re PR c/82210 (Having _Alignas in a struct with VLAs causes writing to one array to overwrite another)

	PR c/82210
	* stor-layout.c (place_field): For variable length fields, adjust
	offset_align afterwards not just based on the field's alignment,
	but also on the size.

	* gcc.c-torture/execute/pr82210.c: New test.

From-SVN: r257635

4 files changed, 58 insertions, 0 deletions

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index f60fb1d..c58d419 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,5 +1,10 @@
 2018-02-13  Jakub Jelinek  <jakub@redhat.com>
 
+	PR c/82210
+	* stor-layout.c (place_field): For variable length fields, adjust
+	offset_align afterwards not just based on the field's alignment,
+	but also on the size.
+
 	PR middle-end/84309
 	* match.pd (pow(C,x) -> exp(log(C)*x)): Use exp2s and log2s instead
 	of exps and logs in the use_exp2 case.
diff --git a/gcc/stor-layout.c b/gcc/stor-layout.c
index 8c415ebb..5fdf81a 100644
--- a/gcc/stor-layout.c
+++ b/gcc/stor-layout.c
@@ -1622,6 +1622,30 @@ place_field (record_layout_info rli, tree field)
 	= size_binop (PLUS_EXPR, rli->offset, DECL_SIZE_UNIT (field));
       rli->bitpos = bitsize_zero_node;
       rli->offset_align = MIN (rli->offset_align, desired_align);
+
+      if (!multiple_of_p (bitsizetype, DECL_SIZE (field),
+			  bitsize_int (rli->offset_align)))
+	{
+	  tree type = strip_array_types (TREE_TYPE (field));
+	  /* The above adjusts offset_align just based on the start of the
+	     field.  The field might not have a size that is a multiple of
+	     that offset_align though.  If the field is an array of fixed
+	     sized elements, assume there can be any multiple of those
+	     sizes.  If it is a variable length aggregate or array of
+	     variable length aggregates, assume worst that the end is
+	     just BITS_PER_UNIT aligned.  */
+	  if (TREE_CODE (TYPE_SIZE (type)) == INTEGER_CST)
+	    {
+	      if (TREE_INT_CST_LOW (TYPE_SIZE (type)))
+		{
+		  unsigned HOST_WIDE_INT sz
+		    = least_bit_hwi (TREE_INT_CST_LOW (TYPE_SIZE (type)));
+		  rli->offset_align = MIN (rli->offset_align, sz);
+		}
+	    }
+	  else
+	    rli->offset_align = MIN (rli->offset_align, BITS_PER_UNIT);
+	}
     }
   else if (targetm.ms_bitfield_layout_p (rli->t))
     {
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index cd5ab81..19c0832 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,5 +1,8 @@
 2018-02-13  Jakub Jelinek  <jakub@redhat.com>
 
+	PR c/82210
+	* gcc.c-torture/execute/pr82210.c: New test.
+
 	PR middle-end/84309
 	* gcc.dg/pr84309-2.c: New test.
 
diff --git a/gcc/testsuite/gcc.c-torture/execute/pr82210.c b/gcc/testsuite/gcc.c-torture/execute/pr82210.c
new file mode 100644
index 0000000..48fb715
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/execute/pr82210.c
@@ -0,0 +1,26 @@
+/* PR c/82210 */
+
+void
+foo (int size)
+{
+  int i;
+  struct S {
+    __attribute__((aligned (16))) struct T { short c; } a[size];
+    int b[size];
+  } s;
+
+  for (i = 0; i < size; i++)
+    s.a[i].c = 0x1234;
+  for (i = 0; i < size; i++)
+    s.b[i] = 0;
+  for (i = 0; i < size; i++)
+    if (s.a[i].c != 0x1234 || s.b[i] != 0)
+      __builtin_abort ();
+}
+
+int
+main ()
+{
+  foo (15);
+  return 0;
+}