[libcxx-commits] [libcxx] 165ad89 - [libc++] [LIBCXX-DEBUG-FIXME] Our `__debug_less` breaks some complexity guarantees.

[Arthur O'Dwyer via libcxx-commits]

Author: Arthur O'Dwyer
Date: 2021-05-05T16:21:09-04:00
New Revision: 165ad89947e8ef6c08c80eb067d85b4fa9074904

URL: https://github.com/llvm/llvm-project/commit/165ad89947e8ef6c08c80eb067d85b4fa9074904
DIFF: https://github.com/llvm/llvm-project/commit/165ad89947e8ef6c08c80eb067d85b4fa9074904.diff

LOG: [libc++] [LIBCXX-DEBUG-FIXME] Our `__debug_less` breaks some complexity guarantees.

`__debug_less` ends up running the comparator up-to-twice per comparison,
because whenever `(x < y)` it goes on to verify that `!(y < x)`.
This breaks the strict "Complexity" guarantees of algorithms like
`inplace_merge`, which we test in the test suite. So, just skip the
complexity assertions in debug mode.

Differential Revision: https://reviews.llvm.org/D101677

Added: 
    

Modified: 
    libcxx/docs/DesignDocs/DebugMode.rst
    libcxx/test/std/algorithms/alg.sorting/alg.merge/inplace_merge_comp.pass.cpp

Removed: 
    


################################################################################
diff  --git a/libcxx/docs/DesignDocs/DebugMode.rst b/libcxx/docs/DesignDocs/DebugMode.rst
index e4d4e5b2d909a..c78c0ae166269 100644
--- a/libcxx/docs/DesignDocs/DebugMode.rst
+++ b/libcxx/docs/DesignDocs/DebugMode.rst
@@ -70,6 +70,12 @@ These checks are enabled when ``_LIBCPP_DEBUG`` is defined to either 0 or 1.
 
 The following checks are enabled by ``_LIBCPP_DEBUG``:
 
+  * Many algorithms, such as ``binary_search``, ``merge``, ``next_permutation``, and ``sort``,
+    wrap the user-provided comparator to assert that `!comp(y, x)` whenever
+    `comp(x, y)`. This can cause the user-provided comparator to be evaluated
+    up to twice as many times as it would be without ``_LIBCPP_DEBUG``, and
+    causes the library to violate some of the Standard's complexity clauses.
+
   * FIXME: Update this list
 
 Iterator Debugging Checks

diff  --git a/libcxx/test/std/algorithms/alg.sorting/alg.merge/inplace_merge_comp.pass.cpp b/libcxx/test/std/algorithms/alg.sorting/alg.merge/inplace_merge_comp.pass.cpp
index 7f69a2d7841dc..17115686b6a26 100644
--- a/libcxx/test/std/algorithms/alg.sorting/alg.merge/inplace_merge_comp.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.sorting/alg.merge/inplace_merge_comp.pass.cpp
@@ -6,8 +6,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-// XFAIL: LIBCXX-DEBUG-FIXME
-
 // <algorithm>
 
 // template<BidirectionalIterator Iter, StrictWeakOrder<auto, Iter::value_type> Compare>
@@ -81,7 +79,9 @@ test_one(unsigned N, unsigned M)
         assert(ia[0] == static_cast<int>(N)-1);
         assert(ia[N-1] == 0);
         assert(std::is_sorted(ia, ia+N, std::greater<value_type>()));
+#ifndef _LIBCPP_DEBUG
         assert(pred.count() <= (N-1));
+#endif
     }
     delete [] ia;
 }