Re: C/C++ PATCH to implement -Wmultistatement-macros (PR c/80116)

[Marek Polacek <polacek@redhat.com>]

On Thu, Jun 08, 2017 at 01:24:09PM -0400, David Malcolm wrote:
> On Thu, 2017-06-08 at 18:49 +0200, Marek Polacek wrote:
> > This is the hopefully last incarnation of the patch.  The change from
> > the
> > last time[0] is simpy that I've added a new test and the warning has
> > been
> > renamed to -Wmultistatement-macros.
> > 
> > David - any another comments?
> 
> Thanks for working on this; looks useful.
> 
> The new name is more accurate, but is rather long; oh well.  As part of
> -Wall, users won't typically have to type it, so that's OK.
 
Yeah, I'm not in love with the name either, but it's the best we could come up
with.

> [...]
> 
> > diff --git gcc/c-family/c-warn.c gcc/c-family/c-warn.c
> > index 35321a6..d883330 100644
> > --- gcc/c-family/c-warn.c
> > +++ gcc/c-family/c-warn.c
> [...]
> 
> > +  if (warning_at (body_loc, OPT_Wmultistatement_macros,
> > +		  "macro expands to multiple statements"))
> > +    inform (guard_loc, "some parts of macro expansion are not
> > guarded by "
> > +	    "this conditional");
> 
> Is the guard necessarily a "conditional"?  I take a "conditional" to
> mean an "if"; the guard could be a "for" or a "while" (or an "else",
> which still seems something of a stretch to me to call a
> "conditional").
> 
> Suggestion: word "this conditional" as "this %qs clause" and either (a)
> rework the code in c-indentation.c's guard_tinfo_to_string so that it's
> shared between these two warnings (i.e. to go from a RID_ to a const
> char *), or (b) just pass in a const char * identifying the guard
> clause token.
 
I prefer (a) so I tweaked guard_tinfo_to_string.  Of course, the name is
not that accurate anymore...

> > diff --git gcc/c-family/c.opt gcc/c-family/c.opt
> > index 37bb236..9dbe211 100644
> > --- gcc/c-family/c.opt
> > +++ gcc/c-family/c.opt
> > @@ -698,6 +698,10 @@ Wmissing-field-initializers
> >  C ObjC C++ ObjC++ Var(warn_missing_field_initializers) Warning
> > EnabledBy(Wextra)
> >  Warn about missing fields in struct initializers.
> >  
> > +Wmultistatement-macros
> > +C ObjC C++ ObjC++ Var(warn_multistatement_macros) Warning
> > LangEnabledBy(C ObjC C++ ObjC++,Wall)
> > +Warn about macros expanding to multiple statements in a body of a
> > conditional such as if, else, while, or for.
> 
> Likewise; is "conditional" the right word here?  Also, whether of not
> the statements are actually "in" the body of the guard is the issue
> here.
> 
> How about:
> 
> "Warn about unsafe multiple statement macros that appear to be guarded
> by a clause such as if, else, while, or for, in which only the first
> statement is actually guarded after the macro is expanded."
> 
> or somesuch?
 
I think this is too long for c.opt, so I used:
"Warn about unsafe macros expanding to multiple statements used as a body
of a clause such as if, else, while, switch, or for."

And I forgot to mention "switch" there.

> > diff --git gcc/doc/invoke.texi gcc/doc/invoke.texi
> > index c116882..2fe16dd 100644
> > --- gcc/doc/invoke.texi
> > +++ gcc/doc/invoke.texi
> > @@ -4496,6 +4497,29 @@ This warning is enabled by @option{-Wall}.
> >  @opindex Wno-missing-include-dirs
> >  Warn if a user-supplied include directory does not exist.
> >  
> > +@item -Wmultistatement-macros
> > +@opindex Wmultistatement-macros
> > +@opindex Wno-multistatement-macros
> > +Warn about macros expanding to multiple statements in a body of a
> > conditional,
> > +such as @code{if}, @code{else}, @code{for}, or @code{while}.
> 
> (as above).

But I used the longer form here.

> > +For example:
> > +
> > +@smallexample
> > +#define DOIT x++; y++
> > +if (c)
> > +  DOIT;
> > +@end smallexample
> > +
> > +will increment @code{y} unconditionally, not just when @code{c}
> > holds.
> > +The can usually be fixed by wrapping the macro in a do-while loop:
> > +@smallexample
> > +#define DOIT do @{ x++; y++; @} while (0)
> > +if (c)
> > +  DOIT;
> > +@end smallexample
> > +
> > +This warning is enabled by @option{-Wall} in C and C++.
> > +
> >  @item -Wparentheses
> >  @opindex Wparentheses
> >  @opindex Wno-parentheses
> 
> Hope this is constructive

As usually ;).

Updated version of the patch here.  Thanks,

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2017-06-13  Marek Polacek  <polacek@redhat.com>

	PR c/80116
	* c-common.h (warn_for_multistatement_macros): Declare.
	* c-warn.c: Include "c-family/c-indentation.h".
	(warn_for_multistatement_macros): New function.
	* c.opt (Wmultistatement-macros): New option.
	* c-indentation.c (guard_tinfo_to_string): No longer static.
	Change the parameter type to "enum rid".  Handle RID_SWITCH.
	* c-indentation.h (guard_tinfo_to_string): Declare.

	* c-parser.c (c_parser_if_body): Set the location of the
	body of the conditional after parsing all the labels.  Call
	warn_for_multistatement_macros.
	(c_parser_else_body): Likewise.
	(c_parser_switch_statement): Likewise.
	(c_parser_while_statement): Likewise.
	(c_parser_for_statement): Likewise.
	(c_parser_statement): Add a default argument.  Save the location
	after labels have been parsed.
	(c_parser_c99_block_statement): Likewise.

