[PATCH 2/6] FIXME: WIP on non-jit part of must-tail-call

[David Malcolm <dmalcolm@redhat.com>]

This patch implements support for marking CALL_EXPRs
as being mandatory for tail-call-optimization.

Currently this flag isn't accessible from any frontend,
so the patch uses a plugin for testing the functionality.

TODO:
- lots of FIXMEs
- lots more test coverage
- needs better blurb
- needs ChangeLog
- needs bootstrap&regrtest
---
 gcc/calls.c                                        | 97 ++++++++++++++++++----
 gcc/cfgexpand.c                                    |  1 +
 gcc/gimple-pretty-print.c                          |  2 +
 gcc/gimple.c                                       |  1 +
 gcc/gimple.h                                       | 19 +++++
 gcc/print-tree.c                                   |  2 +-
 gcc/testsuite/gcc.dg/plugin/must-tail-call-1.c     | 22 +++++
 gcc/testsuite/gcc.dg/plugin/must-tail-call-2.c     | 15 ++++
 .../gcc.dg/plugin/must_tail_call_plugin.c          | 76 +++++++++++++++++
 gcc/testsuite/gcc.dg/plugin/plugin.exp             |  3 +
 gcc/tree-core.h                                    |  3 +
 gcc/tree.h                                         |  4 +
 12 files changed, 229 insertions(+), 16 deletions(-)
 create mode 100644 gcc/testsuite/gcc.dg/plugin/must-tail-call-1.c
 create mode 100644 gcc/testsuite/gcc.dg/plugin/must-tail-call-2.c
 create mode 100644 gcc/testsuite/gcc.dg/plugin/must_tail_call_plugin.c

diff --git a/gcc/calls.c b/gcc/calls.c
index ac8092c..c7c3260 100644
--- a/gcc/calls.c
+++ b/gcc/calls.c
@@ -1102,6 +1102,19 @@ store_unaligned_arguments_into_pseudos (struct arg_data *args, int num_actuals)
       }
 }
 
+/* Issue an error if CALL_EXPR was flagged as requiring
+   tall-call optimization.  */
+
+static void
+cannot_tail_call (tree call_expr, const char *reason)
+{
+  gcc_assert (TREE_CODE (call_expr) == CALL_EXPR);
+  if (!CALL_EXPR_MUST_TAIL_CALL (call_expr))
+    return;
+
+  error_at (EXPR_LOCATION (call_expr), "cannot tail-call: %s", reason);
+}
+
 /* Fill in ARGS_SIZE and ARGS array based on the parameters found in
    CALL_EXPR EXP.
 
@@ -1343,7 +1356,10 @@ initialize_argument_information (int num_actuals ATTRIBUTE_UNUSED,
 	      /* We can't use sibcalls if a callee-copied argument is
 		 stored in the current function's frame.  */
 	      if (!call_from_thunk_p && DECL_P (base) && !TREE_STATIC (base))
-		*may_tailcall = false;
+		{
+		  *may_tailcall = false;
+		  cannot_tail_call (exp, "FIXME 1");
+		}
 
 	      args[i].tree_value = build_fold_addr_expr_loc (loc,
 							 args[i].tree_value);
@@ -1406,6 +1422,7 @@ initialize_argument_information (int num_actuals ATTRIBUTE_UNUSED,
 		= build_fold_addr_expr_loc (loc, make_tree (type, copy));
 	      type = TREE_TYPE (args[i].tree_value);
 	      *may_tailcall = false;
+	      cannot_tail_call (exp, "FIXME 2");
 	    }
 	}
 
@@ -2358,48 +2375,74 @@ can_implement_as_sibling_call_p (tree exp,
 				 const args_size &args_size)
 {
   if (!targetm.have_sibcall_epilogue ())
-    return false;
+    {
+      cannot_tail_call (exp, "FIXME: targetm.have_sibcall_epilogue");
+      return false;
+    }
 
