3 Commits

Author	SHA1	Message	Date
Joel Brobecker	4a94e36819	Automatic Copyright Year update after running gdb/copyright.py ... This commit brings all the changes made by running gdb/copyright.py as per GDB's Start of New Year Procedure. For the avoidance of doubt, all changes in this commits were performed by the script.	2022-01-01 19:13:23 +04:00
Simon Marchi	8b6a69b2f3	gdb: use intrusive list for step-over chain ... The threads that need a step-over are currently linked using an hand-written intrusive doubly-linked list, so that seems a very good candidate for intrusive_list, convert it. For this, we have a use case of appending a list to another one (in start_step_over). Based on the std::list and Boost APIs, add a splice method. However, only support splicing the other list at the end of the `this` list, since that's all we need. Add explicit default assignment operators to reference_to_pointer_iterator, which are otherwise implicitly deleted. This is needed because to define thread_step_over_list_safe_iterator, we wrap reference_to_pointer_iterator inside a basic_safe_iterator, and basic_safe_iterator needs to be able to copy-assign the wrapped iterator. The move-assignment operator is therefore not needed, only the copy-assignment operator is. But for completeness, add both. Change-Id: I31b2ff67c7b78251314646b31887ef1dfebe510c	2021-07-12 20:46:52 -04:00
Pedro Alves	bf80931081	gdb: introduce intrusive_list, make thread_info use it ... GDB currently has several objects that are put in a singly linked list, by having the object's type have a "next" pointer directly. For example, struct thread_info and struct inferior. Because these are simply-linked lists, and we don't keep track of a "tail" pointer, when we want to append a new element on the list, we need to walk the whole list to find the current tail. It would be nice to get rid of that walk. Removing elements from such lists also requires a walk, to find the "previous" position relative to the element being removed. To eliminate the need for that walk, we could make those lists doubly-linked, by adding a "prev" pointer alongside "next". It would be nice to avoid the boilerplate associated with maintaining such a list manually, though. That is what the new intrusive_list type addresses. With an intrusive list, it's also possible to move items out of the list without destroying them, which is interesting in our case for example for threads, when we exit them, but can't destroy them immediately. We currently keep exited threads on the thread list, but we could change that which would simplify some things. Note that with std::list, element removal is O(N). I.e., with std::list, we need to walk the list to find the iterator pointing to the position to remove. However, we could store a list iterator inside the object as soon as we put the object in the list, to address it, because std::list iterators are not invalidated when other elements are added/removed. However, if you need to put the same object in more than one list, then std::list<object> doesn't work. You need to instead use std::list<object *>, which is less efficient for requiring extra memory allocations. For an example of an object in multiple lists, see the step_over_next/step_over_prev fields in thread_info: /* Step-over chain. A thread is in the step-over queue if these are non-NULL. If only a single thread is in the chain, then these fields point to self. */ struct thread_info *step_over_prev = NULL; struct thread_info *step_over_next = NULL; The new intrusive_list type gives us the advantages of an intrusive linked list, while avoiding the boilerplate associated with manually maintaining it. intrusive_list's API follows the standard container interface, and thus std::list's interface. It is based the API of Boost's intrusive list, here: https://www.boost.org/doc/libs/1_73_0/doc/html/boost/intrusive/list.html Our implementation is relatively simple, while Boost's is complicated and intertwined due to a lot of customization options, which our version doesn't have. The easiest way to use an intrusive_list is to make the list's element type inherit from intrusive_node. This adds a prev/next pointers to the element type. However, to support putting the same object in more than one list, intrusive_list supports putting the "node" info as a field member, so you can have more than one such nodes, one per list. As a first guinea pig, this patch makes the per-inferior thread list use intrusive_list using the base class method. Unlike Boost's implementation, ours is not a circular list. An earlier version of the patch was circular: the intrusive_list type included an intrusive_list_node "head". In this design, a node contained pointers to the previous and next nodes, not the previous and next elements. This wasn't great for when debugging GDB with GDB, as it was difficult to get from a pointer to the node to a pointer to the element. With the design proposed in this patch, nodes contain pointers to the previous and next elements, making it easy to traverse the list by hand and inspect each element. The intrusive_list object contains pointers to the first and last elements of the list. They are nullptr if the list is empty. Each element's node contains a pointer to the previous and next elements. The first element's previous pointer is nullptr and the last element's next pointer is nullptr. Therefore, if there's a single element in the list, both its previous and next pointers are nullptr. To differentiate such an element from an element that is not linked into a list, the previous and next pointers contain a special value (-1) when the node is not linked. This is necessary to be able to reliably tell if a given node is currently linked or not. A begin() iterator points to the first item in the list. An end() iterator contains nullptr. This makes iteration until end naturally work, as advancing past the last element will make the iterator contain nullptr, making it equal to the end iterator. If the list is empty, a begin() iterator will contain nullptr from the start, and therefore be immediately equal to the end. Iterating on an intrusive_list yields references to objects (e.g. `thread_info&`). The rest of GDB currently expects iterators and ranges to yield pointers (e.g. `thread_info*`). To bridge the gap, add the reference_to_pointer_iterator type. It is used to define inf_threads_iterator. Add a Python pretty-printer, to help inspecting intrusive lists when debugging GDB with GDB. Here's an example of the output: (top-gdb) p current_inferior_.m_obj.thread_list $1 = intrusive list of thread_info = {0x61700002c000, 0x617000069080, 0x617000069400, 0x61700006d680, 0x61700006eb80} It's not possible with current master, but with this patch [1] that I hope will be merged eventually, it's possible to index the list and access the pretty-printed value's children: (top-gdb) p current_inferior_.m_obj.thread_list[1] $2 = (thread_info *) 0x617000069080 (top-gdb) p current_inferior_.m_obj.thread_list[1].ptid $3 = { m_pid = 406499, m_lwp = 406503, m_tid = 0 } Even though iterating the list in C++ yields references, the Python pretty-printer yields pointers. The reason for this is that the output of printing the thread list above would be unreadable, IMO, if each thread_info object was printed in-line, since they contain so much information. I think it's more useful to print pointers, and let the user drill down as needed. [1] https://sourceware.org/pipermail/gdb-patches/2021-April/178050.html Co-Authored-By: Simon Marchi <simon.marchi@efficios.com> Change-Id: I3412a14dc77f25876d742dab8f44e0ba7c7586c0	2021-07-12 20:46:52 -04:00