	* parser.c (cp_parser_statement): Add a default argument.  Save the
	location of the expression-statement after labels have been parsed.
	(cp_parser_implicitly_scoped_statement): Set the location of the
	body of the conditional after parsing all the labels.  Call
	warn_for_multistatement_macros.
	(cp_parser_already_scoped_statement): Likewise.

	* doc/invoke.texi: Document -Wmultistatement-macros.

	* c-c++-common/Wmultistatement-macros-1.c: New test.
	* c-c++-common/Wmultistatement-macros-2.c: New test.
	* c-c++-common/Wmultistatement-macros-3.c: New test.
	* c-c++-common/Wmultistatement-macros-4.c: New test.
	* c-c++-common/Wmultistatement-macros-5.c: New test.
	* c-c++-common/Wmultistatement-macros-6.c: New test.
	* c-c++-common/Wmultistatement-macros-7.c: New test.
	* c-c++-common/Wmultistatement-macros-8.c: New test.
	* c-c++-common/Wmultistatement-macros-9.c: New test.
	* c-c++-common/Wmultistatement-macros-10.c: New test.
	* c-c++-common/Wmultistatement-macros-11.c: New test.

diff --git gcc/c-family/c-common.h gcc/c-family/c-common.h
index 79072e6..f046a37 100644
--- gcc/c-family/c-common.h
+++ gcc/c-family/c-common.h
@@ -1539,6 +1539,8 @@ extern bool maybe_warn_shift_overflow (location_t, tree, tree);
 extern void warn_duplicated_cond_add_or_warn (location_t, tree, vec<tree> **);
 extern bool diagnose_mismatched_attributes (tree, tree);
 extern tree do_warn_duplicated_branches_r (tree *, int *, void *);
+extern void warn_for_multistatement_macros (location_t, location_t,
+					    location_t, enum rid);
 
 /* In c-attribs.c.  */
 extern bool attribute_takes_identifier_p (const_tree);
diff --git gcc/c-family/c-indentation.c gcc/c-family/c-indentation.c
index 8300788..7ca21e8 100644
--- gcc/c-family/c-indentation.c
+++ gcc/c-family/c-indentation.c
@@ -542,10 +542,10 @@ should_warn_for_misleading_indentation (const token_indent_info &guard_tinfo,
 
 /* Return the string identifier corresponding to the given guard token.  */
 
-static const char *
-guard_tinfo_to_string (const token_indent_info &guard_tinfo)
+const char *
+guard_tinfo_to_string (enum rid keyword)
 {
-  switch (guard_tinfo.keyword)
+  switch (keyword)
     {
     case RID_FOR:
       return "for";
@@ -557,6 +557,8 @@ guard_tinfo_to_string (const token_indent_info &guard_tinfo)
       return "while";
     case RID_DO:
       return "do";
+    case RID_SWITCH:
+      return "switch";
     default:
       gcc_unreachable ();
     }
@@ -605,10 +607,10 @@ warn_for_misleading_indentation (const token_indent_info &guard_tinfo,
     {
       if (warning_at (guard_tinfo.location, OPT_Wmisleading_indentation,
 		      "this %qs clause does not guard...",
-		      guard_tinfo_to_string (guard_tinfo)))
+		      guard_tinfo_to_string (guard_tinfo.keyword)))
 	inform (next_tinfo.location,
 		"...this statement, but the latter is misleadingly indented"
 		" as if it were guarded by the %qs",
-		guard_tinfo_to_string (guard_tinfo));
+		guard_tinfo_to_string (guard_tinfo.keyword));
     }
 }
diff --git gcc/c-family/c-indentation.h gcc/c-family/c-indentation.h
index a436697..e4cad26 100644
--- gcc/c-family/c-indentation.h
+++ gcc/c-family/c-indentation.h
@@ -48,5 +48,7 @@ extern void
 warn_for_misleading_indentation (const token_indent_info &guard_tinfo,
 				 const token_indent_info &body_tinfo,
 				 const token_indent_info &next_tinfo);
+extern const char *
+guard_tinfo_to_string (enum rid keyword);
 
 #endif  /* ! GCC_C_INDENTATION_H  */
diff --git gcc/c-family/c-warn.c gcc/c-family/c-warn.c
index 35321a6..72af573 100644
--- gcc/c-family/c-warn.c
+++ gcc/c-family/c-warn.c
@@ -31,6 +31,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "asan.h"
 #include "gcc-rich-location.h"
 #include "gimplify.h"
+#include "c-family/c-indentation.h"
 