   /* Doing sibling call optimization needs some work, since
      structure_value_addr can be allocated on the stack.
      It does not seem worth the effort since few optimizable
      sibling calls will return a structure.  */
   if (structure_value_addr != NULL_RTX)
-    return false;
+    {
+      cannot_tail_call (exp, "FIXME: structure_value_addr");
+      return false;
+    }
 
 #ifdef REG_PARM_STACK_SPACE
   /* If outgoing reg parm stack space changes, we can not do sibcall.  */
   if (OUTGOING_REG_PARM_STACK_SPACE (funtype)
       != OUTGOING_REG_PARM_STACK_SPACE (TREE_TYPE (current_function_decl))
       || (reg_parm_stack_space != REG_PARM_STACK_SPACE (current_function_decl)))
-    return false;
+    {
+      cannot_tail_call (exp, "FIXME: outgoing reg parm stack space changes");
+      return false;
+    }
 #endif
 
   /* Check whether the target is able to optimize the call
      into a sibcall.  */
   if (!targetm.function_ok_for_sibcall (fndecl, exp))
-    return false;
+    {
+      cannot_tail_call (exp, "FIXME: targetm.function_ok_for_sibcall");
+      return false;
+    }
 
   /* Functions that do not return exactly once may not be sibcall
      optimized.  */
   if (flags & (ECF_RETURNS_TWICE | ECF_NORETURN))
-    return false;
+    {
+      cannot_tail_call (exp, "FIXME: ECF_RETURNS_TWICE | ECF_NORETURN");
+      return false;
+    }
 
   if (TYPE_VOLATILE (TREE_TYPE (TREE_TYPE (addr))))
-    return false;
+    {
+      cannot_tail_call (exp,
+			"FIXME: TYPE_VOLATILE (TREE_TYPE (TREE_TYPE (addr))))");
+      return false;
+    }
 
   /* If the called function is nested in the current one, it might access
      some of the caller's arguments, but could clobber them beforehand if
      the argument areas are shared.  */
   if (fndecl && decl_function_context (fndecl) == current_function_decl)
-    return false;
+    {
+      cannot_tail_call (exp, "FIXME");
+      return false;
+    }
 
   /* If this function requires more stack slots than the current
      function, we cannot change it into a sibling call.
      crtl->args.pretend_args_size is not part of the
      stack allocated by our caller.  */
   if (args_size.constant > (crtl->args.size - crtl->args.pretend_args_size))
-    return false;
+    {
+      cannot_tail_call (exp,
+			"callee required more stack slots than the caller");
+      return false;
+    }
 
   /* If the callee pops its own arguments, then it must pop exactly
      the same number of arguments as the current function.  */
@@ -2407,10 +2450,16 @@ can_implement_as_sibling_call_p (tree exp,
       != targetm.calls.return_pops_args (current_function_decl,
 					 TREE_TYPE (current_function_decl),
 					 crtl->args.size))
-    return false;
+    {
+      cannot_tail_call (exp, "FIXME");
+      return false;
+    }
 
   if (!lang_hooks.decls.ok_for_sibcall (fndecl))
-    return false;
+    {
+      cannot_tail_call (exp, "lang_hooks.decls.ok_for_sibcall");
+      return false;
+    }
 
   /* All checks passed.  */
   return true;
@@ -2444,6 +2493,7 @@ expand_call (tree exp, rtx target, int ignore)
   /* The type of the function being called.  */
   tree fntype;
   bool try_tail_call = CALL_EXPR_TAILCALL (exp);
+  bool must_tail_call = CALL_EXPR_MUST_TAIL_CALL (exp);
   int pass;
 
   /* Register in which non-BLKmode value will be returned,
@@ -2811,10 +2861,18 @@ expand_call (tree exp, rtx target, int ignore)
       || dbg_cnt (tail_call) == false)
     try_tail_call = 0;
 
+  /* If the user has marked the function as requiring tail-call
+     optimization, attempt it.  */
+  if (must_tail_call)
+    try_tail_call = 1;
+
   /*  Rest of purposes for tail call optimizations to fail.  */
   if (try_tail_call)
-    try_tail_call = can_implement_as_sibling_call_p (exp, structure_value_addr, funtype,
-						     reg_parm_stack_space, fndecl,
+    try_tail_call = can_implement_as_sibling_call_p (exp,
+						     structure_value_addr,
+						     funtype,
+						     reg_parm_stack_space,
+						     fndecl,
 						     flags, addr, args_size);
 
