Re: [PATCH] define auto_vec copy ctor and assignment (PR 90904)

[Martin Sebor <msebor@gmail.com>]

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On 4/27/21 8:04 AM, Richard Biener wrote:
> On Tue, Apr 27, 2021 at 3:59 PM Martin Sebor <msebor@gmail.com> wrote:
>>
>> On 4/27/21 1:58 AM, Richard Biener wrote:
>>> On Tue, Apr 27, 2021 at 2:46 AM Martin Sebor via Gcc-patches
>>> <gcc-patches@gcc.gnu.org> wrote:
>>>>
>>>> PR 90904 notes that auto_vec is unsafe to copy and assign because
>>>> the class manages its own memory but doesn't define (or delete)
>>>> either special function.  Since I first ran into the problem,
>>>> auto_vec has grown a move ctor and move assignment from
>>>> a dynamically-allocated vec but still no copy ctor or copy
>>>> assignment operator.
>>>>
>>>> The attached patch adds the two special functions to auto_vec along
>>>> with a few simple tests.  It makes auto_vec safe to use in containers
>>>> that expect copyable and assignable element types and passes bootstrap
>>>> and regression testing on x86_64-linux.
>>>
>>> The question is whether we want such uses to appear since those
>>> can be quite inefficient?  Thus the option is to delete those operators?
>>
>> I would strongly prefer the generic vector class to have the properties
>> expected of any other generic container: copyable and assignable.  If
>> we also want another vector type with this restriction I suggest to add
>> another "noncopyable" type and make that property explicit in its name.
>> I can submit one in a followup patch if you think we need one.
> 
> I'm not sure (and not strictly against the copy and assign).  Looking around
> I see that vec<> does not do deep copying.  Making auto_vec<> do it
> might be surprising (I added the move capability to match how vec<>
> is used - as "reference" to a vector)

The vec base classes are special: they have no ctors at all (because
of their use in unions).  That's something we might have to live with
but it's not a model to follow in ordinary containers.

The auto_vec class was introduced to fill the need for a conventional
sequence container with a ctor and dtor.  The missing copy ctor and
assignment operators were an oversight, not a deliberate feature.
This change fixes that oversight.

The revised patch also adds a copy ctor/assignment to the auto_vec
primary template (that's also missing it).  In addition, it adds
a new class called auto_vec_ncopy that disables copying and
assignment as you prefer.  It also disables copying for
the auto_string_vec class.

Martin

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PR middle-end/90904 - vec assignment and copying undefined

gcc/ChangeLog:

	PR middle-end/90904
	* vec.c (test_copy_assign): New function.
	(vec_c_tests): Call it.
	* vec.h (vec_assign): New function.
	(auto_vec copy ctor): Define.
	(auto_vec::operator=): Define.
	(auto_vec_no_copy): New class template.
	(auto_string_vec): Disable copying/assignment.

diff --git a/gcc/vec.c b/gcc/vec.c
index f9dbb2cac31..f15530d1e43 100644
--- a/gcc/vec.c
+++ b/gcc/vec.c
@@ -317,6 +317,86 @@ test_safe_push ()
   ASSERT_EQ (7, v[2]);
 }
 
+
+/* Verify that auto_vec copy ctor and assignment work correctly.  */
+
+template <size_t N>
+void test_copy_assign ()
+{
+  typedef auto_vec <int, N> test_vec;
+
+  test_vec a0;
+  test_vec b0 (0);
+  test_vec c0 (7);
+  ASSERT_EQ (0, b0.length ());
+  ASSERT_EQ (0, c0.length ());
+
+  a0 = a0;
+  ASSERT_EQ (0, a0.length ());
+  b0 = a0;
+  ASSERT_EQ (0, b0.length ());
+  c0 = a0;
+  ASSERT_EQ (0, c0.length ());
+
+  test_vec a3;
+  a3.safe_push (5);
+  a3.safe_push (6);
+  a3.safe_push (7);
+
+  test_vec b3 (a3);
+  ASSERT_EQ (3, b3.length ());
+  ASSERT_EQ (5, b3[0]);
+  ASSERT_EQ (6, b3[1]);
+  ASSERT_EQ (7, b3[2]);
+
+  test_vec c3;
+  c3 = b3;
+  ASSERT_EQ (3, c3.length ());
+  ASSERT_EQ (5, c3[0]);
+  ASSERT_EQ (6, c3[1]);
+  ASSERT_EQ (7, c3[2]);
+
+  test_vec d4;
+  d4.safe_push (1);
+  d4.safe_push (2);
+  d4.safe_push (3);
+  d4.safe_push (4);
+
+  c3 = d4;
+  ASSERT_EQ (4, c3.length ());
+  ASSERT_EQ (1, c3[0]);
+  ASSERT_EQ (2, c3[1]);
+  ASSERT_EQ (3, c3[2]);
+  ASSERT_EQ (4, c3[3]);
+
+  d4 = a3;
+  ASSERT_EQ (3, d4.length ());
+  ASSERT_EQ (5, d4[0]);
+  ASSERT_EQ (6, d4[1]);
+  ASSERT_EQ (7, d4[2]);
+
+  a3 = b0;
+  ASSERT_EQ (0, a3.length ());
+
+  b3 = b0;
+  ASSERT_EQ (0, b3.length ());
+
+  c3 = c0;
+  ASSERT_EQ (0, c3.length ());
+
+  b0 = d4;
+  ASSERT_EQ (3, b0.length ());
+  ASSERT_EQ (5, b0[0]);
+  ASSERT_EQ (6, b0[1]);
+  ASSERT_EQ (7, b0[2]);
+
+  c0 = d4;
+  ASSERT_EQ (3, c0.length ());
+  ASSERT_EQ (5, c0[0]);
+  ASSERT_EQ (6, c0[1]);
+  ASSERT_EQ (7, c0[2]);
+}
+
 /* Verify that vec::truncate works correctly.  */
 