 /* Print a warning if a constant expression had overflow in folding.
    Invoke this function on every expression that the language
@@ -2401,3 +2402,91 @@ do_warn_duplicated_branches_r (tree *tp, int *, void *)
     do_warn_duplicated_branches (*tp);
   return NULL_TREE;
 }
+
+/* Implementation of -Wmultistatement-macros.  This warning warns about
+   cases when a macro expands to multiple statements not wrapped in
+   do {} while (0) or ({ }) and is used as a body of if/else/for/while
+   conditionals.  For example,
+
+   #define DOIT x++; y++
+
+   if (c)
+     DOIT;
+
+   will increment y unconditionally.
+
+   BODY_LOC is the location of the first token in the body after labels
+   have been parsed, NEXT_LOC is the location of the next token after the
+   body of the conditional has been parsed, and GUARD_LOC is the location
+   of the conditional.  */
+
+void
+warn_for_multistatement_macros (location_t body_loc, location_t next_loc,
+				location_t guard_loc, enum rid keyword)
+{
+  if (!warn_multistatement_macros)
+    return;
+
+  /* Ain't got time to waste.  We only care about macros here.  */
+  if (!from_macro_expansion_at (body_loc)
+      || !from_macro_expansion_at (next_loc))
+    return;
+
+  /* Let's skip macros defined in system headers.  */
+  if (in_system_header_at (body_loc)
+      || in_system_header_at (next_loc))
+    return;
+
+  /* Find the actual tokens in the macro definition.  BODY_LOC and
+     NEXT_LOC have to come from the same spelling location, but they
+     will resolve to different locations in the context of the macro
+     definition.  */
+  location_t body_loc_exp
+    = linemap_resolve_location (line_table, body_loc,
+				LRK_MACRO_DEFINITION_LOCATION, NULL);
+  location_t next_loc_exp
+    = linemap_resolve_location (line_table, next_loc,
+				LRK_MACRO_DEFINITION_LOCATION, NULL);
+  location_t guard_loc_exp
+    = linemap_resolve_location (line_table, guard_loc,
+				LRK_MACRO_DEFINITION_LOCATION, NULL);
+
+  /* These are some funky cases we don't want to warn about.  */
+  if (body_loc_exp == guard_loc_exp
+      || next_loc_exp == guard_loc_exp
+      || body_loc_exp == next_loc_exp)
+    return;
+
+  /* Find the macro map for the macro expansion BODY_LOC.  */
+  const line_map *map = linemap_lookup (line_table, body_loc);
+  const line_map_macro *macro_map = linemap_check_macro (map);
+
+  /* Now see if the following token is coming from the same macro
+     expansion.  If it is, it's a problem, because it should've been
+     parsed at this point.  We only look at odd-numbered indexes
+     within the MACRO_MAP_LOCATIONS array, i.e. the spelling locations
+     of the tokens.  */
+  bool found_guard = false;
+  bool found_next = false;
+  for (unsigned int i = 1;
+       i < 2 * MACRO_MAP_NUM_MACRO_TOKENS (macro_map);
+       i += 2)
+    {
+      if (MACRO_MAP_LOCATIONS (macro_map)[i] == next_loc_exp)
+	found_next = true;
+      if (MACRO_MAP_LOCATIONS (macro_map)[i] == guard_loc_exp)
+	found_guard = true;
+    }
+
+  /* The conditional itself must not come from the same expansion, because
+     we don't want to warn about
+     #define IF if (x) x++; y++
+     and similar.  */
+  if (!found_next || found_guard)
+    return;
+
+  if (warning_at (body_loc, OPT_Wmultistatement_macros,
+		  "macro expands to multiple statements"))
+    inform (guard_loc, "some parts of macro expansion are not guarded by "
+	    "this %qs clause", guard_tinfo_to_string (keyword));
+}
diff --git gcc/c-family/c.opt gcc/c-family/c.opt
index 37bb236..34b4a61 100644
--- gcc/c-family/c.opt
+++ gcc/c-family/c.opt
@@ -698,6 +698,10 @@ Wmissing-field-initializers
 C ObjC C++ ObjC++ Var(warn_missing_field_initializers) Warning EnabledBy(Wextra)
 Warn about missing fields in struct initializers.
 
+Wmultistatement-macros
+C ObjC C++ ObjC++ Var(warn_multistatement_macros) Warning LangEnabledBy(C ObjC C++ ObjC++,Wall)
+Warn about unsafe macros expanding to multiple statements used as a body of a clause such as if, else, while, switch, or for.
+
 Wmultiple-inheritance
 C++ ObjC++ Var(warn_multiple_inheritance) Warning
 Warn on direct multiple inheritance.
diff --git gcc/c/c-parser.c gcc/c/c-parser.c
index 6f954f2..f8fbc92 100644
--- gcc/c/c-parser.c
+++ gcc/c/c-parser.c
@@ -1218,9 +1218,11 @@ static void c_parser_initval (c_parser *, struct c_expr *,
 static tree c_parser_compound_statement (c_parser *);
 static void c_parser_compound_statement_nostart (c_parser *);
 static void c_parser_label (c_parser *);
-static void c_parser_statement (c_parser *, bool *);
+static void c_parser_statement (c_parser *, bool *, location_t * = NULL);
 static void c_parser_statement_after_labels (c_parser *, bool *,
 					     vec<tree> * = NULL);
+static tree c_parser_c99_block_statement (c_parser *, bool *,
+					  location_t * = NULL);
 static void c_parser_if_statement (c_parser *, bool *, vec<tree> *);
 static void c_parser_switch_statement (c_parser *, bool *);
 static void c_parser_while_statement (c_parser *, bool, bool *);
@@ -5204,9 +5206,11 @@ c_parser_label (c_parser *parser)
    implement -Wparentheses.  */
 
 static void
-c_parser_statement (c_parser *parser, bool *if_p)
+c_parser_statement (c_parser *parser, bool *if_p, location_t *loc_after_labels)
 {
   c_parser_all_labels (parser);
+  if (loc_after_labels)
+    *loc_after_labels = c_parser_peek_token (parser)->location;
   c_parser_statement_after_labels (parser, if_p, NULL);
 }
 
@@ -5466,11 +5470,12 @@ c_parser_paren_condition (c_parser *parser)
    implement -Wparentheses.  */
 
 static tree
-c_parser_c99_block_statement (c_parser *parser, bool *if_p)
+c_parser_c99_block_statement (c_parser *parser, bool *if_p,
+			      location_t *loc_after_labels)
 {
   tree block = c_begin_compound_stmt (flag_isoc99);
   location_t loc = c_parser_peek_token (parser)->location;
-  c_parser_statement (parser, if_p);
+  c_parser_statement (parser, if_p, loc_after_labels);
   return c_end_compound_stmt (loc, block, flag_isoc99);
 }
 
