[Bug c++/82952] Hang compiling with g++ -fsanitize=undefined -Wduplicated-branches

[reichelt at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82952

Volker Reichelt <reichelt at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |reichelt at gcc dot gnu.org

--- Comment #6 from Volker Reichelt <reichelt at gcc dot gnu.org> ---
Here's a short example that shows that something exponential is going on:

============================================
struct A
{
  virtual ~A() {}
  A& operator<<(int) { return *this; }
};

void foo(A& a, bool b)
{
  if (b)
    a <<  1 <<  2 <<  3 <<  4 <<  5
      <<  6 <<  7 <<  8 <<  9 << 10
      << 11 << 12 << 13 << 14 << 15;
}
============================================

Increasing the number of calls to the operator<< increases the compile
time roughly by a factor of 4 as the following table shows:

calls | compile time
------+-------------
  10  |     0.25s
  11  |     0.95s
  12  |     3.7 s
  13  |    14.8 s
  14  |    59   s
  15  |   234   s

For 20 calls the estimated compile time would be almost 3 days.
With any of the two options removed, the compile time for 20 calls
is way below 0.1s.

This happens since GCC 7.1.0 which introduced the "-Wduplicated-branches"
option.