可变参数模板函数调用上的 lambda 包装器引起的 gcc 分段错误

Question

我今天在 g++6.2 和 g++7.0 上花了好几个小时 trying to understand why this code 段错误，同时愉快地按计划在 clang++3.9 上工作（和 4.0).

我将问题简化为 85 lines self-contained code snippet，在正常执行时不会出现段错误，但总是报告错误在 UBSAN 下。

问题 is reproducible on wandbox，通过使用 g++7 编译，启用优化并将 -fsanitize=undefined 作为额外标志传递。

这是 UBSAN 的报告：

prog.cc: In function 'int main()':
prog.cc:61:49: warning: 'ns#0' is used uninitialized in this function [-Wuninitialized]
         ([&] { ([&] { n.execute(ns...); })(); })();
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
prog.cc:28:10: note: 'ns#0' was declared here
     auto execute(TNode& n, TNodes&... ns)
          ^~~~~~~
prog.cc:30:9: runtime error: member call on null pointer of type 'struct node_then'

g++ 声称 ns#0 在 "lambda gibberish" 中未初始化（模拟原始片段中的 for_tuple） .现在，一些非常有趣的事情发生了：

如果我删除 "lambda gibberish"，将 行 61 转换为
```
 n.execute(ns...);
```
然后 UBSAN 停止抱怨。
如果我将捕获列表从[&]更改为[&n, &ns...]，UBSAN也会停止抱怨：
```
 ([&](auto) { ([&n, &ns...] { n.execute(ns...); })(); })(0);
```
...等什么？这与 [&] 有何不同？

将上述发现应用于原始代码片段 fixes the segfaults。

这是 g++ 错误吗？还是我的代码中有任何未定义的行为？

Answer 1

这与什么无关with temporaries：这是一个 gcc7.0 优化器错误。这是一个更简单的复制器：

#include <utility>

struct root
{
  template <typename TNode, typename... TNodes>
  void start(TNode n, TNodes... ns)
  {
    n->execute(ns...);
  }
};

template <typename TParent>
struct node_then
{
  TParent *_p;

  node_then(TParent *p) : _p{ p }
  {
  }

  auto execute()
  {
  }

  template <typename TNode, typename... TNodes>
  auto execute(TNode n, TNodes... ns)
  {
    n->execute(ns...);
  }

  template <typename... TNodes>
  auto start(TNodes... ns)
  {
    _p->start(this, ns...);
  }
};

template <typename TParent>
struct node_wait_all
{
  TParent *_p;

  node_wait_all(TParent *p) : _p{ p }
  {
  }

  template <typename TNode, typename... TNodes>
  auto execute(TNode n, TNodes... ns)
  {
    ([&] { ([&] { n->execute(ns...); })(); })();
  }

  template <typename... TNodes>
  auto start(TNodes... ns)
  {
    _p->start(this, ns...);
  }
};


int main()
{
  //node_wait_all<root> obj(new root());
  //node_then<node_wait_all<root>> obj2(new node_wait_all<root>(new root()));
  node_then<node_then<node_wait_all<root>>> obj3(new node_then<node_wait_all<root>>(new node_wait_all<root>(new root())));
  obj3.start();
}

输出：

prog.cc: In function 'int main()':
prog.cc:67:1: internal compiler error: in visit_ref_for_mod_analysis, at ipa-prop.c:2308
 }
 ^
0x96c4d6 visit_ref_for_mod_analysis
    /home/heads/gcc/gcc-source/gcc/ipa-prop.c:2308
0x8f615d walk_stmt_load_store_addr_ops(gimple*, void*, bool (*)(gimple*, tree_node*, tree_node*, void*), bool (*)(gimple*, tree_node*, tree_node*, void*), bool (*)(gimple*, tree_node*, tree_node*, void*))
    /home/heads/gcc/gcc-source/gcc/gimple-walk.c:817
0x9761a2 ipa_analyze_params_uses_in_bb
    /home/heads/gcc/gcc-source/gcc/ipa-prop.c:2335
0x9761a2 analysis_dom_walker::before_dom_children(basic_block_def*)
    /home/heads/gcc/gcc-source/gcc/ipa-prop.c:2415
0x10c8472 dom_walker::walk(basic_block_def*)
    /home/heads/gcc/gcc-source/gcc/domwalk.c:265
0x977ceb ipa_analyze_node(cgraph_node*)
    /home/heads/gcc/gcc-source/gcc/ipa-prop.c:2486
0x1108f0a ipcp_generate_summary
    /home/heads/gcc/gcc-source/gcc/ipa-cp.c:5036
0xa4759c execute_ipa_summary_passes(ipa_opt_pass_d*)
    /home/heads/gcc/gcc-source/gcc/passes.c:2167
0x7d6b45 ipa_passes
    /home/heads/gcc/gcc-source/gcc/cgraphunit.c:2311
0x7d6b45 symbol_table::compile()
    /home/heads/gcc/gcc-source/gcc/cgraphunit.c:2425
0x7d8616 symbol_table::compile()
    /home/heads/gcc/gcc-source/gcc/cgraphunit.c:2587
0x7d8616 symbol_table::finalize_compilation_unit()
    /home/heads/gcc/gcc-source/gcc/cgraphunit.c:2584
Please submit a full bug report,
with preprocessed source if appropriate.
Please include the complete backtrace with any bug report.
See <http://gcc.gnu.org/bugs.html> for instructions.

Link: http://melpon.org/wandbox/permlink/E11fOumFJda6OW6m

为了帮助理解这段代码，我使用了一个强大的调试工具：paint.exe

可变参数模板函数调用上的 lambda 包装器引起的 gcc 分段错误

Segmentation fault on gcc caused by lambda wrapper over variadic template function call

c++

segmentation-fault

undefined-behavior

c++14

ubsan