@@ -5492,6 +5497,7 @@ c_parser_if_body (c_parser *parser, bool *if_p,
 {
   tree block = c_begin_compound_stmt (flag_isoc99);
   location_t body_loc = c_parser_peek_token (parser)->location;
+  location_t body_loc_after_labels = UNKNOWN_LOCATION;
   token_indent_info body_tinfo
     = get_token_indent_info (c_parser_peek_token (parser));
 
@@ -5508,11 +5514,18 @@ c_parser_if_body (c_parser *parser, bool *if_p,
   else if (c_parser_next_token_is (parser, CPP_OPEN_BRACE))
     add_stmt (c_parser_compound_statement (parser));
   else
-    c_parser_statement_after_labels (parser, if_p);
+    {
+      body_loc_after_labels = c_parser_peek_token (parser)->location;
+      c_parser_statement_after_labels (parser, if_p);
+    }
 
   token_indent_info next_tinfo
     = get_token_indent_info (c_parser_peek_token (parser));
   warn_for_misleading_indentation (if_tinfo, body_tinfo, next_tinfo);
+  if (body_loc_after_labels != UNKNOWN_LOCATION
+      && next_tinfo.type != CPP_SEMICOLON)
+    warn_for_multistatement_macros (body_loc_after_labels, next_tinfo.location,
+				    if_tinfo.location, RID_IF);
 
   return c_end_compound_stmt (body_loc, block, flag_isoc99);
 }
@@ -5530,6 +5543,7 @@ c_parser_else_body (c_parser *parser, const token_indent_info &else_tinfo,
   tree block = c_begin_compound_stmt (flag_isoc99);
   token_indent_info body_tinfo
     = get_token_indent_info (c_parser_peek_token (parser));
+  location_t body_loc_after_labels = UNKNOWN_LOCATION;
 
   c_parser_all_labels (parser);
   if (c_parser_next_token_is (parser, CPP_SEMICOLON))
@@ -5542,11 +5556,18 @@ c_parser_else_body (c_parser *parser, const token_indent_info &else_tinfo,
       c_parser_consume_token (parser);
     }
   else
-    c_parser_statement_after_labels (parser, NULL, chain);
+    {
+      body_loc_after_labels = c_parser_peek_token (parser)->location;
+      c_parser_statement_after_labels (parser, NULL, chain);
+    }
 
   token_indent_info next_tinfo
     = get_token_indent_info (c_parser_peek_token (parser));
   warn_for_misleading_indentation (else_tinfo, body_tinfo, next_tinfo);
+  if (body_loc_after_labels != UNKNOWN_LOCATION
+      && next_tinfo.type != CPP_SEMICOLON)
+    warn_for_multistatement_macros (body_loc_after_labels, next_tinfo.location,
+				    else_tinfo.location, RID_ELSE);
 
   return c_end_compound_stmt (body_loc, block, flag_isoc99);
 }
@@ -5732,7 +5753,13 @@ c_parser_switch_statement (c_parser *parser, bool *if_p)
   c_start_case (switch_loc, switch_cond_loc, expr, explicit_cast_p);
   save_break = c_break_label;
   c_break_label = NULL_TREE;
-  body = c_parser_c99_block_statement (parser, if_p);
+  location_t loc_after_labels;
+  bool open_brace_p = c_parser_peek_token (parser)->type == CPP_OPEN_BRACE;
+  body = c_parser_c99_block_statement (parser, if_p, &loc_after_labels);
+  location_t next_loc = c_parser_peek_token (parser)->location;
+  if (!open_brace_p && c_parser_peek_token (parser)->type != CPP_SEMICOLON)
+    warn_for_multistatement_macros (loc_after_labels, next_loc, switch_loc,
+				    RID_SWITCH);
   c_finish_case (body, ce.original_type);
   if (c_break_label)
     {
@@ -5783,7 +5810,8 @@ c_parser_while_statement (c_parser *parser, bool ivdep, bool *if_p)
   token_indent_info body_tinfo
     = get_token_indent_info (c_parser_peek_token (parser));
 
-  body = c_parser_c99_block_statement (parser, if_p);
+  location_t loc_after_labels;
+  body = c_parser_c99_block_statement (parser, if_p, &loc_after_labels);
   c_finish_loop (loc, cond, NULL, body, c_break_label, c_cont_label, true);
   add_stmt (c_end_compound_stmt (loc, block, flag_isoc99));
   c_parser_maybe_reclassify_token (parser);
@@ -5792,6 +5820,10 @@ c_parser_while_statement (c_parser *parser, bool ivdep, bool *if_p)
     = get_token_indent_info (c_parser_peek_token (parser));
   warn_for_misleading_indentation (while_tinfo, body_tinfo, next_tinfo);
 
+  if (next_tinfo.type != CPP_SEMICOLON)
+    warn_for_multistatement_macros (loc_after_labels, next_tinfo.location,
+				    while_tinfo.location, RID_WHILE);
+
   c_break_label = save_break;
   c_cont_label = save_cont;
 }
@@ -6076,7 +6108,8 @@ c_parser_for_statement (c_parser *parser, bool ivdep, bool *if_p)
   token_indent_info body_tinfo
     = get_token_indent_info (c_parser_peek_token (parser));
 
