[gcc r12-4062] libstdc++: Do not allocate a zero-size vector<bool> [PR 100153]

[Jonathan Wakely]

https://gcc.gnu.org/g:681707ec28d56494fa61a80c62500724d55f8586

commit r12-4062-g681707ec28d56494fa61a80c62500724d55f8586
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Apr 20 16:16:13 2021 +0100

    libstdc++: Do not allocate a zero-size vector<bool> [PR 100153]
    
    The vector<bool>::shrink_to_fit() implementation will allocate new
    storage even if the vector is empty. That then leads to the
    end-of-storage pointer being non-null and equal to the _M_start._M_p
    pointer, which means that _M_end_addr() has undefined behaviour.
    
    The fix is to stop doing a useless zero-sized allocation in
    shrink_to_fit(), so that _M_start._M_p and _M_end_of_storage are both
    null after an empty vector shrinks.
    
    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/100153
            * include/bits/vector.tcc (vector<bool>::_M_shrink_to_fit()):
            When size() is zero just deallocate and reset.

Diff:
---
 libstdc++-v3/include/bits/vector.tcc | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/vector.tcc b/libstdc++-v3/include/bits/vector.tcc
index caee5cbfc2f..16366e03c86 100644
--- a/libstdc++-v3/include/bits/vector.tcc
+++ b/libstdc++-v3/include/bits/vector.tcc
@@ -944,7 +944,13 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 	return false;
       __try
 	{
-	  _M_reallocate(size());
+	  if (size_type __n = size())
+	    _M_reallocate(__n);
+	  else
+	    {
+	      this->_M_deallocate();
+	      this->_M_impl._M_reset();
+	    }
 	  return true;
 	}
       __catch(...)