libstdc++: allocate_at_least ask only what it reports (P0401)

allocate_at_least is rounding up the allocation request size to
its default alignment, which may be more than an integral
multiple of the object size requested. When the memory is freed,
what the container reports it is freeing differs from the amount
that was allocated. This patch rounds the request size back down
to what will be reported to the caller.

The algorithm to compute the allocation is altered in response
to findings on godbolt.org, which indicate dropping to uint8 to
perform the division is a pessimization everywhere other than
x86. The new version emits code for multiplication, instead.

In addition, the remaining -m32 test that failed under the new
allocation method is fixed, and guards are added for building
with -fno-aligned-new and for -fno-sized-deallocation.

Tested on x86 -m64/-m32.

libstdc++-v3/ChangeLog:
	* include/bits/new_allocator.h (allocate_at_least): Reduce
	allocation to match what is reported.
	* testsuite/20_util/allocator/allocate_at_least2.cc: Add tests.
	* testsuite/23_containers/vector/modifiers/insert_vs_emplace.cc:
	Fix, for -m32 and new allocation results.

diff --git a/libstdc++-v3/include/bits/new_allocator.h b/libstdc++-v3/include/bits/new_allocator.h
index 4524355a4a0f6634a0e8b11c0248b0bead8ed501..e7666b5831c1de2196beafc5874dba619af28084 100644 (file)
--- a/libstdc++-v3/include/bits/new_allocator.h
+++ b/libstdc++-v3/include/bits/new_allocator.h
@@ -177,6 +177,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       [[nodiscard]] constexpr std::allocation_result<_Tp*, size_t>
       allocate_at_least(size_t __n)
       {
+#if __cpp_aligned_new
        if ! consteval
          {
            if constexpr (requires { sizeof(_Tp); })
@@ -184,19 +185,28 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
            if constexpr ( sizeof(_Tp) <  __STDCPP_DEFAULT_NEW_ALIGNMENT__)
              {
                _S_check_allocation_limit(__n);
-               const size_t __need = __n * sizeof(_Tp);
+               size_t __bytes = __n * sizeof(_Tp);
                const size_t __mask = __STDCPP_DEFAULT_NEW_ALIGNMENT__ - 1;
-               size_t __ask = (__need + __mask) & ~__mask;
-               // Avoid rounding up to and asking for 2^63 bytes (PR108377):
-               __ask -= __ask >> (__SIZE_WIDTH__ - 1);
-               auto* __p = static_cast<_Tp*>(_GLIBCXX_OPERATOR_NEW(__ask));
-               using _U8 = const unsigned char;
-               static_assert(sizeof(_Tp) <= ~_U8());
-               // Use 8-bit division for minimal latency:
-               _U8 __spare = __ask - __need, __size = sizeof(_Tp);
-               return { __p , __n + __spare / __size };
+               size_t __max = (__bytes + __mask) & ~__mask;
+               // Avoid seeming to ask for 2^63 bytes (PR108377):
+               __max -= __max >> (__SIZE_WIDTH__ - 1);
+               auto __spare = static_cast<unsigned>(__max - __bytes);
+               if constexpr (sizeof(_Tp) < (__mask + 1) / 2)
+                 {
+                   auto __bonus = __spare / sizeof(_Tp);
+                   __n += __bonus;
+                   __bytes += __bonus * sizeof(_Tp);
+                 }
+               else if (sizeof(_Tp) <= __spare)
+                 {
+                   __n += 1;
+                   __bytes += sizeof(_Tp);
+                 }
+               void* __p = _GLIBCXX_OPERATOR_NEW(__bytes);
+               return { static_cast<_Tp*>(__p), __n };
              }
          }
+#endif
        return { allocate(__n), __n };
       }
 #endif

diff --git a/libstdc++-v3/testsuite/20_util/allocator/allocate_at_least2.cc b/libstdc++-v3/testsuite/20_util/allocator/allocate_at_least2.cc
new file mode 100644 (file)
index 0000000..4f8645e
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/allocator/allocate_at_least2.cc
@@ -0,0 +1,66 @@
+// { dg-do run { target c++23 } }
+
+#include <memory>
+#include <stdlib.h>
+#include <testsuite_hooks.h>
+
+std::size_t gn = 0;
+void* operator new(std::size_t n)
+{
+  gn = n;
+  return ::malloc(n);
+}
+
+// Failing to define ops delete, too, would generate warnings.
+
+void operator delete(void* p) noexcept
+{ ::free(p); }
+
+#if __cpp_sized_deallocation
+void operator delete(void* p, std::size_t) noexcept
+{ ::free(p); }
+#endif
+
+template <unsigned size, unsigned n>
+void test_nm()
+{
+  struct A { char a[size]; };
+  std::allocator<A> alloc;
+  using alloc_traits = std::allocator_traits<std::allocator<A>>;
+  auto [p, m] = alloc_traits::allocate_at_least(alloc, n);
+
+#if __cpp_aligned_new
+  unsigned mod = __STDCPP_DEFAULT_NEW_ALIGNMENT__;
+  unsigned max = ((n * size) + mod - 1) & ~(mod - 1);
+  unsigned count = max / sizeof(A);
+#else
+  unsigned count = n;
+#endif
+  VERIFY(m == count);
+  VERIFY(gn == count * sizeof(A));
+  VERIFY(p != nullptr);  // named it, use it.
+}
+
+void test()
+{                  //  m  gn
+  test_nm<1,3>();  // 16  16
+  test_nm<2,3>();  //  8  16
+  test_nm<3,3>();  //  5  15
+  test_nm<4,3>();  //  4  16
+  test_nm<5,3>();  //  3  15
+  test_nm<6,3>();  //  5  30
+  test_nm<7,3>();  //  4  28
+  test_nm<8,3>();  //  4  32
+  test_nm<9,3>();  //  3  27
+  test_nm<10,3>(); //  3  30
+  test_nm<11,3>(); //  4  44
+  test_nm<12,3>(); //  4  48
+  test_nm<13,3>(); //  3  39
+  test_nm<14,3>(); //  3  42
+  test_nm<15,3>(); //  3  45
+}
+
+int main()
+{
+  test();
+}