   /* Check if caller and callee disagree in promotion of function
@@ -2845,7 +2903,10 @@ expand_call (tree exp, rtx target, int ignore)
 		  && (caller_unsignedp != callee_unsignedp
 		      || GET_MODE_BITSIZE (caller_mode)
 			 < GET_MODE_BITSIZE (callee_mode)))))
-	try_tail_call = 0;
+	{
+	  try_tail_call = 0;
+	  cannot_tail_call (exp, "FIXME 3");
+	}
     }
 
   /* Ensure current function's preferred stack boundary is at least
@@ -3743,7 +3804,13 @@ expand_call (tree exp, rtx target, int ignore)
       crtl->tail_call_emit = true;
     }
   else
-    emit_insn (normal_call_insns);
+    {
+      emit_insn (normal_call_insns);
+      if (try_tail_call)
+	/* Ideally we'd emit a message for all of the ways that it could
+	   have failed.  */
+	cannot_tail_call (exp, "FIXME");
+    }
 
   currently_expanding_call--;
 
diff --git a/gcc/cfgexpand.c b/gcc/cfgexpand.c
index 77a1964..2a27691 100644
--- a/gcc/cfgexpand.c
+++ b/gcc/cfgexpand.c
@@ -2626,6 +2626,7 @@ expand_call_stmt (gcall *stmt)
     TREE_NOTHROW (exp) = 1;
 
   CALL_EXPR_TAILCALL (exp) = gimple_call_tail_p (stmt);
+  CALL_EXPR_MUST_TAIL_CALL (exp) = gimple_call_must_tail_p (stmt);
   CALL_EXPR_RETURN_SLOT_OPT (exp) = gimple_call_return_slot_opt_p (stmt);
   if (decl
       && DECL_BUILT_IN_CLASS (decl) == BUILT_IN_NORMAL
diff --git a/gcc/gimple-pretty-print.c b/gcc/gimple-pretty-print.c
index 4b0dc7c..1806a8c 100644
--- a/gcc/gimple-pretty-print.c
+++ b/gcc/gimple-pretty-print.c
@@ -742,6 +742,8 @@ dump_gimple_call (pretty_printer *buffer, gcall *gs, int spc, int flags)
     pp_string (buffer, " [return slot optimization]");
   if (gimple_call_tail_p (gs))
     pp_string (buffer, " [tail call]");
+  if (gimple_call_must_tail_p (gs))
+    pp_string (buffer, " [must tail call]");
 
   if (fn == NULL)
     return;
diff --git a/gcc/gimple.c b/gcc/gimple.c
index 1a22e82..92f873b 100644
--- a/gcc/gimple.c
+++ b/gcc/gimple.c
@@ -359,6 +359,7 @@ gimple_build_call_from_tree (tree t)
   /* Carry all the CALL_EXPR flags to the new GIMPLE_CALL.  */
   gimple_call_set_chain (call, CALL_EXPR_STATIC_CHAIN (t));
   gimple_call_set_tail (call, CALL_EXPR_TAILCALL (t));
+  gimple_call_set_must_tail (call, CALL_EXPR_MUST_TAIL_CALL (t));
   gimple_call_set_return_slot_opt (call, CALL_EXPR_RETURN_SLOT_OPT (t));
   if (fndecl
       && DECL_BUILT_IN_CLASS (fndecl) == BUILT_IN_NORMAL
diff --git a/gcc/gimple.h b/gcc/gimple.h
index 6d15dab..9855db1 100644
--- a/gcc/gimple.h
+++ b/gcc/gimple.h
@@ -145,6 +145,7 @@ enum gf_mask {
     GF_CALL_INTERNAL		= 1 << 6,
     GF_CALL_CTRL_ALTERING       = 1 << 7,
     GF_CALL_WITH_BOUNDS 	= 1 << 8,
+    GF_CALL_MUST_TAIL_CALL	= 1 << 9,
     GF_OMP_PARALLEL_COMBINED	= 1 << 0,
     GF_OMP_PARALLEL_GRID_PHONY = 1 << 1,
     GF_OMP_TASK_TASKLOOP	= 1 << 0,
@@ -3209,6 +3210,24 @@ gimple_call_tail_p (gcall *s)
   return (s->subcode & GF_CALL_TAILCALL) != 0;
 }
 
