[PATCH v2 09/10] Introduce strub: strubm (mode assignment) pass

[Alexandre Oliva <oliva@adacore.com>]

This middle fragment of ipa-strub.cc covers strub mode selection and
assignment logic, and most of the pass that performs that assignment.

+/* Return TRUE iff NODE calls builtin va_start.  */
+
+static bool
+calls_builtin_va_start_p (cgraph_node *node)
+{
+  bool result = false;
+
+  for (cgraph_edge *e = node->callees; e; e = e->next_callee)
+    {
+      tree cdecl = e->callee->decl;
+      if (fndecl_built_in_p (cdecl, BUILT_IN_VA_START))
+	return true;
+    }
+
+  return result;
+}
+
+/* Return TRUE iff NODE calls builtin apply_args, and optionally REPORT it.  */
+
+static bool
+calls_builtin_apply_args_p (cgraph_node *node, bool report = false)
+{
+  bool result = false;
+
+  for (cgraph_edge *e = node->callees; e; e = e->next_callee)
+    {
+      tree cdecl = e->callee->decl;
+      if (!fndecl_built_in_p (cdecl, BUILT_IN_APPLY_ARGS))
+	continue;
+
+      result = true;
+
+      if (!report)
+	break;
+
+      sorry_at (gimple_location (e->call_stmt),
+		"at-calls %<strub%> does not support call to %qD",
+		cdecl);
+    }
+
+  return result;
+}
+
+/* Return TRUE iff NODE carries the always_inline attribute.  */
+
+static inline bool
+strub_always_inline_p (cgraph_node *node)
+{
+  return lookup_attribute ("always_inline", DECL_ATTRIBUTES (node->decl));
+}
+
+/* Return TRUE iff NODE is potentially eligible for any strub-enabled mode, and
+   optionally REPORT the reasons for ineligibility.  */
+
+static inline bool
+can_strub_p (cgraph_node *node, bool report = false)
+{
+  bool result = true;
+
+  if (!report && strub_always_inline_p (node))
+    return result;
+
+  if (lookup_attribute ("noipa", DECL_ATTRIBUTES (node->decl)))
+    {
+      result = false;
+
+      if (!report)
+	return result;
+
+      sorry_at (DECL_SOURCE_LOCATION (node->decl),
+		"%qD is not eligible for %<strub%>"
+		" because of attribute %<noipa%>",
+		node->decl);
+    }
+
+  /* We can't, and don't want to vectorize the watermark and other
+     strub-introduced parms.  */
+  if (lookup_attribute ("simd", DECL_ATTRIBUTES (node->decl)))
+    {
+      result = false;
+
+      if (!report)
+	return result;
+
+      sorry_at (DECL_SOURCE_LOCATION (node->decl),
+		"%qD is not eligible for %<strub%>"
+		" because of attribute %<simd%>",
+		node->decl);
+    }
+
+  return result;
+}
+
+/* Return TRUE iff NODE is eligible for at-calls strub, and optionally REPORT
+   the reasons for ineligibility.  Besides general non-eligibility for
+   strub-enabled modes, at-calls rules out calling builtin apply_args.  */
+
+static bool
+can_strub_at_calls_p (cgraph_node *node, bool report = false)
+{
+  bool result = !report || can_strub_p (node, report);
+
+  if (!result && !report)
+    return result;
+
+  return !calls_builtin_apply_args_p (node, report);
+}
+
+/* Return TRUE iff the called function (pointer or, if available,
+   decl) undergoes a significant type conversion for the call.  Strub
+   mode changes between function types, and other non-useless type
+   conversions, are regarded as significant.  When the function type
+   is overridden, the effective strub mode for the call is that of the
+   call fntype, rather than that of the pointer or of the decl.
