[PATCH 2/2] Avoid default-initializing auto_vec storage, fix vec

[Richard Biener via Gcc-patches]

The following avoids default-initializing auto_vec storage for
non-POD T since that's not what the allocated storage fallback
will do and it's also not expected for existing cases like

  auto_vec<std::pair<unsigned, unsigned>, 64> elts;

which exist to optimize the allocation.

It also fixes the array accesses done by vec<vl_embed> to not
use its own m_vecdata member but instead access the container
provided storage via pointer arithmetic.

This seems to work but it also somehow breaks genrecog which now
goes OOM with this change.  I'm going to see if the testsuite
shows anything, but maybe it's obvious from a second eye what
I did wrong ...

Comments welcome of course.

Thanks,
Richard.

        * vec.h (vec<T, A, vl_embed>::m_vecdata): Remove.
        (vec<T, A, vl_embed>::m_vecpfx): Align as T to avoid
        changing alignment of vec<T, A, vl_embed> and simplifying
        address.
        (vec<T, A, vl_embed>::address): Compute as this + 1.
        (vec<T, A, vl_embed>::embedded_size): Use sizeof the
        vector instead of the offset of the m_vecdata member.
        (auto_vec<T, N>): Turn m_data storage into
        uninitialized unsigned char aligned as T.
        * vec.cc (test_auto_alias): New.
        (vec_cc_tests): Call it.
---
 gcc/vec.cc | 17 +++++++++++++++++
 gcc/vec.h  | 11 +++++------
 2 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/gcc/vec.cc b/gcc/vec.cc
index 511e6dff50d..2128f6666b1 100644
--- a/gcc/vec.cc
+++ b/gcc/vec.cc
@@ -568,6 +568,22 @@ test_auto_delete_vec ()
   ASSERT_EQ (dtor_count, 2);
 }
 
+/* Verify accesses to m_vecdata are done indirectly.  */
+
+static void
+test_auto_alias ()
+{
+  volatile int i = 1;
+  auto_vec<int, 8> v;
+  v.quick_grow (2);
+  v[0] = 1;
+  v[1] = 2;
+  int val;
+  for (int ix = i; v.iterate (ix, &val); ix++)
+    ASSERT_EQ (val, 2);
+  ASSERT_EQ (val, 0);
+}
+
 /* Run all of the selftests within this file.  */
 
 void
@@ -587,6 +603,7 @@ vec_cc_tests ()
   test_qsort ();
   test_reverse ();
   test_auto_delete_vec ();
+  test_auto_alias ();
 }
 
 } // namespace selftest
diff --git a/gcc/vec.h b/gcc/vec.h
index 5a2ee9c0294..b680efebe7a 100644
--- a/gcc/vec.h
+++ b/gcc/vec.h
@@ -586,8 +586,8 @@ public:
   unsigned allocated (void) const { return m_vecpfx.m_alloc; }
   unsigned length (void) const { return m_vecpfx.m_num; }
   bool is_empty (void) const { return m_vecpfx.m_num == 0; }
-  T *address (void) { return m_vecdata; }
-  const T *address (void) const { return m_vecdata; }
+  T *address (void) { return reinterpret_cast <T *> (this + 1); }
+  const T *address (void) const { return reinterpret_cast <const T *> (this + 
1); }
   T *begin () { return address (); }
   const T *begin () const { return address (); }
   T *end () { return address () + length (); }
@@ -631,8 +631,7 @@ public:
 
   /* FIXME - These fields should be private, but we need to cater to
             compilers that have stricter notions of PODness for types.  */
-  vec_prefix m_vecpfx;
-  T m_vecdata[1];
+  alignas (T) vec_prefix m_vecpfx;
 };
 
 
@@ -1313,7 +1312,7 @@ vec<T, A, vl_embed>::embedded_size (unsigned alloc)
                                    vec, vec_embedded>::type vec_stdlayout;
   static_assert (sizeof (vec_stdlayout) == sizeof (vec), "");
   static_assert (alignof (vec_stdlayout) == alignof (vec), "");
-  return offsetof (vec_stdlayout, m_vecdata) + alloc * sizeof (T);
+  return sizeof (vec_stdlayout) + alloc * sizeof (T);
 }
 
 
@@ -1588,7 +1587,7 @@ public:
 
 private:
   vec<T, va_heap, vl_embed> m_auto;
-  T m_data[MAX (N - 1, 1)];
+  alignas(T) unsigned char m_data[sizeof (T) * N];
 };
 
 /* auto_vec is a sub class of vec whose storage is released when it is
-- 
2.35.3