[gcc r13-6332] Avoid default-initializing auto_vec<T, N> storage, fix vec<vl_embed>

[gcc r13-6332] Avoid default-initializing auto_vec<T, N> storage, fix vec<vl_embed>

[Richard Biener]

https://gcc.gnu.org/g:2dd68cddbc745de2c4a4b4982673122716c8231c

commit r13-6332-g2dd68cddbc745de2c4a4b4982673122716c8231c
Author: Richard Biener <rguenther@suse.de>
Date:   Thu Feb 23 11:56:17 2023 +0100

    Avoid default-initializing auto_vec<T, N> storage, fix vec<vl_embed>
    
    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.
    
            * 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>::m_data): Turn storage into
            uninitialized unsigned char.
            (auto_vec<T, N>::auto_vec): Allow allocation of one
            stack member.  Initialize m_vec in a special way to
            avoid later stringop overflow diagnostics.
            * vec.cc (test_auto_alias): New.
            (vec_cc_tests): Call it.

Diff:
---
 gcc/vec.cc | 17 +++++++++++++++++
 gcc/vec.h  | 31 ++++++++++++++++++++-----------
 2 files changed, 37 insertions(+), 11 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 2b36f065234..36918915701 100644
--- a/gcc/vec.h
+++ b/gcc/vec.h
@@ -586,8 +586,9 @@ 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 (); }
@@ -629,10 +630,10 @@ public:
   friend struct va_gc_atomic;
   friend struct va_heap;
 
-  /* FIXME - These fields should be private, but we need to cater to
+  /* FIXME - This field 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];
+  /* Align m_vecpfx to simplify address ().  */
+  alignas (T) alignas (vec_prefix) vec_prefix m_vecpfx;
 };
 
 
@@ -1315,7 +1316,7 @@ 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);
 }
 
 
@@ -1559,8 +1560,14 @@ class auto_vec : public vec<T, va_heap>
 public:
   auto_vec ()
   {
-    m_auto.embedded_init (MAX (N, 2), 0, 1);
-    this->m_vec = &m_auto;
+    m_auto.embedded_init (N, 0, 1);
+    /* ???  Instead of initializing m_vec from &m_auto directly use an
+       expression that avoids refering to a specific member of 'this'
+       to derail the -Wstringop-overflow diagnostic code, avoiding
+       the impression that data accesses are supposed to be to the
+       m_auto member storage.  */
+    size_t off = (char *) &m_auto - (char *) this;
+    this->m_vec = (vec<T, va_heap, vl_embed> *) ((char *) this + off);
   }
 
   auto_vec (size_t s CXX_MEM_STAT_INFO)
@@ -1571,8 +1578,10 @@ public:
 	return;
       }
 
-    m_auto.embedded_init (MAX (N, 2), 0, 1);
-    this->m_vec = &m_auto;
+    m_auto.embedded_init (N, 0, 1);
+    /* ???  See above.  */
+    size_t off = (char *) &m_auto - (char *) this;
+    this->m_vec = (vec<T, va_heap, vl_embed> *) ((char *) this + off);
   }
 
   ~auto_vec ()
@@ -1590,7 +1599,7 @@ public:
 
 private:
   vec<T, va_heap, vl_embed> m_auto;
-  T m_data[MAX (N - 1, 1)];
+  unsigned char m_data[sizeof (T) * N];
 };
 
 /* auto_vec is a sub class of vec whose storage is released when it is