[gcc r13-1016] PR middle-end/105874: Use EXPAND_MEMORY to fix ada bootstrap.

[gcc r13-1016] PR middle-end/105874: Use EXPAND_MEMORY to fix ada bootstrap.

[Roger Sayle]

https://gcc.gnu.org/g:b6e1373bd34aebbb512a03ea9a4e3c7acd955382

commit r13-1016-gb6e1373bd34aebbb512a03ea9a4e3c7acd955382
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Wed Jun 8 20:43:03 2022 +0100

    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.

Diff:
---
 gcc/expr.cc                         | 11 ++++++++---
 gcc/testsuite/g++.dg/opt/pr105874.C | 30 ++++++++++++++++++++++++++++++
 2 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/gcc/expr.cc b/gcc/expr.cc
index 85cb4141195..c37a9990536 100644
--- a/gcc/expr.cc
+++ b/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.
diff --git a/gcc/testsuite/g++.dg/opt/pr105874.C b/gcc/testsuite/g++.dg/opt/pr105874.C
new file mode 100644
index 00000000000..58699a6f03a
--- /dev/null
+++ b/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" } } */