+   Functions called with type overrides cannot undergo type changes;
+   it's as if their address was taken, so they're considered
+   non-viable for implicit at-calls strub mode.  */
+
+static inline bool
+strub_call_fntype_override_p (const gcall *gs)
+{
+  if (gimple_call_internal_p (gs))
+    return false;
+  tree fn_type = TREE_TYPE (TREE_TYPE (gimple_call_fn (gs)));
+  if (tree decl = gimple_call_fndecl (gs))
+    fn_type = TREE_TYPE (decl);
+
+  /* We do NOT want to take the mode from the decl here.  This
+     function is used to tell whether we can change the strub mode of
+     a function, and whether the effective mode for the call is to be
+     taken from the decl or from an overrider type.  When the strub
+     mode is explicitly declared, or overridden with a type cast, the
+     difference will be noticed in function types.  However, if the
+     strub mode is implicit due to e.g. strub variables or -fstrub=*
+     command-line flags, we will adjust call types along with function
+     types.  In either case, the presence of type or strub mode
+     overriders in calls will prevent a function from having its strub
+     modes changed in ways that would imply type changes, but taking
+     strub modes from decls would defeat this, since we set strub
+     modes and then call this function to tell whether the original
+     type was overridden to decide whether to adjust the call.  We
+     need the answer to be about the type, not the decl.  */
+  enum strub_mode mode = get_strub_mode_from_type (fn_type);
+  return (get_strub_mode_from_type (gs->u.fntype) != mode
+	  || !useless_type_conversion_p (gs->u.fntype, fn_type));
+}
+
+/* Return TRUE iff NODE is called directly with a type override.  */
+
+static bool
+called_directly_with_type_override_p (cgraph_node *node, void *)
+{
+  for (cgraph_edge *e = node->callers; e; e = e->next_caller)
+    if (strub_call_fntype_override_p (e->call_stmt))
+      return true;
+
+  return false;
+}
+
+/* Return TRUE iff NODE or any other nodes aliased to it are called
+   with type overrides.  We can't safely change the type of such
+   functions.  */
+
+static bool
+called_with_type_override_p (cgraph_node *node)
+{
+  return (node->call_for_symbol_thunks_and_aliases
+	  (called_directly_with_type_override_p, NULL, true, true));
+}
+
+/* Symbolic macro for the max number of arguments that internal strub may add to
+   a function.  */
+
+#define STRUB_INTERNAL_MAX_EXTRA_ARGS 3
+
+/* We can't perform internal strubbing if the function body involves certain
+   features:
+
+   - a non-default __builtin_va_start (e.g. x86's __builtin_ms_va_start) is
+   currently unsupported because we can't discover the corresponding va_copy and
+   va_end decls in the wrapper, and we don't convey the alternate variable
+   arguments ABI to the modified wrapped function.  The default
+   __builtin_va_start is supported by calling va_start/va_end at the wrapper,
+   that takes variable arguments, passing a pointer to the va_list object to the
+   wrapped function, that runs va_copy from it where the original function ran
+   va_start.
+
+   __builtin_next_arg is currently unsupported because the wrapped function
+   won't be a variable argument function.  We could process it in the wrapper,
+   that remains a variable argument function, and replace calls in the wrapped
+   body, but we currently don't.
+
+   __builtin_return_address is rejected because it's generally used when the
+   actual caller matters, and introducing a wrapper breaks such uses as those in
+   the unwinder.  */
+
+static bool
+can_strub_internally_p (cgraph_node *node, bool report = false)
+{
+  bool result = !report || can_strub_p (node, report);
+
+  if (!result && !report)
+    return result;
+
+  if (!report && strub_always_inline_p (node))
+    return result;
+
+  /* Since we're not changing the function identity proper, just
+     moving its full implementation, we *could* disable
+     fun->cannot_be_copied_reason and/or temporarily drop a noclone
+     attribute, but we'd have to prevent remapping of the labels.  */
+  if (lookup_attribute ("noclone", DECL_ATTRIBUTES (node->decl)))
+    {
+      result = false;
+
+      if (!report)
+	return result;
+
+      sorry_at (DECL_SOURCE_LOCATION (node->decl),
+		"%qD is not eligible for internal %<strub%>"
+		" because of attribute %<noclone%>",
+		node->decl);
+    }
+
+  if (node->has_gimple_body_p ())
+    {
+      for (cgraph_edge *e = node->callees; e; e = e->next_callee)
+	{
+	  tree cdecl = e->callee->decl;
+	  if (!((fndecl_built_in_p (cdecl, BUILT_IN_VA_START)
+		 && cdecl != builtin_decl_explicit (BUILT_IN_VA_START))
+		|| fndecl_built_in_p (cdecl, BUILT_IN_NEXT_ARG)
+		|| fndecl_built_in_p (cdecl, BUILT_IN_RETURN_ADDRESS)))
+	    continue;
+
+	  result = false;
+
+	  if (!report)
+	    return result;
+
+	  sorry_at (gimple_location (e->call_stmt),
