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

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(-)

[I slightly improve the patch covering a few more cases where
tree-tailcall gives up, especially with -O1 and -Os.
Here's the updated version.]

Give better error messages for musttail
    
When musttail is set, make tree-tailcall give error messages
when it cannot handle a call. This avoids vague "other reasons"
error messages later at expand time when it sees a musttail
function not marked tail call.
    
gcc/ChangeLog:
    
      * tree-tailcall.cc (maybe_error_musttail): Add.
        (bb_get_succ_edge_count): Add.
        (find_tail_calls): Add error messages. Keep searching
        for basic blocks with multiple BBs if all but one is EH
        only.

diff --git a/gcc/tree-tailcall.cc b/gcc/tree-tailcall.cc
index 094856de22ef..7268e8138529 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 +403,36 @@ 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);
+    }
+}
+
+/* Count succ edges for BB and return in NUM_OTHER and NUM_EH.  */
+
+static void
+bb_get_succ_edge_count (basic_block bb, int &num_other, int &num_eh)
+{
+  edge e;
+  edge_iterator ei;
+  num_eh = 0;
+  num_other = 0;
+  FOR_EACH_EDGE (e, ei, bb->succs)
+    if (e->flags & EDGE_EH)
+      num_eh++;
+    else
+      num_other++;
+}
+
 /* 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;
@@ -426,8 +457,16 @@ find_tail_calls (basic_block bb, struct tailcall **ret, bool only_musttail)
   tree var;
 
   if (!single_succ_p (bb))
-    return;
+    {
+      int num_eh, num_other;
+      bb_get_succ_edge_count (bb, num_eh, num_other);
+      /* Allow a single EH edge so that we can give a better
+	 error message later.  */
+      if (!(num_eh == 1 && num_other == 1))
+	return;
+    }
 
+  bool bad_stmt = false;
   for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi); gsi_prev (&gsi))
     {
       stmt = gsi_stmt (gsi);
@@ -448,6 +487,11 @@ find_tail_calls (basic_block bb, struct tailcall **ret, bool only_musttail)
 	  /* Handle only musttail calls when not optimizing.  */
 	  if (only_musttail && !gimple_call_must_tail_p (call))
 	    return;
+	  if (bad_stmt)
+	    {
+	      maybe_error_musttail (call, "Memory reference or volatile after call");
+	      return;
+	    }
 	  ass_var = gimple_call_lhs (call);
 	  break;
 	}
@@ -462,7 +506,9 @@ find_tail_calls (basic_block bb, struct tailcall **ret, bool only_musttail)
       /* If the statement references memory or volatile operands, fail.  */
       if (gimple_references_memory_p (stmt)
 	  || gimple_has_volatile_ops (stmt))
-	return;
+	{
+	  bad_stmt = true;
+	}
     }
 
   if (gsi_end_p (gsi))
@@ -489,13 +535,21 @@ 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;
+    }
 
   /* 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)
+  if ((stmt_could_throw_p (cfun, stmt)
       && !stmt_can_throw_external (cfun, stmt))
+	|| !single_succ_p (bb))
+  {
+    maybe_error_musttail (call,
+			  "call may throw exception that does not propagate");
     return;
+  }
 
   /* If the function returns a value, then at present, the tail call
      must return the same type of value.  There is conceptually a copy
@@ -524,7 +578,10 @@ 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 +662,7 @@ 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 +671,7 @@ 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;
 		}
 	    }
@@ -658,17 +717,21 @@ find_tail_calls (basic_block bb, struct tailcall **ret, bool only_musttail)
 	continue;
 
       if (gimple_code (stmt) != GIMPLE_ASSIGN)
-	return;
+	{
+	  maybe_error_musttail (call, "unhandled code after call");
+	  return;
+	}
 
       /* This is a gimple assign. */
       par ret = process_assignment (as_a <gassign *> (stmt), gsi,
 				    &tmp_m, &tmp_a, &ass_var, to_move_defs);
-      if (ret == FAIL)
-	return;
+      if (ret == FAIL || (ret == TRY_MOVE && !tail_recursion))
+	{
+	  maybe_error_musttail (call, "return value changed after call");
+	  return;
+	}
       else if (ret == TRY_MOVE)
 	{
-	  if (! tail_recursion)
-	    return;
 	  /* Do not deal with checking dominance, the real fix is to
 	     do path isolation for the transform phase anyway, removing
 	     the need to compute the accumulators with new stmts.  */
@@ -716,16 +779,25 @@ 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.  */
   if (!tail_recursion && (m || a))
-    return;
+    {
+      maybe_error_musttail (call, "operations after non tail recursive call");
+      return;
+    }
 
   /* For pointers only allow additions.  */
   if (m && POINTER_TYPE_P (TREE_TYPE (DECL_RESULT (current_function_decl))))
-    return;
+    {
+      maybe_error_musttail (call, "tail recursion with pointers can only use additions");
+      return;
+    }
 
   /* Move queued defs.  */
   if (tail_recursion)