varasm: Fix output_constructor where a RANGE_EXPR index needs to skip some elts [PR94303]

The following testcase is miscompiled, because output_constructor doesn't
output the initializer correctly.  The FE creates {[1...2] = 9} in this
case, and we emit .long 9; long 9; .zero 8 instead of the expected
.zero 8; .long 9; .long 9.  If the CONSTRUCTOR is {[1] = 9, [2] = 9},
output_constructor_regular_field has code to notice that the current
location (local->total_bytes) is smaller than the location we want to write
to (1*sizeof(elt)) and will call assemble_zeros to skip those.  But
RANGE_EXPRs are handled by a different function which didn't do this,
so for RANGE_EXPRs we emitted them properly only if local->total_bytes
was always equal to the location where the RANGE_EXPR needs to start.

2020-03-25  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/94303
	* varasm.c (output_constructor_array_range): If local->index
	RANGE_EXPR doesn't start at the current location in the constructor,
	skip needed number of bytes using assemble_zeros or assert we don't
	go backwards.

	PR middle-end/94303
	* g++.dg/torture/pr94303.C: New test.

4 files changed, 46 insertions, 0 deletions

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index cde4653..3a1bfce 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,5 +1,11 @@
 2020-03-25  Jakub Jelinek  <jakub@redhat.com>
 
+	PR middle-end/94303
+	* varasm.c (output_constructor_array_range): If local->index
+	RANGE_EXPR doesn't start at the current location in the constructor,
+	skip needed number of bytes using assemble_zeros or assert we don't
+	go backwards.
+
 	PR c++/94223
 	* langhooks.c (lhd_set_decl_assembler_name): Use a static ulong
 	counter instead of DECL_UID.
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 048f830..b68779f 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,5 +1,8 @@
 2020-03-25  Jakub Jelinek  <jakub@redhat.com>
 
+	PR middle-end/94303
+	* g++.dg/torture/pr94303.C: New test.
+
 	PR c++/94223
 	* g++.dg/opt/pr94223.C: New test.
 
diff --git a/gcc/testsuite/g++.dg/torture/pr94303.C b/gcc/testsuite/g++.dg/torture/pr94303.C
new file mode 100644
index 0000000..45b90a2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/torture/pr94303.C
@@ -0,0 +1,17 @@
+// PR middle-end/94303
+// { dg-do run }
+
+struct A {
+  int d = 9;
+  A () = default;
+  A (int x) : d(x) {}
+  void foo () { if (d < 1) __builtin_abort (); }
+};
+
+A a[3] = { 1 };
+
+int
+main ()
+{
+  a[2].foo ();
+}
diff --git a/gcc/varasm.c b/gcc/varasm.c
index dc6da6c..271a67a 100644
--- a/gcc/varasm.c
+++ b/gcc/varasm.c
@@ -5152,6 +5152,26 @@ struct oc_local_state {
 static void
 output_constructor_array_range (oc_local_state *local)
 {
+  /* Perform the index calculation in modulo arithmetic but
+     sign-extend the result because Ada has negative DECL_FIELD_OFFSETs
+     but we are using an unsigned sizetype.  */
+  unsigned prec = TYPE_PRECISION (sizetype);
+  offset_int idx = wi::sext (wi::to_offset (TREE_OPERAND (local->index, 0))
+			     - wi::to_offset (local->min_index), prec);
+  tree valtype = TREE_TYPE (local->val);
+  HOST_WIDE_INT fieldpos
+    = (idx * wi::to_offset (TYPE_SIZE_UNIT (valtype))).to_short_addr ();
+
+  /* Advance to offset of this element.  */
+  if (fieldpos > local->total_bytes)
+    {
+      assemble_zeros (fieldpos - local->total_bytes);
+      local->total_bytes = fieldpos;
+    }
+  else
+    /* Must not go backwards.  */
+    gcc_assert (fieldpos == local->total_bytes);
+
   unsigned HOST_WIDE_INT fieldsize
     = int_size_in_bytes (TREE_TYPE (local->type));