-  body = c_parser_c99_block_statement (parser, if_p);
+  location_t loc_after_labels;
+  body = c_parser_c99_block_statement (parser, if_p, &loc_after_labels);
 
   if (is_foreach_statement)
     objc_finish_foreach_loop (loc, object_expression, collection_expression, body, c_break_label, c_cont_label);
@@ -6089,6 +6122,10 @@ c_parser_for_statement (c_parser *parser, bool ivdep, bool *if_p)
     = get_token_indent_info (c_parser_peek_token (parser));
   warn_for_misleading_indentation (for_tinfo, body_tinfo, next_tinfo);
 
+  if (next_tinfo.type != CPP_SEMICOLON)
+    warn_for_multistatement_macros (loc_after_labels, next_tinfo.location,
+				    for_tinfo.location, RID_FOR);
+
   c_break_label = save_break;
   c_cont_label = save_cont;
 }
diff --git gcc/cp/parser.c gcc/cp/parser.c
index d02ad36..1107c03 100644
--- gcc/cp/parser.c
+++ gcc/cp/parser.c
@@ -2102,7 +2102,7 @@ static void cp_parser_lambda_body
 /* Statements [gram.stmt.stmt]  */
 
 static void cp_parser_statement
-  (cp_parser *, tree, bool, bool *, vec<tree> * = NULL);
+  (cp_parser *, tree, bool, bool *, vec<tree> * = NULL, location_t * = NULL);
 static void cp_parser_label_for_labeled_statement
 (cp_parser *, tree);
 static tree cp_parser_expression_statement
@@ -10535,7 +10535,8 @@ cp_parser_lambda_body (cp_parser* parser, tree lambda_expr)
 
 static void
 cp_parser_statement (cp_parser* parser, tree in_statement_expr,
-		     bool in_compound, bool *if_p, vec<tree> *chain)
+		     bool in_compound, bool *if_p, vec<tree> *chain,
+		     location_t *loc_after_labels)
 {
   tree statement, std_attrs = NULL_TREE;
   cp_token *token;
@@ -10728,6 +10729,10 @@ cp_parser_statement (cp_parser* parser, tree in_statement_expr,
 	  if (cp_parser_parse_definitely (parser))
 	    return;
 	}
+      /* All preceding labels have been parsed at this point.  */
+      if (loc_after_labels != NULL)
+	*loc_after_labels = statement_location;
+
       /* Look for an expression-statement instead.  */
       statement = cp_parser_expression_statement (parser, in_statement_expr);
 
@@ -12268,6 +12273,7 @@ cp_parser_implicitly_scoped_statement (cp_parser* parser, bool *if_p,
 {
   tree statement;
   location_t body_loc = cp_lexer_peek_token (parser->lexer)->location;
+  location_t body_loc_after_labels = UNKNOWN_LOCATION;
   token_indent_info body_tinfo
     = get_token_indent_info (cp_lexer_peek_token (parser->lexer));
 
@@ -12297,7 +12303,8 @@ cp_parser_implicitly_scoped_statement (cp_parser* parser, bool *if_p,
       /* Create a compound-statement.  */
       statement = begin_compound_stmt (0);
       /* Parse the dependent-statement.  */
-      cp_parser_statement (parser, NULL_TREE, false, if_p, chain);
+      cp_parser_statement (parser, NULL_TREE, false, if_p, chain,
+			   &body_loc_after_labels);
       /* Finish the dummy compound-statement.  */
       finish_compound_stmt (statement);
     }
@@ -12306,6 +12313,11 @@ cp_parser_implicitly_scoped_statement (cp_parser* parser, bool *if_p,
     = get_token_indent_info (cp_lexer_peek_token (parser->lexer));
   warn_for_misleading_indentation (guard_tinfo, body_tinfo, next_tinfo);
 
+  if (body_loc_after_labels != UNKNOWN_LOCATION
+      && next_tinfo.type != CPP_SEMICOLON)
+    warn_for_multistatement_macros (body_loc_after_labels, next_tinfo.location,
+				    guard_tinfo.location, guard_tinfo.keyword);
+
   /* Return the statement.  */
   return statement;
 }
@@ -12324,11 +12336,18 @@ cp_parser_already_scoped_statement (cp_parser* parser, bool *if_p,
     {
       token_indent_info body_tinfo
 	= get_token_indent_info (cp_lexer_peek_token (parser->lexer));
+      location_t loc_after_labels;
 
-      cp_parser_statement (parser, NULL_TREE, false, if_p);
+      cp_parser_statement (parser, NULL_TREE, false, if_p, NULL,
+			   &loc_after_labels);
       token_indent_info next_tinfo
 	= get_token_indent_info (cp_lexer_peek_token (parser->lexer));
       warn_for_misleading_indentation (guard_tinfo, body_tinfo, next_tinfo);
+
+      if (next_tinfo.type != CPP_SEMICOLON)
+	warn_for_multistatement_macros (loc_after_labels, next_tinfo.location,
+					guard_tinfo.location,
+					guard_tinfo.keyword);
     }
   else
     {
diff --git gcc/doc/invoke.texi gcc/doc/invoke.texi
index 5d41649..ec59682 100644
--- gcc/doc/invoke.texi
+++ gcc/doc/invoke.texi
@@ -293,7 +293,7 @@ Objective-C and Objective-C++ Dialects}.
 -Wmain  -Wmaybe-uninitialized  -Wmemset-elt-size  -Wmemset-transposed-args @gol
 -Wmisleading-indentation  -Wmissing-braces @gol
 -Wmissing-field-initializers  -Wmissing-include-dirs @gol
--Wno-multichar  -Wnonnull  -Wnonnull-compare @gol
+-Wno-multichar  -Wmultistatement-macros  -Wnonnull  -Wnonnull-compare @gol
 -Wnormalized=@r{[}none@r{|}id@r{|}nfc@r{|}nfkc@r{]} @gol
 -Wnull-dereference  -Wodr  -Wno-overflow  -Wopenmp-simd  @gol
 -Woverride-init-side-effects  -Woverlength-strings @gol
@@ -3824,6 +3824,7 @@ Options} and @ref{Objective-C and Objective-C++ Dialect Options}.
 -Wmemset-transposed-args @gol
 -Wmisleading-indentation @r{(only for C/C++)} @gol
 -Wmissing-braces @r{(only for C/ObjC)} @gol
+-Wmultistatement-macros  @gol
 -Wnarrowing @r{(only for C++)}  @gol
 -Wnonnull  @gol
 -Wnonnull-compare  @gol
@@ -4496,6 +4497,32 @@ This warning is enabled by @option{-Wall}.
 @opindex Wno-missing-include-dirs
 Warn if a user-supplied include directory does not exist.
 