 static void
@@ -549,6 +629,8 @@ vec_c_tests ()
 {
   test_quick_push ();
   test_safe_push ();
+  test_copy_assign<0> ();
+  test_copy_assign<2> ();
   test_truncate ();
   test_safe_grow_cleared ();
   test_pop ();
diff --git a/gcc/vec.h b/gcc/vec.h
index 24df2db0eeb..f5dd5bb329a 100644
--- a/gcc/vec.h
+++ b/gcc/vec.h
@@ -1509,7 +1509,41 @@ public:
    want to ask for internal storage for vectors on the stack because if the
    size of the vector is larger than the internal storage that space is wasted.
    */
+
 template<typename T, size_t N = 0>
+class auto_vec;
+
+/* Safely assign elements from SRC to DST, invoking the copy assignment
+   operator on the initial elements and the copy ctor on the excess.  */
+
+template<typename T, size_t N>
+auto_vec<T, N>& vec_assign (auto_vec<T, N> &dst, const auto_vec<T, N> &src)
+{
+  unsigned n0 = dst.length ();
+  const unsigned n1 = src.length ();
+  if (n0 < n1)
+    dst.safe_grow (n1);
+  else
+    {
+      dst.truncate (n1);
+      n0 = n1;
+    }
+
+  T *pdst = dst.address ();
+  const T *psrc = src.address ();
+
+  /* Copy-assign over the first N0 elements.  */
+  for (unsigned i = 0; i != n0; ++i)
+    pdst[i] = psrc[i];
+
+  if (n0 < n1)
+    /* Copy-construct elements in excess of N0.  */
+    vec_copy_construct (pdst + n0, psrc + n0, n1 - n0);
+
+  return dst;
+}
+
+template<typename T, size_t N /* = 0 */>
 class auto_vec : public vec<T, va_heap>
 {
 public:
@@ -1536,11 +1570,34 @@ public:
     this->release ();
   }
 
+  /* Copy ctor.  */
+  auto_vec (const auto_vec &);
+
+  /* Copy asssignment.  */
+  auto_vec& operator= (const auto_vec &r)
+  {
+    return vec_assign (*this, r);
+  }
+
 private:
   vec<T, va_heap, vl_embed> m_auto;
   T m_data[MAX (N - 1, 1)];
 };
 
+/* Copy elements from R to *THIS.  */
+
+template<typename T, size_t N>
+auto_vec<T, N>::auto_vec (const auto_vec& r)
+{
+  const unsigned n = r.length ();
+  this->create (n);
+  if (n)
+    {
+      vec_copy_construct (this->address (), r.address (), n);
+      this->m_vec->m_vecpfx.m_num = n;
+    }
+}
+
 /* auto_vec is a sub class of vec whose storage is released when it is
   destroyed. */
 template<typename T>
@@ -1551,12 +1608,24 @@ public:
   auto_vec (size_t n CXX_MEM_STAT_INFO) { this->create (n PASS_MEM_STAT); }
   ~auto_vec () { this->release (); }
 
+  /* Copy ctor.  */
+  auto_vec (const auto_vec&);
+
+  /* Move ctor from vec that's not auto_vec.  */
   auto_vec (vec<T, va_heap>&& r)
     {
       gcc_assert (!r.using_auto_storage ());
       this->m_vec = r.m_vec;
       r.m_vec = NULL;
     }
+
+  /* Copy asssignment.  */
+  auto_vec& operator= (const auto_vec &rhs)
+  {
+    return vec_assign (*this, rhs);
+  }
+
+  /* Move asssignment from vec that's not auto_vec.  */
   auto_vec& operator= (vec<T, va_heap>&& r)
     {
       gcc_assert (!r.using_auto_storage ());
@@ -1567,6 +1636,35 @@ public:
     }
 };
 
+/* Copy elements from R to *THIS.  */
+
+template<typename T>
+auto_vec<T, 0>::auto_vec (const auto_vec& r)
+{
+  const unsigned n = r.length ();
+  this->create (n);
+  if (n)
+    {
+      vec_copy_construct (this->address (), r.address (), n);
+      this->m_vec->m_vecpfx.m_num = n;
+    }
+}
+
+/* Same as auto_vec<T, 0> but with deleted copy ctor and assignment
+   operator to detect uses of these special functions in contexts
+   where they may be inefficient.  */
+
+template<typename T>
+class auto_vec_no_copy: public auto_vec<T, 0>
+{
+public:
+  /* Inherit all ctors.  */
+  using auto_vec<T, 0>::auto_vec;
+
+private:
+  /* Prevent copying and assignment.  */
+  DISABLE_COPY_AND_ASSIGN (auto_vec_no_copy);
+};
 
 /* Allocate heap memory for pointer V and create the internal vector
    with space for NELEMS elements.  If NELEMS is 0, the internal
@@ -1587,7 +1685,11 @@ vec_alloc (vec<T> *&v, unsigned nelems CXX_MEM_STAT_INFO)
 class auto_string_vec : public auto_vec <char *>
 {
  public:
+  auto_string_vec (): auto_vec () { }
   ~auto_string_vec ();
+
+private:
+  DISABLE_COPY_AND_ASSIGN (auto_string_vec);
 };
 
 /* A subclass of auto_vec <T *> that deletes all of its elements on