PR middle-end/105874: Use EXPAND_MEMORY to fix ada bootstrap.

Many thanks to Tamar Christina for filing PR middle-end/105874 indicating
that SPECcpu 2017's Leela is failing on x86_64 due to a miscompilation
of FastBoard::is_eye.  This function is much smaller and easier to work
with than my previous hunt for the cause of the Ada bootstrap failures
due to miscompilation somewhere in GCC (or one of the 131 places that
the problematic form of optimization triggers during an ada bootstrap).

It turns out the source of the miscompilation introduced by my recent
patch is the distinction (during RTL expansion) of l-values and r-values.
According to the documentation above expand_modifier, EXPAND_MEMORY
should be used for lvalues (when a memory is required), and EXPAND_NORMAL
for rvalues when a constant is permissible.  In what I'd like to consider
a latent bug, the recursive call to expand_expr_real on line 11188 of
expr.cc, in the case handling ARRAY_REF, COMPONENT_REF, BIT_FIELD_REF
and ARRARY_RANGE_REF was passing EXPAND_NORMAL when it really required
(the semantics of) EXPAND_MEMORY.  All the time that VAR_DECLs were
being returned as memory this was fine, but as soon as we're able to
optimize sort arrays into immediate constants, bad things happen.

In the test case from Leela, we notice that the array s_eyemask
always has DImode constant value { 4, 64 }, which is useful as
an rvalue, but not when we need to index it as an lvalue, as in
s_eyemask[color].  This also explains why everything being accepted
by immediate_const_ctor_p (during an ada bootstrap) looks reasonable,
what's incorrect is that we don't know how these structs/arrays are
to be used.

The fix is to ensure that we call expand_expr with EXPAND_MEMORY
when processing the VAR_DECL's returned by get_inner_reference.

2022-06-08  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR middle-end/105874
	* expr.cc (expand_expr_real_1) <normal_inner_ref>:  New local
	variable tem_modifier for calculating the expand_modifier enum to
	use for expanding tem.  If tem is a VAR_DECL, use EXPAND_MEMORY.

gcc/testsuite/ChangeLog
	PR middle-end/105874
	* g++.dg/opt/pr105874.C: New test case.

gcc/expr.cc

@@ -11181,6 +11181,13 @@ expand_expr_real_1 (tree exp, rtx target, machine_mode tmode,
 	   infinitely recurse.  */
 	gcc_assert (tem != exp);
 
+	/* If tem is a VAR_DECL, we need a memory reference.  */
+	enum expand_modifier tem_modifier = modifier;
+	if (tem_modifier == EXPAND_SUM)
+	  tem_modifier = EXPAND_NORMAL;
+	if (TREE_CODE (tem) == VAR_DECL)
+	  tem_modifier = EXPAND_MEMORY;
+
 	/* If TEM's type is a union of variable size, pass TARGET to the inner
 	   computation, since it will need a temporary and TARGET is known
 	   to have to do.  This occurs in unchecked conversion in Ada.  */
@@ -11192,9 +11199,7 @@ expand_expr_real_1 (tree exp, rtx target, machine_mode tmode,
 				   != INTEGER_CST)
 			       && modifier != EXPAND_STACK_PARM
 			       ? target : NULL_RTX),
-			      VOIDmode,
-			      modifier == EXPAND_SUM ? EXPAND_NORMAL : modifier,
-			      NULL, true);
+			      VOIDmode, tem_modifier, NULL, true);
 
 	/* If the field has a mode, we want to access it in the
 	   field's mode, not the computed mode.

gcc/testsuite/g++.dg/opt/pr105874.C

@@ -0,0 +1,30 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -std=c++11" } */
+#include <array>
+
+static constexpr int NBR_SHIFT = 4;
+
+static constexpr int MAXBOARDSIZE = 25;
+
+static constexpr int MAXSQ = ((MAXBOARDSIZE + 2) * (MAXBOARDSIZE + 2));
+
+enum square_t : char {
+        BLACK = 0, WHITE = 1, EMPTY = 2, INVAL = 3
+    };
+
+const std::array<int, 2> s_eyemask = {
+    4 * (1 << (NBR_SHIFT * BLACK)),
+    4 * (1 << (NBR_SHIFT * WHITE))
+};
+
+/* counts of neighboring stones */
+std::array<unsigned short, MAXSQ> m_neighbours;
+
+int is_eye(const int color, const int i) {
+    /* check for 4 neighbors of the same color */
+    int ownsurrounded = (m_neighbours[i] & s_eyemask[color]);
+
+    return ownsurrounded;
+}
+
+/* { dg-final { scan-assembler "s_eyemask" } } */