+@item -Wmultistatement-macros
+@opindex Wmultistatement-macros
+@opindex Wno-multistatement-macros
+Warn about unsafe multiple statement macros that appear to be guarded
+by a clause such as @code{if}, @code{else}, @code{for}, @code{switch}, or
+@code{while}, in which only the first statement is actually guarded after
+the macro is expanded.
+
+For example:
+
+@smallexample
+#define DOIT x++; y++
+if (c)
+  DOIT;
+@end smallexample
+
+will increment @code{y} unconditionally, not just when @code{c} holds.
+The can usually be fixed by wrapping the macro in a do-while loop:
+@smallexample
+#define DOIT do @{ x++; y++; @} while (0)
+if (c)
+  DOIT;
+@end smallexample
+
+This warning is enabled by @option{-Wall} in C and C++.
+
 @item -Wparentheses
 @opindex Wparentheses
 @opindex Wno-parentheses
diff --git gcc/testsuite/c-c++-common/Wmultistatement-macros-1.c gcc/testsuite/c-c++-common/Wmultistatement-macros-1.c
index e69de29..cdecbb4 100644
--- gcc/testsuite/c-c++-common/Wmultistatement-macros-1.c
+++ gcc/testsuite/c-c++-common/Wmultistatement-macros-1.c
@@ -0,0 +1,118 @@
+/* PR c/80116 */
+/* { dg-options "-Wmultistatement-macros" } */
+/* { dg-do compile } */
+
+#define SWAP(X, Y)	\
+  tmp = X; /* { dg-warning "macro expands to multiple statements" } */ \
+  X = Y;		\
+  Y = tmp
+
+#define STUFF		\
+  if (0) x = y
+
+#define STUFF2		\
+  if (0) x = y; x++
+
+#define STUFF3		\
+  if (x) /* { dg-message "not guarded by this 'if' clause" } */ \
+    SWAP(x, y) /* { dg-message "in expansion of macro .SWAP." } */
+
+#define SET(X, Y)	\
+  (X) = (Y)
+
+#define STUFF4		\
+  if (x)		\
+    SET(x, y);		\
+  SET(x, y)
+
+#define STUFF5		\
+  { tmp = x; x = y; }
+
+#define STUFF6		\
+  x++;;
+
+int x, y, tmp;
+
+void
+fn1 (void)
+{
+  if (x) /* { dg-message "not guarded by this 'if' clause" } */
+    SWAP(x, y); /* { dg-message "in expansion of macro .SWAP." } */
+}
+
+void
+fn2 (void)
+{
+  SWAP(x, y);
+}
+
+void
+fn3 (void)
+{
+  if (x)
+    {
+      SWAP(x, y);
+    }
+}
+
+void
+fn4 (void)
+{
+  if (x)
+  ({ x = 10; x++; });
+}
+
+void
+fn5 (void)
+{
+  if (x) /* { dg-message "not guarded by this 'if' clause" } */
+L1:
+    SWAP (x, y); /* { dg-message "in expansion of macro .SWAP." } */
+  goto L1;
+}
+
+void
+fn6 (void)
+{
+  if (x)
+    SET (x, y);
+  SET (tmp, x);
+}
+
+void
+fn7 (void)
+{
+  STUFF;
+}
+
+void
+fn8 (void)
+{
+  STUFF2;
+}
+
+void
+fn9 (void)
+{
+  STUFF3; /* { dg-message "in expansion of macro .STUFF3." } */
+}
+
+void
+fn10 (void)
+{
+  STUFF4;
+}
+
+void
+fn11 (void)
+{
+  if (x)
+    STUFF5;
+}
+
+void
+fn12 (void)
+{
+  if (x)
+    STUFF6;
+}
diff --git gcc/testsuite/c-c++-common/Wmultistatement-macros-10.c gcc/testsuite/c-c++-common/Wmultistatement-macros-10.c
index e69de29..766ed25 100644
--- gcc/testsuite/c-c++-common/Wmultistatement-macros-10.c
+++ gcc/testsuite/c-c++-common/Wmultistatement-macros-10.c
@@ -0,0 +1,82 @@
+/* PR c/80116 */
+/* { dg-options "-Wmultistatement-macros" } */
+/* { dg-do compile } */
+
+#define SWAP(x, y) \
+  tmp = x; /* { dg-warning "macro expands to multiple statements" } */ \
+  x = y; \
+  y = tmp
+
+#define M1	\
+  switch (x) /* { dg-message "not guarded by this 'switch' clause" } */ \
+    case 1:	\
+    SWAP (x, y) /* { dg-message "in expansion of macro .SWAP." } */
+
+#define M2	\
+  switch (x)	\
+    case 1:	\
+    x++
+
+#define M3	\
+  switch (x)	\
+    case 1:	\
+    x++;;
+
+#define M4	\
+  switch (x) /* { dg-message "not guarded by this 'switch' clause" } */ \
+L1:		\
+    case 1:	\
+    SWAP (x, y) /* { dg-message "in expansion of macro .SWAP." } */
+
+#define INC	\
+  x++;;
+
+int x, y, tmp;
+
+void
+fn0 (void)
+{
+  switch (x) /* { dg-message "not guarded by this 'switch' clause" } */
+    case 1:
+      SWAP (x, y); /* { dg-message "in expansion of macro .SWAP." } */
+
+  switch (x) /* { dg-message "not guarded by this 'switch' clause" } */
+    case 1:
+    case 2:
+    case 3:
+    case 4:
+    case 5:
+      SWAP (x, y); /* { dg-message "in expansion of macro .SWAP." } */
+}
+
+void
+fn1 (void)
+{
+  M1; /* { dg-message "in expansion of macro .M1." } */
+  M2;
+  M3;
+  M4; /* { dg-message "in expansion of macro .M4." } */
+  goto L1;
+}
+
+void
+fn2 (void)
+{
+  switch (x)
+    case 1:
+      INC
+
+  switch (x)
+    case 1:
+      ({ x = 10; x++; });
+}
+
+void
+fn3 (void)
+{
+  switch (x)
+    {
+    case 1:
+      SWAP (x, y);
+    }
+}
diff --git gcc/testsuite/c-c++-common/Wmultistatement-macros-11.c gcc/testsuite/c-c++-common/Wmultistatement-macros-11.c
index e69de29..4f4a123 100644
--- gcc/testsuite/c-c++-common/Wmultistatement-macros-11.c
+++ gcc/testsuite/c-c++-common/Wmultistatement-macros-11.c
@@ -0,0 +1,19 @@
+/* PR c/80116 */
+/* { dg-options "-Wmultistatement-macros" } */
+/* { dg-do compile } */
+
+enum E { A, B };
+
+const char *
+foo (enum E e)
+{
+#define CASE(X) case X: return #X
+  switch (e)
+    {
+      CASE (A);
+      CASE (B);
+    default:
+      return "<unknown>";
+    }
+#undef CASE
+};
diff --git gcc/testsuite/c-c++-common/Wmultistatement-macros-2.c gcc/testsuite/c-c++-common/Wmultistatement-macros-2.c
index e69de29..9fef901 100644
--- gcc/testsuite/c-c++-common/Wmultistatement-macros-2.c
+++ gcc/testsuite/c-c++-common/Wmultistatement-macros-2.c
@@ -0,0 +1,137 @@
+/* PR c/80116 */
+/* { dg-options "-Wmultistatement-macros" } */
+/* { dg-do compile } */
+
+#define SWAP(X, Y)	\
+  tmp = X; /* { dg-warning "macro expands to multiple statements" } */ \
+  X = Y;		\
+  Y = tmp
+
+#define STUFF		\
+  if (0) {} else x = y
+
+#define STUFF2		\
+  if (0) {} else x = y; x++
+
+#define STUFF3		\
+  if (x)		\
+    {}			\
+  else /* { dg-message "not guarded by this 'else' clause" } */ \
+    SWAP(x, y) /* { dg-message "in expansion of macro .SWAP." } */
+
+#define SET(X, Y)	\
+  (X) = (Y)
+
+#define STUFF4		\
+  if (x)		\
+    {}			\
+  else			\
+    SET(x, y);		\
+  SET(x, y)
+
+#define STUFF5		\
+  { tmp = x; x = y; }
+
+#define STUFF6		\
+  x++;;
+
+int x, y, tmp;
+
+void
+fn1 (void)
+{
+  if (x)
+   {
+   }
+  else /* { dg-message "not guarded by this 'else' clause" } */
+    SWAP(x, y); /* { dg-message "in expansion of macro .SWAP." } */
+}
+
+void
+fn2 (void)
+{
+  SWAP(x, y);
+}
+
+void
+fn3 (void)
+{
+  if (x)
+    {
+    }
+  else
+    {
+      SWAP(x, y);
+    }
+}
+
+void
+fn4 (void)
+{
+  if (x)
+    {
+    }
+  else
+    ({ x = 10; x++; });
+}
+
+void
+fn5 (void)
+{
+  if (x)
+    {
+    }
+  else /* { dg-message "not guarded by this 'else' clause" } */
+L1:
+    SWAP (x, y); /* { dg-message "in expansion of macro .SWAP." } */
+  goto L1;
+}
+
+void
+fn6 (void)
+{
+  if (x)
+    {
+    }
+  else
+    SET (x, y);
+  SET (tmp, x);
+}
+
+void
+fn7 (void)
+{
+  STUFF;
+}
+
+void
+fn8 (void)
+{
+  STUFF2;
+}
+
+void
+fn9 (void)
+{
+  STUFF3; /* { dg-message "in expansion of macro .STUFF3." } */
+}
+
+void
+fn10 (void)
+{
+  STUFF4;
+}
+
+void
+fn11 (void)
+{
+  if (x)
+    STUFF5;
+}
+
+void
+fn12 (void)
+{
+  if (x)
+    STUFF6;
+}
diff --git gcc/testsuite/c-c++-common/Wmultistatement-macros-3.c gcc/testsuite/c-c++-common/Wmultistatement-macros-3.c
index e69de29..d130a35 100644
--- gcc/testsuite/c-c++-common/Wmultistatement-macros-3.c
+++ gcc/testsuite/c-c++-common/Wmultistatement-macros-3.c
@@ -0,0 +1,12 @@
+/* PR c/80116 */
+/* { dg-options "-Wmultistatement-macros" } */
+/* { dg-do compile } */
+
+#define CHECK(X) if (!(X)) __builtin_abort ()
+
+void
+fn (int i)
+{
+  CHECK (i == 1);
+  CHECK (i == 2);
+}
