dunpealer at gmail.com> wrote:
>>>
>>> If it's trying to create 4 billion non-existing elements per vector, there's probably no need to sample. It explains the lost time pretty well.
>>>
>>> Let me know if you want me to file a bug. I'm encountering other issues, for instance sometimes trying to `p some_name`, I get:
>>>
>>> error: Couldn't materialize struct: size of variable some_name disagrees with the ValueObject's size
>>> Errored out in Execute, couldn't PrepareToExecuteJITExpression
>>>
>>> Perhaps lldb simply isn't production ready for non-OSX platforms?
>>>
>>>
>>>> On Mon, Nov 4, 2013 at 2:14 PM, Enrico Granata <egranata at apple.com> wrote:
>>>> Replies inlined
>>>>
>>>> Enrico Granata
>>>> 📩 egranata@.com
>>>> ☎️ 27683
>>>>
>>>>> On Nov 4, 2013, at 1:48 PM, Dun Peal <dunpealer at gmail.com> wrote:
>>>>>
>>>>> In my case, it's a vector of vectors of pairs of ints, i.e. vector<vector<pair<int,int>>>.
>>>>>
>>>>> I'm not sure what "a sample of lldb taken while it's sitting there" means. Sorry, I'm an LLVM newbie.
>>>>
>>>> If you are on OSX, it simply means typing sample lldb at a bash prompt :)
>>>> It will periodically look at the state of LLDB and generate a report of what is most likely taking up all that time
>>>>
>>>> On Linux/Windows/.. I suppose there are equivalent facilities. Google is your friend. A process sample has nothing to do with LLVM specifically.
>>>>
>>>>> I have reproduced the problem with minimal code, posted below. Two interesting observations:
>>>>>
>>>>> 1) For some reason, lldb prints each vector<pair<int,int>> as:
>>>>>
>>>>> [0] = size=4294967295 {
>>>>> [0] = (first = 0, second = 1)
>>>>> [1] = (first = 1, second = 2)
>>>>> [2] = (first = 2, second = 3)
>>>>> [3] = (first = 3, second = 4)
>>>>> ...
>>>>> }
>>>>>
>>>>> Since each of those vectors is exactly 4 pairs, it is printed in its entirety, so I'm not sure why there's an ellipsis there.
>>>>
>>>> It looks like something is wrong with this data structure and we believe its size to be the large number
>>>> That value is not just a placeholder, it’s how many elements LLDB actually thinks are in the vector!
>>>> Most likely we end up realizing those are not valid and so we omit printing them, but we still believe they exist, and since you likely asked to see all of them, we are trying to create 4 billion elements and failing. Here your 300 seconds
>>>> Why we end up with malformed data like that is an interesting question. I will try to repro
>>>>
>>>>> 2) The times I quoted above are surprisingly preserved with this sample code. For example, printing the first 256 elements is still about 8 seconds. Printing all 300 elements, which is only about 20% more, takes 300 seconds, i.e. almost x40 the time! Curious.
>>>>>
>>>>> #include <iostream>
>>>>> #include <vector>
>>>>>
>>>>> using namespace std;
>>>>>
>>>>> int main() {
>>>>> vector<vector<pair<int,int> > > vec;
>>>>> for (int i=0; i < 300; ++i) {
>>>>> vector<pair<int,int> > v;
>>>>> for (int j=0; j < 4; ++j) {
>>>>> v.push_back(make_pair(i+j, i+j+1));
>>>>> }
>>>>> vec.push_back(v);
>>>>> }
>>>>> return 0; // to reproduce, put a breakpoint in this line, and `p vec`
>>>>> }
>>>>>
>>>>>
>>>>>> On Mon, Nov 4, 2013 at 12:49 PM, Enrico Granata <egranata at apple.com> wrote:
>>>>>> Yes please. Possibly with a sample of lldb taken while it's sitting there.
>>>>>> From your email, it sounds like the repro case is just a vector of pairs of int and int, with about 400 elements. Is that all?
>>>>>>
>>>>>> Sent from the iPhone of
>>>>>> Enrico Granata <egranata@🍎.com>
>>>>>>
>>>>>>> On Nov 4, 2013, at 12:42 PM, Dun Peal <dunpealer at gmail.com> wrote:
>>>>>>>
>>>>>>> Thanks! This works, though surprisingly slow; I just printed a vector<vector<pair<int,int>>> of 384 elements, and it blocked for about 390 seconds (6:30 minutes!) before rendering.
>>>>>>>
>>>>>>> The print only blocks for about 8 seconds when rendering the first 256 elements (i.e. without the settings change).
>>>>>>>
>>>>>>> This is LLDB 3.4 from the LLVM aptitude repo, running on a high end Xubuntu Linux 13.04 developer workstation.
>>>>>>>
>>>>>>> This is obviously a major usability issue for me with LLDB. Should I file a bug for this?
>>>>>>>
>>>>>>>
>>>>>>>> On Mon, Nov 4, 2013 at 10:05 AM, Greg Clayton <gclayton at apple.com> wrote:
>>>>>>>> (lldb) settings show target.max-children-count
>>>>>>>> target.max-children-count (int) = 256
>>>>>>>> (lldb) settings set target.max-children-count 10000
>>>>>>>>
>>>>>>>>
>>>>>>>> You can then add this line to your ~/.lldbinit file if you want the setting to always be increased.
>>>>>>>>
>>>>>>>>
>>>>>>>> On Nov 3, 2013, at 7:57 PM, Dun Peal <dunpealer at gmail.com> wrote:
>>>>>>>>
>>>>>>>> > Hi,
>>>>>>>> >
>>>>>>>> > When I do:
>>>>>>>> >
>>>>>>>> > (lldb) p some_vector
>>>>>>>> >
>>>>>>>> > It seems LLDB only actually prints the first 256 values. How do I get it to print the entire vector?
>>>>>>>> >
>>>>>>>> > Thanks, D.
>>>>>>>> > _______________________________________________
>>>>>>>> > lldb-dev mailing list
>>>>>>>> > lldb-dev at cs.uiuc.edu
>>>>>>>> > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> lldb-dev mailing list
>>>>>>> lldb-dev at cs.uiuc.edu
>>>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
>
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