+/* FIXME.  */
+
+static inline void
+gimple_call_set_must_tail (gcall *s, bool must_tail_p)
+{
+  if (must_tail_p)
+    s->subcode |= GF_CALL_MUST_TAIL_CALL;
+  else
+    s->subcode &= ~GF_CALL_MUST_TAIL_CALL;
+}
+
+/* FIXME.  */
+
+static inline bool
+gimple_call_must_tail_p (gcall *s)
+{
+  return (s->subcode & GF_CALL_MUST_TAIL_CALL) != 0;
+}
 
 /* If RETURN_SLOT_OPT_P is true mark GIMPLE_CALL S as valid for return
    slot optimization.  This transformation uses the target of the call
diff --git a/gcc/print-tree.c b/gcc/print-tree.c
index aa6593f..58b6118 100644
--- a/gcc/print-tree.c
+++ b/gcc/print-tree.c
@@ -324,7 +324,7 @@ print_node (FILE *file, const char *prefix, tree node, int indent)
   if (TREE_PROTECTED (node))
     fputs (" protected", file);
   if (TREE_STATIC (node))
-    fputs (" static", file);
+    fputs (code == CALL_EXPR ? " must-tail-call" : " static", file);
   if (TREE_DEPRECATED (node))
     fputs (" deprecated", file);
   if (TREE_VISITED (node))
diff --git a/gcc/testsuite/gcc.dg/plugin/must-tail-call-1.c b/gcc/testsuite/gcc.dg/plugin/must-tail-call-1.c
new file mode 100644
index 0000000..6d74955
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/plugin/must-tail-call-1.c
@@ -0,0 +1,22 @@
+extern void abort (void);
+
+int __attribute__((noinline,noclone))
+callee (int i)
+{
+  return i * i;
+}
+
+int __attribute__((noinline,noclone))
+caller (int i)
+{
+  return callee (i + 1);
+}
+
+int
+main (int argc, const char **argv)
+{
+  int result = caller (5);
+  if (result != 36)
+    abort ();
+  return 0;
+}
diff --git a/gcc/testsuite/gcc.dg/plugin/must-tail-call-2.c b/gcc/testsuite/gcc.dg/plugin/must-tail-call-2.c
new file mode 100644
index 0000000..264a792
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/plugin/must-tail-call-2.c
@@ -0,0 +1,15 @@
+struct box { char dummy[64]; int i; };
+
+struct box __attribute__((noinline,noclone))
+returns_struct (int i)
+{
+  struct box b;
+  b.i = i * i;
+  return b;
+}
+
+int __attribute__((noinline,noclone))
+test_1 (int i)
+{
+  return returns_struct (i * 5).i; /* { dg-error "cannot tail-call: FIXME: structure_value_addr" } */
+}
diff --git a/gcc/testsuite/gcc.dg/plugin/must_tail_call_plugin.c b/gcc/testsuite/gcc.dg/plugin/must_tail_call_plugin.c
new file mode 100644
index 0000000..5294f28
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/plugin/must_tail_call_plugin.c
@@ -0,0 +1,76 @@
+/* { dg-options "-O" } */
+
+/* Mark all CALL_EXPRs not within "main" as requiring tail-call. */
+
+#include "gcc-plugin.h"
+#include "config.h"
+#include "system.h"
+#include "coretypes.h"
+#include "tm.h"
+#include "tree.h"
+#include "stringpool.h"
+#include "toplev.h"
+#include "basic-block.h"
+#include "hash-table.h"
+#include "vec.h"
+#include "ggc.h"
+#include "basic-block.h"
+#include "tree-ssa-alias.h"
+#include "internal-fn.h"
+#include "gimple-fold.h"
+#include "tree-eh.h"
+#include "gimple-expr.h"
+#include "is-a.h"
+#include "tree.h"
+#include "tree-pass.h"
+#include "intl.h"
+#include "plugin-version.h"
+
+int plugin_is_GPL_compatible;
+
+tree
+cb_walk_tree_fn (tree * tp, int * walk_subtrees,
+		 void * data ATTRIBUTE_UNUSED)
+{
+  if (TREE_CODE (*tp) != CALL_EXPR)
+    return NULL_TREE;
+
+  tree call_expr = *tp;
+
+  /* Forcibly mark the CALL_EXPR as requiring tail-call optimization.  */
+  CALL_EXPR_MUST_TAIL_CALL (call_expr) = 1;
+  
+  return NULL_TREE;
+}
+
+static void
+callback (void *gcc_data, void *user_data)
+{
+  tree fndecl = (tree)gcc_data;
+  gcc_assert (TREE_CODE (fndecl) == FUNCTION_DECL);
+
+  /* Don't mark calls inside "main".  */
+  tree decl_name = DECL_NAME (fndecl);
+  if (decl_name)
+    if (0 == strcmp (IDENTIFIER_POINTER (decl_name), "main"))
+      return;
+
+  walk_tree (&DECL_SAVED_TREE (fndecl), cb_walk_tree_fn, NULL, NULL);
+}
+
+int
+plugin_init (struct plugin_name_args *plugin_info,
+	     struct plugin_gcc_version *version)
+{
+  const char *plugin_name = plugin_info->base_name;
+
+  if (!plugin_default_version_check (version, &gcc_version))
+    return 1;
+
+  register_callback (plugin_name,
+		     PLUGIN_PRE_GENERICIZE,
+		     callback,
+		     NULL);
+
+  return 0;
+}
diff --git a/gcc/testsuite/gcc.dg/plugin/plugin.exp b/gcc/testsuite/gcc.dg/plugin/plugin.exp
index fd1e98e..62f6797 100644
--- a/gcc/testsuite/gcc.dg/plugin/plugin.exp
+++ b/gcc/testsuite/gcc.dg/plugin/plugin.exp
@@ -74,6 +74,9 @@ set plugin_test_list [list \
     { location_overflow_plugin.c \
 	  location-overflow-test-1.c \
 	  location-overflow-test-2.c } \
+    { must_tail_call_plugin.c \
+	  must-tail-call-1.c \
+	  must-tail-call-2.c } \
 ]
 