+		    "%qD is not eligible for internal %<strub%> "
+		    "because it calls %qD",
+		    node->decl, cdecl);
+	}
+
+      struct function *fun = DECL_STRUCT_FUNCTION (node->decl);
+      if (fun->has_nonlocal_label)
+	{
+	  result = false;
+
+	  if (!report)
+	    return result;
+
+	  sorry_at (DECL_SOURCE_LOCATION (node->decl),
+		    "%qD is not eligible for internal %<strub%> "
+		    "because it contains a non-local goto target",
+		    node->decl);
+	}
+
+      if (fun->has_forced_label_in_static)
+	{
+	  result = false;
+
+	  if (!report)
+	    return result;
+
+	  sorry_at (DECL_SOURCE_LOCATION (node->decl),
+		    "%qD is not eligible for internal %<strub%> "
+		    "because the address of a local label escapes",
+		    node->decl);
+	}
+
+      /* Catch any other case that would prevent versioning/cloning
+	 so as to also have it covered above.  */
+      gcc_checking_assert (!result /* || !node->has_gimple_body_p () */
+			   || tree_versionable_function_p (node->decl));
+
+
+      /* Label values references are not preserved when copying.  If referenced
+	 in nested functions, as in 920415-1.c and 920721-4.c their decls get
+	 remapped independently.  The exclusion below might be too broad, in
+	 that we might be able to support correctly cases in which the labels
+	 are only used internally in a function, but disconnecting forced labels
+	 from their original declarations is undesirable in general.  */
+      basic_block bb;
+      FOR_EACH_BB_FN (bb, DECL_STRUCT_FUNCTION (node->decl))
+	for (gimple_stmt_iterator gsi = gsi_start_bb (bb);
+	     !gsi_end_p (gsi); gsi_next (&gsi))
+	  {
+	    glabel *label_stmt = dyn_cast <glabel *> (gsi_stmt (gsi));
+	    tree target;
+
+	    if (!label_stmt)
+	      break;
+
+	    target = gimple_label_label (label_stmt);
+
+	    if (!FORCED_LABEL (target))
+	      continue;
+
+	    result = false;
+
+	    if (!report)
+	      return result;
+
+	    sorry_at (gimple_location (label_stmt),
+		      "internal %<strub%> does not support forced labels");
+	  }
+    }
+
+  if (list_length (TYPE_ARG_TYPES (TREE_TYPE (node->decl)))
+      >= (((HOST_WIDE_INT) 1 << IPA_PARAM_MAX_INDEX_BITS)
+	  - STRUB_INTERNAL_MAX_EXTRA_ARGS))
+    {
+      result = false;
+
+      if (!report)
+	return result;
+
+      sorry_at (DECL_SOURCE_LOCATION (node->decl),
+		"%qD has too many arguments for internal %<strub%>",
+		node->decl);
+    }
+
+  return result;
+}
+
+/* Return TRUE iff NODE has any strub-requiring local variable, or accesses (as
+   in reading) any variable through a strub-requiring type.  */
+
+static bool
+strub_from_body_p (cgraph_node *node)
+{
+  if (!node->has_gimple_body_p ())
+    return false;
+
+  /* If any local variable is marked for strub...  */
+  unsigned i;
+  tree var;
+  FOR_EACH_LOCAL_DECL (DECL_STRUCT_FUNCTION (node->decl),
+		       i, var)
+    if (get_strub_mode_from_type (TREE_TYPE (var))
+	!= STRUB_DISABLED)
+      return true;
+
+  /* Now scan the body for loads with strub-requiring types.
+     ??? Compound types don't propagate the strub requirement to
+     component types.  */
+  basic_block bb;
+  FOR_EACH_BB_FN (bb, DECL_STRUCT_FUNCTION (node->decl))
+    for (gimple_stmt_iterator gsi = gsi_start_bb (bb);
+	 !gsi_end_p (gsi); gsi_next (&gsi))
+      {
+	gimple *stmt = gsi_stmt (gsi);
+
+	if (!gimple_assign_load_p (stmt))
+	  continue;
+
+	tree rhs = gimple_assign_rhs1 (stmt);
+	if (get_strub_mode_from_type (TREE_TYPE (rhs))
+	    != STRUB_DISABLED)
+	  return true;
+      }
+
+  return false;
+}
+
+/* Return TRUE iff node is associated with a builtin that should be callable
+   from strub contexts.  */
+
+static inline bool
+strub_callable_builtin_p (cgraph_node *node)
+{
+  if (DECL_BUILT_IN_CLASS (node->decl) != BUILT_IN_NORMAL)
+    return false;
+
+  enum built_in_function fcode = DECL_FUNCTION_CODE (node->decl);
+
+  switch (fcode)
+    {
+    case BUILT_IN_NONE:
+      gcc_unreachable ();
+
+      /* This temporarily allocates stack for the call, and we can't reasonably
+	 update the watermark for that.  Besides, we don't check the actual call
+	 target, nor its signature, and it seems to be overkill to as much as
+	 try to do so.  */
+    case BUILT_IN_APPLY:
+      return false;
+
+      /* Conversely, this shouldn't be called from within strub contexts, since
+	 the caller may have had its signature modified.  STRUB_INTERNAL is ok,
+	 the call will remain in the STRUB_WRAPPER, and removed from the
+	 STRUB_WRAPPED clone.  */
+    case BUILT_IN_APPLY_ARGS:
+      return false;
+
+      /* ??? Make all other builtins callable.  We wish to make any builtin call
+	 the compiler might introduce on its own callable.  Anything that is
+	 predictable enough as to be known not to allow stack data that should
+	 be strubbed to unintentionally escape to non-strub contexts can be
+	 allowed, and pretty much every builtin appears to fit this description.
