c++: small tweak for cp_fold_r

This patch is an optimization tweak for cp_fold_r.  If we cp_fold_r the
COND_EXPR's op0 first, we may be able to evaluate it to a constant if -O.
cp_fold has:

3143         if (callee && DECL_DECLARED_CONSTEXPR_P (callee)
3144             && !flag_no_inline)
...
3151             r = maybe_constant_value (x, /*decl=*/NULL_TREE,

flag_no_inline is 1 for -O0:

1124   if (opts->x_optimize == 0)
1125     {
1126       /* Inlining does not work if not optimizing,
1127          so force it not to be done.  */
1128       opts->x_warn_inline = 0;
1129       opts->x_flag_no_inline = 1;
1130     }

but otherwise it's 0 and cp_fold will maybe_constant_value calls to
constexpr functions.  And if it doesn't, then folding the COND_EXPR
will keep both arms, and we can avoid calling maybe_constant_value.

gcc/cp/ChangeLog:

	* cp-gimplify.cc (cp_fold_r): Don't call maybe_constant_value.

diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
index a282c3930a37f151621a744389789f6ff65f355d..33e9411f10c6e2b8a8b325f5f144a99d065ca94c 100644 (file)
--- a/gcc/cp/cp-gimplify.cc
+++ b/gcc/cp/cp-gimplify.cc
@@ -1152,13 +1152,12 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void *data_)
          auto then_fn = cp_fold_r, else_fn = cp_fold_r;
          /* See if we can figure out if either of the branches is dead.  If it
             is, we don't need to do everything that cp_fold_r does.  */
-         tree cond = maybe_constant_value (TREE_OPERAND (stmt, 0));
-         if (integer_zerop (cond))
+         cp_walk_tree (&TREE_OPERAND (stmt, 0), cp_fold_r, data, nullptr);
+         if (integer_zerop (TREE_OPERAND (stmt, 0)))
            then_fn = cp_fold_immediate_r;
-         else if (TREE_CODE (cond) == INTEGER_CST)
+         else if (integer_nonzerop (TREE_OPERAND (stmt, 0)))
            else_fn = cp_fold_immediate_r;
 
-         cp_walk_tree (&TREE_OPERAND (stmt, 0), cp_fold_r, data, nullptr);
          if (TREE_OPERAND (stmt, 1))
            cp_walk_tree (&TREE_OPERAND (stmt, 1), then_fn, data,
                          nullptr);