 foreach plugin_test $plugin_test_list {
diff --git a/gcc/tree-core.h b/gcc/tree-core.h
index ea832fa..078e006 100644
--- a/gcc/tree-core.h
+++ b/gcc/tree-core.h
@@ -998,6 +998,9 @@ struct GTY(()) tree_base {
        SSA_NAME_ANTI_RANGE_P in
 	   SSA_NAME
 
+       MUST_TAIL_CALL in
+	   CALL_EXPR
+
    public_flag:
 
        TREE_OVERFLOW in
diff --git a/gcc/tree.h b/gcc/tree.h
index 37324bf..bfbb646 100644
--- a/gcc/tree.h
+++ b/gcc/tree.h
@@ -661,6 +661,10 @@ extern void omp_clause_range_check_failed (const_tree, const char *, int,
 #define CALL_EXPR_TAILCALL(NODE) \
   (CALL_EXPR_CHECK (NODE)->base.addressable_flag)
 
+/* FIXME.  */
+#define CALL_EXPR_MUST_TAIL_CALL(NODE) \
+  (CALL_EXPR_CHECK (NODE)->base.static_flag)
+
 /* Used as a temporary field on a CASE_LABEL_EXPR to indicate that the
    CASE_LOW operand has been processed.  */
 #define CASE_LOW_SEEN(NODE) \
-- 
1.8.5.3