+	 The exceptions to this rule seem to be rare, and only available as
+	 explicit __builtin calls, so let's keep it simple and allow all of
+	 them...  */
+    default:
+      return true;
+    }
+}
+
+/* Compute the strub mode to be used for NODE.  STRUB_ATTR should be the strub
+   attribute,found for NODE, if any.  */
+
+static enum strub_mode
+compute_strub_mode (cgraph_node *node, tree strub_attr)
+{
+  enum strub_mode req_mode = get_strub_mode_from_attr (strub_attr);
+
+  gcc_checking_assert (flag_strub >= -2 && flag_strub <= 3);
+
+  /* Symbolic encodings of the -fstrub-* flags.  */
+  /* Enable strub when explicitly requested through attributes to functions or
+     variables, reporting errors if the requests cannot be satisfied.  */
+  const bool strub_flag_auto = flag_strub < 0;
+  /* strub_flag_auto with strub call verification; without this, functions are
+     implicitly callable.  */
+  const bool strub_flag_strict = flag_strub < -1;
+  /* Disable strub altogether, ignore attributes entirely.  */
+  const bool strub_flag_disabled = flag_strub == 0;
+  /* On top of _auto, also enable strub implicitly for functions that can
+     safely undergo at-calls strubbing.  Internal mode will still be used in
+     functions that request it explicitly with attribute strub(2), or when the
+     function body requires strubbing and at-calls strubbing is not viable.  */
+  const bool strub_flag_at_calls = flag_strub == 1;
+  /* On top of default, also enable strub implicitly for functions that can
+     safely undergo internal strubbing.  At-calls mode will still be used in
+     functions that requiest it explicitly with attribute strub() or strub(1),
+     or when the function body requires strubbing and internal strubbing is not
+     viable.  */
+  const bool strub_flag_internal = flag_strub == 2;
+  /* On top of default, also enable strub implicitly for functions that can
+     safely undergo strubbing in either mode.  When both modes are viable,
+     at-calls is preferred.  */
+  const bool strub_flag_either = flag_strub == 3;
+  /* Besides the default behavior, enable strub implicitly for all viable
+     functions.  */
+  const bool strub_flag_viable = flag_strub > 0;
+
+  /* The consider_* variables should be TRUE if selecting the corresponding
+     strub modes would be consistent with requests from attributes and command
+     line flags.  Attributes associated with functions pretty much mandate a
+     selection, and should report an error if not satisfied; strub_flag_auto
+     implicitly enables some viable strub mode if that's required by references
+     to variables marked for strub; strub_flag_viable enables strub if viable
+     (even when favoring one mode, body-requested strub can still be satisfied
+     by either mode), and falls back to callable, silently unless variables
+     require strubbing.  */
+
+  const bool consider_at_calls
+    = (!strub_flag_disabled
+       && (strub_attr
+	   ? req_mode == STRUB_AT_CALLS
+	   : true));
+  const bool consider_internal
+    = (!strub_flag_disabled
+       && (strub_attr
+	   ? req_mode == STRUB_INTERNAL
+	   : true));
+
+  const bool consider_callable
+    = (!strub_flag_disabled
+       && (strub_attr
+	   ? req_mode == STRUB_CALLABLE
+	   : (!strub_flag_strict
+	      || strub_callable_builtin_p (node))));
+
+  /* This is a shorthand for either strub-enabled mode.  */
+  const bool consider_strub
+    = (consider_at_calls || consider_internal);
+
+  /* We can cope with always_inline functions even with noipa and noclone,
+     because we just leave them alone.  */
+  const bool is_always_inline
+    = strub_always_inline_p (node);
+
+  /* Strubbing in general, and each specific strub mode, may have its own set of
+     requirements.  We require noipa for strubbing, either because of cloning
+     required for internal strub, or because of caller enumeration required for
+     at-calls strub.  We don't consider the at-calls mode eligible if it's not
+     even considered, it has no further requirements.  Internal mode requires
+     cloning and the absence of certain features in the body and, like at-calls,
+     it's not eligible if it's not even under consideration.
+
+     ??? Do we need target hooks for further constraints?  E.g., x86's
+     "interrupt" attribute breaks internal strubbing because the wrapped clone
+     carries the attribute and thus isn't callable; in this case, we could use a
+     target hook to adjust the clone instead.  */
+  const bool strub_eligible
+    = (consider_strub
+       && (is_always_inline || can_strub_p (node)));
+  const bool at_calls_eligible
+    = (consider_at_calls && strub_eligible
+       && can_strub_at_calls_p (node));
+  const bool internal_eligible
+    = (consider_internal && strub_eligible
+       && (is_always_inline
+	   || can_strub_internally_p (node)));
+
+  /* In addition to the strict eligibility requirements, some additional
+     constraints are placed on implicit selection of certain modes.  These do
+     not prevent the selection of a mode if explicitly specified as part of a
+     function interface (the strub attribute), but they may prevent modes from
+     being selected by the command line or by function bodies.  The only actual
+     constraint is on at-calls mode: since we change the function's exposed
+     signature, we won't do it implicitly if the function can possibly be used
+     in ways that do not expect the signature change, e.g., if the function is
+     available to or interposable by other units, if its address is taken,
+     etc.  */
+  const bool at_calls_viable
+    = (at_calls_eligible
+       && (strub_attr
+	   || (node->has_gimple_body_p ()
+	       && (!node->externally_visible
+		   || (node->binds_to_current_def_p ()
+		       && node->can_be_local_p ()))
+	       && node->only_called_directly_p ()
+	       && !called_with_type_override_p (node))));
+  const bool internal_viable
+    = (internal_eligible);
+
+  /* Shorthand.  */
+  const bool strub_viable
+    = (at_calls_viable || internal_viable);
+
+  /* We wish to analyze the body, to look for implicit requests for strub, both
+     to implicitly enable it when the body calls for it, and to report errors if
+     the body calls for it but neither mode is viable (even if that follows from
+     non-eligibility because of the explicit specification of some non-strubbing
+     mode).  We can refrain from scanning the body only in rare circumstances:
+     when strub is enabled by a function attribute (scanning might be redundant
+     in telling us to also enable it), and when we are enabling strub implicitly
+     but there are non-viable modes: we want to know whether strubbing is
+     required, to fallback to another mode, even if we're only enabling a
+     certain mode, or, when either mode would do, to report an error if neither
+     happens to be viable.  */
+  const bool analyze_body
+    = (strub_attr
+       ? !consider_strub
+       : (strub_flag_auto
+	  || (strub_flag_viable && (!at_calls_viable && !internal_viable))
+	  || (strub_flag_either && !strub_viable)));
+
+  /* Cases in which strubbing is enabled or disabled by strub_flag_auto.
