Tighten check for whether a sibcall references local variables

[Richard Sandiford]

This loop:

  /* Make sure the tail invocation of this function does not refer
     to local variables.  */
  FOR_EACH_LOCAL_DECL (cfun, idx, var)
    {
      if (TREE_CODE (var) != PARM_DECL
          && auto_var_in_fn_p (var, cfun->decl)
          && (ref_maybe_used_by_stmt_p (call, var)
              || call_may_clobber_ref_p (call, var)))
        return;
    }

triggered even for local variables that are passed by value.
This meant that we didn't allow local aggregates to be passed
to a sibling call but did (for example) allow global aggregates
to be passed.

I think the loop is really checking for indirect references,
so should be able to skip any variables that never have their
address taken.

Tested on aarch64-linux-gnu and x86_64-linux-gnu.  OK to install?

Thanks,
Richard


gcc/
        * tree-tailcall.c (find_tail_calls): Allow calls to reference
        local variables if all references are known to be direct.

gcc/testsuite/
        * gcc.dg/tree-ssa/tailcall-8.c: New test.

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/tailcall-8.c 
b/gcc/testsuite/gcc.dg/tree-ssa/tailcall-8.c
new file mode 100644
index 0000000..ffeabe5
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/tailcall-8.c
@@ -0,0 +1,80 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-tailc-details" } */
+
+struct s { int x; };
+void f_direct (struct s);
+void f_indirect (struct s *);
+void f_void (void);
+
+/* Tail call.  */
+void
+g1 (struct s param)
+{
+  f_direct (param);
+}
+
+/* Tail call.  */
+void
+g2 (struct s *param_ptr)
+{
+  f_direct (*param_ptr);
+}
+
+/* Tail call.  */
+void
+g3 (struct s *param_ptr)
+{
+  f_indirect (param_ptr);
+}
+
+/* Tail call.  */
+void
+g4 (struct s *param_ptr)
+{
+  f_indirect (param_ptr);
+  f_void ();
+}
+
+/* Tail call.  */
+void
+g5 (struct s param)
+{
+  struct s local = param;
+  f_direct (local);
+}
+
+/* Tail call.  */
+void
+g6 (struct s param)
+{
+  struct s local = param;
+  f_direct (local);
+  f_void ();
+}
+
+/* Not a tail call.  */
+void
+g7 (struct s param)
+{
+  struct s local = param;
+  f_indirect (&local);
+}
+
+/* Not a tail call.  */
+void
+g8 (struct s *param_ptr)
+{
+  struct s local = *param_ptr;
+  f_indirect (&local);
+}
+
+/* Not a tail call.  */
+void
+g9 (struct s *param_ptr)
+{
+  struct s local = *param_ptr;
+  f_indirect (&local);
+  f_void ();
+}
+
+/* { dg-final { scan-tree-dump-times "Found tail call" 6 "tailc" } } */
diff --git a/gcc/tree-tailcall.c b/gcc/tree-tailcall.c
index f97541d..66a0a4c 100644
--- a/gcc/tree-tailcall.c
+++ b/gcc/tree-tailcall.c
@@ -504,12 +504,14 @@ find_tail_calls (basic_block bb, struct tailcall **ret)
        tail_recursion = true;
     }
 
-  /* Make sure the tail invocation of this function does not refer
-     to local variables.  */
+  /* Make sure the tail invocation of this function does not indirectly
+     refer to local variables.  (Passing variables directly by value
+     is OK.)  */
   FOR_EACH_LOCAL_DECL (cfun, idx, var)
     {
       if (TREE_CODE (var) != PARM_DECL
          && auto_var_in_fn_p (var, cfun->decl)
+         && may_be_aliased (var)
          && (ref_maybe_used_by_stmt_p (call, var)
              || call_may_clobber_ref_p (call, var)))
        return;