libstdc++: handle small max_blocks_per_chunk in pool resources [PR 94160]

When a pool resource is constructed with max_blocks_per_chunk=1 it ends
up creating a pool with blocks_per_chunk=0 which means it never
allocates anything. Instead it returns null pointers, which should be
impossible.

To avoid this problem, round the max_blocks_per_chunk value to a
multiple of four, so it's never smaller than four.

libstdc++-v3/ChangeLog:

	PR libstdc++/94160
	* src/c++17/memory_resource.cc (munge_options): Round
	max_blocks_per_chunk to a multiple of four.
	(__pool_resource::_M_alloc_pools()): Simplify slightly.
	* testsuite/20_util/unsynchronized_pool_resource/allocate.cc:
	Check that valid pointers are returned when small values are
	used for max_blocks_per_chunk.

(cherry picked from commit 30b41cfbb2dade63e52465234a725d1d02fe70aa)

diff --git a/libstdc++-v3/src/c++17/memory_resource.cc b/libstdc++-v3/src/c++17/memory_resource.cc
index 95352b2353780da158b3715989526bc100ae1614..75ae24e0a06217434a7ddf83290510db9d61115a 100644 (file)
--- a/libstdc++-v3/src/c++17/memory_resource.cc
+++ b/libstdc++-v3/src/c++17/memory_resource.cc
@@ -873,7 +873,18 @@ namespace pmr
       }
     else
       {
-       // TODO round to preferred granularity ?
+       // Round to preferred granularity.
+       if (opts.max_blocks_per_chunk < size_t(-4))
+         {
+           // round up
+           opts.max_blocks_per_chunk += 3;
+           opts.max_blocks_per_chunk &= ~size_t(3);
+         }
+       else
+         {
+           // round down
+           opts.max_blocks_per_chunk &= ~size_t(3);
+         }
       }
 
     if (opts.max_blocks_per_chunk > chunk::max_blocks_per_chunk())
@@ -1013,11 +1024,9 @@ namespace pmr
          : pool_sizes[i];
 
        // Decide on initial number of blocks per chunk.
-       // Always have at least 16 blocks per chunk:
-       const size_t min_blocks_per_chunk = 16;
-       // But for smaller blocks, use a larger initial size:
-       size_t blocks_per_chunk
-         = std::max(1024 / block_size, min_blocks_per_chunk);
+       // At least 16 blocks per chunk seems reasonable,
+       // more for smaller blocks:
+       size_t blocks_per_chunk = std::max(size_t(16), 1024 / block_size);
        // But don't exceed the requested max_blocks_per_chunk:
        blocks_per_chunk
          = std::min(blocks_per_chunk, _M_opts.max_blocks_per_chunk);

diff --git a/libstdc++-v3/testsuite/20_util/unsynchronized_pool_resource/allocate.cc b/libstdc++-v3/testsuite/20_util/unsynchronized_pool_resource/allocate.cc
index 5bf20cf262c04aa9cd83e4a09cff6fc4810c3bcb..ef5f921211de7404f1144ff0acb5d09de1efc145 100644 (file)
--- a/libstdc++-v3/testsuite/20_util/unsynchronized_pool_resource/allocate.cc
+++ b/libstdc++-v3/testsuite/20_util/unsynchronized_pool_resource/allocate.cc
@@ -239,6 +239,25 @@ test06()
   }
 }
 
+void
+test08()
+{
+  std::pmr::pool_options opts;
+  opts.largest_required_pool_block = 64;
+
+  // PR libstdc++/94160
+  // max_blocks_per_chunk=1 causes pool resources to return null pointers
+  for (int i = 0; i < 8; ++i)
+  {
+    opts.max_blocks_per_chunk = i;
+    std::pmr::unsynchronized_pool_resource upr(opts);
+    auto* p = (int*)upr.allocate(4);
+    VERIFY( p != nullptr );
+    *p = i;
+    upr.deallocate(p, 4);
+  }
+}
+
 int
 main()
 {
@@ -248,4 +267,5 @@ main()
   test04();
   test05();
   test06();
+  test08();
 }