+     Unsatisfiable requests ought to be reported.  */
+  const bool strub_required
+    = ((strub_attr && consider_strub)
+       || (analyze_body && strub_from_body_p (node)));
+
+  /* Besides the required cases, we want to abide by the requests to enabling on
+     an if-viable basis.  */
+  const bool strub_enable
+    = (strub_required
+       || (strub_flag_at_calls && at_calls_viable)
+       || (strub_flag_internal && internal_viable)
+       || (strub_flag_either && strub_viable));
+
+  /* And now we're finally ready to select a mode that abides by the viability
+     and eligibility constraints, and that satisfies the strubbing requirements
+     and requests, subject to the constraints.  If both modes are viable and
+     strub is to be enabled, pick STRUB_AT_CALLS unless STRUB_INTERNAL was named
+     as preferred.  */
+  const enum strub_mode mode
+    = ((strub_enable && is_always_inline)
+       ? (strub_required ? STRUB_INLINABLE : STRUB_CALLABLE)
+       : (strub_enable && internal_viable
+	  && (strub_flag_internal || !at_calls_viable))
+       ? STRUB_INTERNAL
+       : (strub_enable && at_calls_viable)
+       ? (strub_required && !strub_attr
+	  ? STRUB_AT_CALLS_OPT
+	  : STRUB_AT_CALLS)
+       : consider_callable
+       ? STRUB_CALLABLE
+       : STRUB_DISABLED);
+
+  switch (mode)
+    {
+    case STRUB_CALLABLE:
+      if (is_always_inline)
+	break;
+      /* Fall through.  */
+
+    case STRUB_DISABLED:
+      if (strub_enable && !strub_attr)
+	{
+	  gcc_checking_assert (analyze_body);
+	  error_at (DECL_SOURCE_LOCATION (node->decl),
+		    "%qD requires %<strub%>,"
+		    " but no viable %<strub%> mode was found",
+		    node->decl);
+	  break;
+	}
+      /* Fall through.  */
+
+    case STRUB_AT_CALLS:
+    case STRUB_INTERNAL:
+    case STRUB_INLINABLE:
+      /* Differences from an mode requested through a function attribute are
+	 reported in set_strub_mode_to.  */
+      break;
+
+    case STRUB_AT_CALLS_OPT:
+      /* Functions that select this mode do so because of references to strub
+	 variables.  Even if we choose at-calls as an optimization, the
+	 requirements for internal strub must still be satisfied.  Optimization
+	 options may render implicit at-calls strub not viable (-O0 sets
+	 force_output for static non-inline functions), and it would not be good
+	 if changing optimization options turned a well-formed into an
+	 ill-formed one.  */
+      if (!internal_viable)
+	can_strub_internally_p (node, true);
+      break;
+
+    case STRUB_WRAPPED:
+    case STRUB_WRAPPER:
+    default:
+      gcc_unreachable ();
+    }
+
+  return mode;
+}
+
+/* Set FNDT's strub mode to MODE; FNDT may be a function decl or
+   function type.  If OVERRIDE, do not check whether a mode is already
+   set.  */
+
+static void
+strub_set_fndt_mode_to (tree fndt, enum strub_mode mode, bool override)
+{
+  gcc_checking_assert (override
+		       || !(DECL_P (fndt)
+			    ? get_strub_attr_from_decl (fndt)
+			    : get_strub_attr_from_type (fndt)));
+
+  tree attr = tree_cons (get_identifier ("strub"),
+			 get_strub_mode_attr_value (mode),
+			 NULL_TREE);
+  tree *attrp = NULL;
+  if (DECL_P (fndt))
+    {
+      gcc_checking_assert (FUNC_OR_METHOD_TYPE_P (TREE_TYPE (fndt)));
+      attrp = &DECL_ATTRIBUTES (fndt);
+    }
+  else if (FUNC_OR_METHOD_TYPE_P (fndt))
+    attrp = &TYPE_ATTRIBUTES (fndt);
+  else
+    gcc_unreachable ();
+
+  TREE_CHAIN (attr) = *attrp;
+  *attrp = attr;
+}
+
+/* Set FNDT's strub mode to callable.