diff --git a/libstdc++-v3/testsuite/23_containers/vector/modifiers/insert_vs_emplace.cc b/libstdc++-v3/testsuite/23_containers/vector/modifiers/insert_vs_emplace.cc
index 187e433d9d37ec0712504e58536570dfd8b1be7e..d226490d6f3bcc3fe89c57e4a011ea06a86e3e96 100644 (file)
--- a/libstdc++-v3/testsuite/23_containers/vector/modifiers/insert_vs_emplace.cc
+++ b/libstdc++-v3/testsuite/23_containers/vector/modifiers/insert_vs_emplace.cc
@@ -223,11 +223,12 @@ test03()
 void
 test04()
 {
-  const X::special expected{ 0, 3, 1, 0, 3, 0 };
+  const X::special expected{ 0, 4, 1, 0, 4, 0 };
   X::special ins, emp;
   {
     std::vector<X> v;
-    v.reserve(3);
+    v.reserve(4);
+    v.push_back(X(0,0));
     v.push_back(X(0,0));
     v.push_back(X(0,0));
     v.push_back(X(0,0));
@@ -241,7 +242,8 @@ test04()
   }
   {
     std::vector<X> v;
-    v.reserve(3);
+    v.reserve(4);
+    v.push_back(X(0,0));
     v.push_back(X(0,0));
     v.push_back(X(0,0));
     v.push_back(X(0,0));
@@ -261,11 +263,12 @@ test04()
 void
 test05()
 {
-  const X::special expected{ 0, 3, 0, 0, 4, 0 };
+  const X::special expected{ 0, 4, 0, 0, 5, 0 };
   X::special ins, emp;
   {
     std::vector<X> v;
-    v.reserve(3);
+    v.reserve(4);
+    v.push_back(X(0,0));
     v.push_back(X(0,0));
     v.push_back(X(0,0));
     v.push_back(X(0,0));
@@ -279,7 +282,8 @@ test05()
   }
   {
     std::vector<X> v;
-    v.reserve(3);
+    v.reserve(4);
+    v.push_back(X(0,0));
     v.push_back(X(0,0));
     v.push_back(X(0,0));
     v.push_back(X(0,0));
@@ -299,11 +303,12 @@ test05()
 void
 test06()
 {
-  const X::special expected{ 1, 4, 0, 0, 4, 0 };
+  const X::special expected{ 1, 5, 0, 0, 5, 0 };
   X::special ins, emp;
   {
     std::vector<X> v;
-    v.reserve(3);
+    v.reserve(4);
+    v.push_back(X(0,0));
     v.push_back(X(0,0));
     v.push_back(X(0,0));
     v.push_back(X(0,0));
@@ -316,7 +321,8 @@ test06()
   }
   {
     std::vector<X> v;
-    v.reserve(3);
+    v.reserve(4);
+    v.push_back(X(0,0));
     v.push_back(X(0,0));
     v.push_back(X(0,0));
     v.push_back(X(0,0));
@@ -447,11 +453,12 @@ test09()
 void
 test10()
 {
-  const X::special expected{ 0, 3, 1, 0, 3, 0 };
+  const X::special expected{ 0, 4, 1, 0, 4, 0 };
   X::special ins, emp;
   {
     std::vector<X> v;
-    v.reserve(3);
+    v.reserve(4);
+    v.push_back(X(0,0));
     v.push_back(X(0,0));
     v.push_back(X(0,0));
     v.push_back(X(0,0));
@@ -465,7 +472,8 @@ test10()
   }
   {
     std::vector<X> v;
-    v.reserve(3);
+    v.reserve(4);
+    v.push_back(X(0,0));
     v.push_back(X(0,0));
     v.push_back(X(0,0));
     v.push_back(X(0,0));
@@ -485,11 +493,12 @@ test10()
 void
 test11()
 {
-  const X::special expected{ 0, 3, 0, 0, 4, 0 };
+  const X::special expected{ 0, 4, 0, 0, 5, 0 };
   X::special ins, emp;
   {
     std::vector<X> v;
-    v.reserve(3);
+    v.reserve(4);
+    v.push_back(X(0,0));
     v.push_back(X(0,0));
     v.push_back(X(0,0));
     v.push_back(X(0,0));
@@ -503,7 +512,8 @@ test11()
   }
   {
     std::vector<X> v;
-    v.reserve(3);
+    v.reserve(4);
+    v.push_back(X(0,0));
     v.push_back(X(0,0));
     v.push_back(X(0,0));
     v.push_back(X(0,0));
@@ -523,11 +533,12 @@ test11()
 void
 test12()
 {
-  const X::special expected{ 1, 4, 0, 0, 4, 0 };
+  const X::special expected{ 1, 5, 0, 0, 5, 0 };
   X::special ins, emp;
   {
     std::vector<X> v;
-    v.reserve(3);
+    v.reserve(4);
+    v.push_back(X(0,0));
     v.push_back(X(0,0));
     v.push_back(X(0,0));
     v.push_back(X(0,0));
@@ -540,7 +551,8 @@ test12()
   }
   {
     std::vector<X> v;
-    v.reserve(3);
+    v.reserve(4);
+    v.push_back(X(0,0));
     v.push_back(X(0,0));
     v.push_back(X(0,0));
     v.push_back(X(0,0));