d: Add warning for call expression without side effects

In the last merge of the dmd front-end with upstream (r14-4830), this
warning got removed from the semantic passes.  Reimplement the warning
for the code generation pass instead, where it cannot have an effect on
conditional compilation.

gcc/d/ChangeLog:

	* d-codegen.cc (call_side_effect_free_p): New function.
	* d-tree.h (CALL_EXPR_WARN_IF_UNUSED): New macro.
	(call_side_effect_free_p): New prototype.
	* expr.cc (ExprVisitor::visit (CallExp *)): Set
	CALL_EXPR_WARN_IF_UNUSED on matched call expressions.
	(ExprVisitor::visit (NewExp *)): Don't dereference the result of an
	allocation call here.
	* toir.cc (add_stmt): Emit warning when call expression added to
	statement list without being used.

gcc/testsuite/ChangeLog:

	* gdc.dg/Wunused_value.d: New test.

1 files changed, 12 insertions, 1 deletions

diff --git a/gcc/d/expr.cc b/gcc/d/expr.cc
index 52243e6..72180b1 100644
--- a/gcc/d/expr.cc
+++ b/gcc/d/expr.cc
@@ -1714,6 +1714,12 @@ public:
        build the call expression.  */
     tree exp = d_build_call (tf, callee, object, e->arguments);
 
+    /* Record whether the call expression has no side effects, so we can check
+       for an unused return value later.  */
+    if (TREE_CODE (exp) == CALL_EXPR && CALL_EXPR_FN (exp) != NULL_TREE
+	&& call_side_effect_free_p (e->f, e->e1->type))
+      CALL_EXPR_WARN_IF_UNUSED (exp) = 1;
+
     if (returnvalue != NULL_TREE)
       exp = compound_expr (exp, returnvalue);
 
@@ -2338,7 +2344,12 @@ public:
 		new_call = d_save_expr (new_call);
 		se->type = sd->type;
 		se->sym = new_call;
-		result = compound_expr (build_expr (se), new_call);
+
+		/* Setting `se->sym' would mean that the result of the
+		   constructed struct literal expression is `*(new_call)'.
+		   Strip off the indirect reference, as we don't mean to
+		   compute the value yet.  */
+		result = build_address (build_expr (se));
 	      }
 	    else
 	      result = new_call;