+   FNDT may be a function decl or a function type.  */
+
+void
+strub_make_callable (tree fndt)
+{
+  strub_set_fndt_mode_to (fndt, STRUB_CALLABLE, false);
+}
+
+/* Set NODE to strub MODE.  Report incompatibilities between MODE and the mode
+   requested through explicit attributes, and cases of non-eligibility.  */
+
+static void
+set_strub_mode_to (cgraph_node *node, enum strub_mode mode)
+{
+  tree attr = get_strub_attr_from_decl (node->decl);
+  enum strub_mode req_mode = get_strub_mode_from_attr (attr);
+
+  if (attr)
+    {
+      /* Check for and report incompatible mode changes.  */
+      if (mode != req_mode
+	  && !(req_mode == STRUB_INTERNAL
+	       && (mode == STRUB_WRAPPED
+		   || mode == STRUB_WRAPPER))
+	  && !((req_mode == STRUB_INTERNAL
+		|| req_mode == STRUB_AT_CALLS
+		|| req_mode == STRUB_CALLABLE)
+	       && mode == STRUB_INLINABLE))
+	{
+	  error_at (DECL_SOURCE_LOCATION (node->decl),
+		    "%<strub%> mode %qE selected for %qD, when %qE was requested",
+		    get_strub_mode_attr_parm (mode),
+		    node->decl,
+		    get_strub_mode_attr_parm (req_mode));
+	  if (node->alias)
+	    {
+	      cgraph_node *target = node->ultimate_alias_target ();
+	      if (target != node)
+		error_at (DECL_SOURCE_LOCATION (target->decl),
+			  "the incompatible selection was determined"
+			  " by ultimate alias target %qD",
+			  target->decl);
+	    }
+
+	  /* Report any incompatibilities with explicitly-requested strub.  */
+	  switch (req_mode)
+	    {
+	    case STRUB_AT_CALLS:
+	      can_strub_at_calls_p (node, true);
+	      break;
+
+	    case STRUB_INTERNAL:
+	      can_strub_internally_p (node, true);
+	      break;
+
+	    default:
+	      break;
+	    }
+	}
+
+      /* Drop any incompatible strub attributes leading the decl attribute
+	 chain.  Return if we find one with the mode we need.  */
+      for (;;)
+	{
+	  if (mode == req_mode)
+	    return;
+
+	  if (DECL_ATTRIBUTES (node->decl) != attr)
+	    break;
+
+	  DECL_ATTRIBUTES (node->decl) = TREE_CHAIN (attr);
+	  attr = get_strub_attr_from_decl (node->decl);
+	  if (!attr)
+	    break;
+
+	  req_mode = get_strub_mode_from_attr (attr);
+	}
+    }
+  else if (mode == req_mode)
+    return;
+
+  strub_set_fndt_mode_to (node->decl, mode, attr);
+}
+
+/* Compute and set NODE's strub mode.  */
+
+static void
+set_strub_mode (cgraph_node *node)
+{
+  tree attr = get_strub_attr_from_decl (node->decl);
+
+  if (attr)
+    switch (get_strub_mode_from_attr (attr))
+      {
+	/* These can't have been requested through user attributes, so we must
+	   have already gone through them.  */
+      case STRUB_WRAPPER:
+      case STRUB_WRAPPED:
+      case STRUB_INLINABLE:
+      case STRUB_AT_CALLS_OPT:
+	return;
+
+      case STRUB_DISABLED:
+      case STRUB_AT_CALLS:
+      case STRUB_INTERNAL:
+      case STRUB_CALLABLE:
+	break;
+
+      default:
+	gcc_unreachable ();
+      }
+
+  cgraph_node *xnode = node;
+  if (node->alias)
+    xnode = node->ultimate_alias_target ();
+  /* Weakrefs may remain unresolved (the above will return node) if
+     their targets are not defined, so make sure we compute a strub
+     mode for them, instead of defaulting to STRUB_DISABLED and
+     rendering them uncallable.  */
+  enum strub_mode mode = (xnode != node && !xnode->alias
+			  ? get_strub_mode (xnode)
+			  : compute_strub_mode (node, attr));
+
+  set_strub_mode_to (node, mode);
+}
+
+\f
+/* Non-strub functions shouldn't be called from within strub contexts,
+   except through callable ones.  Always inline strub functions can
+   only be called from strub functions.  */
+
+static bool
+strub_callable_from_p (strub_mode caller_mode, strub_mode callee_mode)
+{
+  switch (caller_mode)
+    {
+    case STRUB_WRAPPED:
+    case STRUB_AT_CALLS_OPT:
+    case STRUB_AT_CALLS:
+    case STRUB_INTERNAL:
+    case STRUB_INLINABLE:
+      break;
+
+    case STRUB_WRAPPER:
+    case STRUB_DISABLED:
+    case STRUB_CALLABLE:
+      return callee_mode != STRUB_INLINABLE;
+
+    default:
+      gcc_unreachable ();
+    }
+
+  switch (callee_mode)
+    {
+    case STRUB_WRAPPED:
+    case STRUB_AT_CALLS:
+    case STRUB_INLINABLE:
+      break;
+
+    case STRUB_AT_CALLS_OPT:
+    case STRUB_INTERNAL:
+    case STRUB_WRAPPER:
+      return (flag_strub >= -1);
+
+    case STRUB_DISABLED:
+      return false;
+
+    case STRUB_CALLABLE:
+      break;
+
+    default:
+      gcc_unreachable ();
+    }
+
+  return true;
+}
+
