[PATCH v6 7/8] Give better error messages for musttail

[Andi Kleen <ak@linux.intel.com>]

When musttail is set, make tree-tailcall to give error messages
when it cannot handle a call. This avoids vague "other reasons"
error messages later at expand time.

This doesn't always work, for example when find_tail_call
walking gives up because the control flow is too complicated
then it won't find the tail call and can't give a suitable
error message.

gcc/ChangeLog:

	* tree-tailcall.cc (maybe_error_musttail): Add.
	(find_tail_calls): Add error messages.
---
 gcc/tree-tailcall.cc | 37 ++++++++++++++++++++++++++++++++++---
 1 file changed, 34 insertions(+), 3 deletions(-)

diff --git a/gcc/tree-tailcall.cc b/gcc/tree-tailcall.cc
index 094856de22ef..1217435c3f90 100644
--- a/gcc/tree-tailcall.cc
+++ b/gcc/tree-tailcall.cc
@@ -43,6 +43,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "common/common-target.h"
 #include "ipa-utils.h"
 #include "tree-ssa-live.h"
+#include "diagnostic-core.h"
 
 /* The file implements the tail recursion elimination.  It is also used to
    analyze the tail calls in general, passing the results to the rtl level
@@ -402,6 +403,20 @@ propagate_through_phis (tree var, edge e)
   return var;
 }
 
+/* Report an error for failing to tail convert must call CALL
+   with error message ERR.  */
+
+static void
+maybe_error_musttail (gcall *call, const char *err)
+{
+  if (gimple_call_must_tail_p (call))
+    {
+      error_at (call->location, "cannot tail-call: %s", err);
+      gimple_call_set_must_tail (call, false); /* Avoid another error.  */
+      gimple_call_set_tail (call, false);
+    }
+}
+
 /* Argument for compute_live_vars/live_vars_at_stmt and what compute_live_vars
    returns.  Computed lazily, but just once for the function.  */
 static live_vars_map *live_vars;
@@ -489,8 +504,14 @@ find_tail_calls (basic_block bb, struct tailcall **ret, bool only_musttail)
   if (ass_var
       && !is_gimple_reg (ass_var)
       && !auto_var_in_fn_p (ass_var, cfun->decl))
-    return;
+    {
+      maybe_error_musttail (call, "complex return value");
+      return;
+    }
 
+  /* This doesn't really work because the BB would have multiple succ edges
+     and is rejected above. Instead just gives a "other reasons" error
+     at expand time.  */
   /* If the call might throw an exception that wouldn't propagate out of
      cfun, we can't transform to a tail or sibling call (82081).  */
   if (stmt_could_throw_p (cfun, stmt)
@@ -524,7 +545,10 @@ find_tail_calls (basic_block bb, struct tailcall **ret, bool only_musttail)
   if (result_decl
       && may_be_aliased (result_decl)
       && ref_maybe_used_by_stmt_p (call, result_decl, false))
-    return;
+    {
+      maybe_error_musttail (call, "tail call must be same type");
+      return;
+    }
 
   /* We found the call, check whether it is suitable.  */
   tail_recursion = false;
@@ -605,6 +629,7 @@ find_tail_calls (basic_block bb, struct tailcall **ret, bool only_musttail)
 	    {
 	      if (local_live_vars)
 		BITMAP_FREE (local_live_vars);
+	      maybe_error_musttail (call, "call invocation refers to locals");
 	      return;
 	    }
 	  else
@@ -613,6 +638,7 @@ find_tail_calls (basic_block bb, struct tailcall **ret, bool only_musttail)
 	      if (bitmap_bit_p (local_live_vars, *v))
 		{
 		  BITMAP_FREE (local_live_vars);
+		  maybe_error_musttail (call, "call invocation refers to locals");
 		  return;
 		}
 	    }
@@ -631,6 +657,8 @@ find_tail_calls (basic_block bb, struct tailcall **ret, bool only_musttail)
   auto_bitmap to_move_defs;
   auto_vec<gimple *> to_move_stmts;
 
+  /* Does not give musttail errors because the frontend won't generate
+     such musttails. */
   abb = bb;
   agsi = gsi;
   while (1)
@@ -716,7 +744,10 @@ find_tail_calls (basic_block bb, struct tailcall **ret, bool only_musttail)
   if (ret_var
       && (ret_var != ass_var
 	  && !(is_empty_type (TREE_TYPE (ret_var)) && !ass_var)))
-    return;
+    {
+      maybe_error_musttail (call, "call must be the same type");
+      return;
+    }
 
   /* If this is not a tail recursive call, we cannot handle addends or
      multiplicands.  */
-- 
2.44.0