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.

1 files changed, 17 insertions, 0 deletions

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 ();
+}