+/* Return TRUE iff CALLEE can be inlined into CALLER.  We wish to avoid inlining
+   WRAPPED functions back into their WRAPPERs.  More generally, we wish to avoid
+   inlining strubbed functions into non-strubbed ones.  CALLER doesn't have to
+   be an immediate caller of CALLEE: the immediate caller may have already been
+   cloned for inlining, and then CALLER may be further up the original call
+   chain.  ???  It would be nice if our own caller would retry inlining callee
+   if caller gets inlined.  */
+
+bool
+strub_inlinable_to_p (cgraph_node *callee, cgraph_node *caller)
+{
+  strub_mode callee_mode = get_strub_mode (callee);
+
+  switch (callee_mode)
+    {
+    case STRUB_WRAPPED:
+    case STRUB_AT_CALLS:
+    case STRUB_INTERNAL:
+    case STRUB_INLINABLE:
+    case STRUB_AT_CALLS_OPT:
+      break;
+
+    case STRUB_WRAPPER:
+    case STRUB_DISABLED:
+    case STRUB_CALLABLE:
+      /* When we consider inlining, we've already verified callability, so we
+	 can even inline callable and then disabled into a strub context.  That
+	 will get strubbed along with the context, so it's hopefully not a
+	 problem.  */
+      return true;
+
+    default:
+      gcc_unreachable ();
+    }
+
+  strub_mode caller_mode = get_strub_mode (caller);
+
+  switch (caller_mode)
+    {
+    case STRUB_WRAPPED:
+    case STRUB_AT_CALLS:
+    case STRUB_INTERNAL:
+    case STRUB_INLINABLE:
+    case STRUB_AT_CALLS_OPT:
+      return true;
+
+    case STRUB_WRAPPER:
+    case STRUB_DISABLED:
+    case STRUB_CALLABLE:
+      break;
+
+    default:
+      gcc_unreachable ();
+    }
+
+  return false;
+}
+
+/* Check that types T1 and T2 are strub-compatible.  Return 1 if the strub modes
+   are the same, 2 if they are interchangeable, and 0 otherwise.  */
+
+int
+strub_comptypes (tree t1, tree t2)
+{
+  if (TREE_CODE (t1) != TREE_CODE (t2))
+    return 0;
+
+  enum strub_mode m1 = get_strub_mode_from_type (t1);
+  enum strub_mode m2 = get_strub_mode_from_type (t2);
+
+  if (m1 == m2)
+    return 1;
+
+  /* We're dealing with types, so only strub modes that can be selected by
+     attributes in the front end matter.  If either mode is at-calls (for
+     functions) or internal (for variables), the conversion is not
+     compatible.  */
+  bool var_p = !FUNC_OR_METHOD_TYPE_P (t1);
+  enum strub_mode mr = var_p ? STRUB_INTERNAL : STRUB_AT_CALLS;
+  if (m1 == mr || m2 == mr)
+    return 0;
+
+  return 2;
+}
+
+/* Return the effective strub mode used for CALL, and set *TYPEP to
+   the effective type used for the call.  The effective type and mode
+   are those of the callee, unless the call involves a typecast.  */
+
+static enum strub_mode
+effective_strub_mode_for_call (gcall *call, tree *typep)
+{
+  tree type;
+  enum strub_mode mode;
+
+  if (strub_call_fntype_override_p (call))
+    {
+      type = gimple_call_fntype (call);
+      mode = get_strub_mode_from_type (type);
+    }
+  else
+    {
+      type = TREE_TYPE (TREE_TYPE (gimple_call_fn (call)));
+      tree decl = gimple_call_fndecl (call);
+      if (decl)
+	mode = get_strub_mode_from_fndecl (decl);
+      else
+	mode = get_strub_mode_from_type (type);
+    }
+
+  if (typep)
+    *typep = type;
+
+  return mode;
+}
+
+/* Create a distinct copy of the type of NODE's function, and change
+   the fntype of all calls to it with the same main type to the new
+   type.  */
+
+static void
+distinctify_node_type (cgraph_node *node)
+{
+  tree old_type = TREE_TYPE (node->decl);
+  tree new_type = build_distinct_type_copy (old_type);
+  tree new_ptr_type = NULL_TREE;
+
+  /* Remap any calls to node->decl that use old_type, or a variant
+     thereof, to new_type as well.  We don't look for aliases, their
+     declarations will have their types changed independently, and
+     we'll adjust their fntypes then.  */
+  for (cgraph_edge *e = node->callers; e; e = e->next_caller)
+    {
+      tree fnaddr = gimple_call_fn (e->call_stmt);
+      gcc_checking_assert (TREE_CODE (fnaddr) == ADDR_EXPR
+			   && TREE_OPERAND (fnaddr, 0) == node->decl);
+      if (strub_call_fntype_override_p (e->call_stmt))
+	continue;
+      if (!new_ptr_type)
+	new_ptr_type = build_pointer_type (new_type);
+      TREE_TYPE (fnaddr) = new_ptr_type;
+      gimple_call_set_fntype (e->call_stmt, new_type);
+    }
+
+  TREE_TYPE (node->decl) = new_type;
+}
+
+/* Return TRUE iff TYPE and any variants have the same strub mode.  */
+
+static bool
+same_strub_mode_in_variants_p (tree type)
+{
+  enum strub_mode mode = get_strub_mode_from_type (type);
+
+  for (tree other = TYPE_MAIN_VARIANT (type);
+       other != NULL_TREE; other = TYPE_NEXT_VARIANT (other))
+    if (type != other && mode != get_strub_mode_from_type (other))
+      return false;
+
+  /* Check that the canonical type, if set, either is in the same
+     variant chain, or has the same strub mode as type.  Also check
+     the variants of the canonical type.  */
+  if (TYPE_CANONICAL (type)
+      && (TYPE_MAIN_VARIANT (TYPE_CANONICAL (type))
+	  != TYPE_MAIN_VARIANT (type)))
+    {
+      if (mode != get_strub_mode_from_type (TYPE_CANONICAL (type)))
+	return false;
+      else
+	return same_strub_mode_in_variants_p (TYPE_CANONICAL (type));
+    }
+
+  return true;
+}
+
+/* Check that strub functions don't call non-strub functions, and that
+   always_inline strub functions are only called by strub
+   functions.  */
+
+static void
+verify_strub ()
+{
+  cgraph_node *node;
+
+  /* It's expected that check strub-wise pointer type compatibility of variables
+     and of functions is already taken care of by front-ends, on account of the
+     attribute's being marked as affecting type identity and of the creation of
+     distinct types.  */
+
+  /* Check that call targets in strub contexts have strub-callable types.  */
+
+  FOR_EACH_FUNCTION_WITH_GIMPLE_BODY (node)
+  {
+    enum strub_mode caller_mode = get_strub_mode (node);
+
+    for (cgraph_edge *e = node->indirect_calls; e; e = e->next_callee)
+      {
+	gcc_checking_assert (e->indirect_unknown_callee);
+
+	enum strub_mode callee_mode
+	  = effective_strub_mode_for_call (e->call_stmt, NULL);
+
+	if (!strub_callable_from_p (caller_mode, callee_mode))
+	  error_at (gimple_location (e->call_stmt),
+		    "indirect non-%<strub%> call in %<strub%> context %qD",
+		    node->decl);
+      }
+
+    for (cgraph_edge *e = node->callees; e; e = e->next_callee)
+      {
+	gcc_checking_assert (!e->indirect_unknown_callee);
+
+	tree callee_fntype;
+	enum strub_mode callee_mode
+	  = effective_strub_mode_for_call (e->call_stmt, &callee_fntype);
+
+	if (!strub_callable_from_p (caller_mode, callee_mode))
+	  {
+	    if (callee_mode == STRUB_INLINABLE)
+	      error_at (gimple_location (e->call_stmt),
+			"calling %<always_inline%> %<strub%> %qD"
+			" in non-%<strub%> context %qD",
+			e->callee->decl, node->decl);
+	    else if (fndecl_built_in_p (e->callee->decl, BUILT_IN_APPLY_ARGS)
+		     && caller_mode == STRUB_INTERNAL)
+	      /* This is ok, it will be kept in the STRUB_WRAPPER, and removed
+		 from the STRUB_WRAPPED's strub context.  */
+	      continue;
+	    else if (!strub_call_fntype_override_p (e->call_stmt))
+	      error_at (gimple_location (e->call_stmt),
+			"calling non-%<strub%> %qD in %<strub%> context %qD",
+			e->callee->decl, node->decl);
+	    else
+	      error_at (gimple_location (e->call_stmt),
+			"calling %qD using non-%<strub%> type %qT"
+			" in %<strub%> context %qD",
+			e->callee->decl, callee_fntype, node->decl);
+	  }
+      }
+  }
+}
+
+namespace {
+
+/* Define a pass to compute strub modes.  */
+const pass_data pass_data_ipa_strub_mode = {
+  SIMPLE_IPA_PASS,
+  "strubm",
+  OPTGROUP_NONE,
+  TV_NONE,
+  PROP_cfg, // properties_required
+  0,	    // properties_provided
+  0,	    // properties_destroyed
+  0,	    // properties_start
+  0,	    // properties_finish
+};
+
+class pass_ipa_strub_mode : public simple_ipa_opt_pass
+{
+public:
+  pass_ipa_strub_mode (gcc::context *ctxt)
+    : simple_ipa_opt_pass (pass_data_ipa_strub_mode, ctxt)
+  {}
+  opt_pass *clone () { return new pass_ipa_strub_mode (m_ctxt); }
+  virtual bool gate (function *) {
+    /* In relaxed (-3) and strict (-4) settings, that only enable strub at a
+       function or variable attribute's request, the attribute handler changes
+       flag_strub to -1 or -2, respectively, if any strub-enabling occurence of
+       the attribute is found.  Therefore, if it remains at -3 or -4, nothing
+       that would enable strub was found, so we can disable it and avoid the
+       overhead.  */
+    if (flag_strub < -2)
+      flag_strub = 0;
+    return flag_strub;
+  }
+  virtual unsigned int execute (function *);
+};
+

-- 
Alexandre Oliva, happy hacker                https://FSFLA.org/blogs/lxo/
   Free Software Activist                       GNU Toolchain Engineer
Disinformation flourishes because many people care deeply about injustice
but very few check the facts.  Ask me about <https://stallmansupport.org>