diff --git gcc/testsuite/c-c++-common/Wmultistatement-macros-4.c gcc/testsuite/c-c++-common/Wmultistatement-macros-4.c
index e69de29..e5cc9c3 100644
--- gcc/testsuite/c-c++-common/Wmultistatement-macros-4.c
+++ gcc/testsuite/c-c++-common/Wmultistatement-macros-4.c
@@ -0,0 +1,14 @@
+/* PR c/80116 */
+/* { dg-options "-Wmultistatement-macros" } */
+/* { dg-do compile } */
+
+#define FN(C)		\
+  void			\
+  fn (void)		\
+  {			\
+    C;			\
+  }
+
+int i;
+
+FN (if (i) ++i)
diff --git gcc/testsuite/c-c++-common/Wmultistatement-macros-5.c gcc/testsuite/c-c++-common/Wmultistatement-macros-5.c
index e69de29..0ac84f5 100644
--- gcc/testsuite/c-c++-common/Wmultistatement-macros-5.c
+++ gcc/testsuite/c-c++-common/Wmultistatement-macros-5.c
@@ -0,0 +1,18 @@
+/* PR c/80116 */
+/* { dg-options "-Wmultistatement-macros" } */
+/* { dg-do compile } */
+
+#define M(N)	\
+L ## N:		\
+  x++; x++ /* { dg-warning "macro expands to multiple statements" } */
+
+int x, y, tmp;
+
+void
+fn1 (void)
+{
+  if (x) /* { dg-message "not guarded by this 'if' clause" } */
+   M (0); /* { dg-message "in expansion of macro .M." } */
+  if (x) /* { dg-message "not guarded by this 'if' clause" } */
+   M (1); /* { dg-message "in expansion of macro .M." } */
+}
diff --git gcc/testsuite/c-c++-common/Wmultistatement-macros-6.c gcc/testsuite/c-c++-common/Wmultistatement-macros-6.c
index e69de29..5ec9cd9 100644
--- gcc/testsuite/c-c++-common/Wmultistatement-macros-6.c
+++ gcc/testsuite/c-c++-common/Wmultistatement-macros-6.c
@@ -0,0 +1,22 @@
+/* PR c/80116 */
+/* { dg-options "-Wmultistatement-macros" } */
+/* { dg-do compile } */
+
+#define M \
+  if (x) x++; x++
+
+void
+f (int x)
+{
+  M;
+  M;
+  M;
+  M;
+  M;
+  M;
+  M;
+  M;
+  M;
+  M;
+  M;
+}
diff --git gcc/testsuite/c-c++-common/Wmultistatement-macros-7.c gcc/testsuite/c-c++-common/Wmultistatement-macros-7.c
index e69de29..d661f14 100644
--- gcc/testsuite/c-c++-common/Wmultistatement-macros-7.c
+++ gcc/testsuite/c-c++-common/Wmultistatement-macros-7.c
@@ -0,0 +1,18 @@
+/* PR c/80116 */
+/* { dg-options "-Wmultistatement-macros" } */
+/* { dg-do compile } */
+
+#define SWAP(X, Y)      \
+  tmp = X; /* { dg-warning "macro expands to multiple statements" } */ \
+  X = Y;                \
+  Y = tmp
+
+#define BODY_AND_IF(COND, X, Y)  \
+  if (COND) SWAP (X, Y) /* { dg-message "in expansion of macro .SWAP." } */
+
+void
+fn (int x, int y)
+{
+  int tmp;
+  BODY_AND_IF (1, x, y); /* { dg-message "in expansion of macro .BODY_AND_IF." } */
+}
diff --git gcc/testsuite/c-c++-common/Wmultistatement-macros-8.c gcc/testsuite/c-c++-common/Wmultistatement-macros-8.c
index e69de29..06522a7 100644
--- gcc/testsuite/c-c++-common/Wmultistatement-macros-8.c
+++ gcc/testsuite/c-c++-common/Wmultistatement-macros-8.c
@@ -0,0 +1,64 @@
+/* PR c/80116 */
+/* { dg-options "-Wmultistatement-macros" } */
+/* { dg-do compile } */
+
+#define SWAP(x, y) \
+  tmp = x; /* { dg-warning "macro expands to multiple statements" } */ \
+  x = y; \
+  y = tmp
+
+#define M1				\
+  for (i = 0; i < 1; ++i) /* { dg-message "not guarded by this 'for' clause" } */ \
+    SWAP (x, y) /* { dg-message "in expansion of macro .SWAP." } */
+
+#define M2				\
+  for (i = 0; i < 1; ++i)		\
+    x++
+
+#define M3				\
+  for (i = 0; i < 1; ++i)		\
+    x++;;
+
+#define M4				\
+  for (i = 0; i < 1; ++i) /* { dg-message "not guarded by this 'for' clause" } */ \
+L1:					\
+    SWAP (x, y) /* { dg-message "in expansion of macro .SWAP." } */
+
+#define INC	\
+  x++;;
+
+int x, y, tmp;
+
+void
+fn0 (void)
+{
+  int i;
+  for (i = 0; i < 1; ++i) /* { dg-message "not guarded by this 'for' clause" } */
+    SWAP (x, y); /* { dg-message "in expansion of macro .SWAP." } */
+
+  for (i = 0; i < 1; ++i) /* { dg-message "not guarded by this 'for' clause" } */
+L:
+    SWAP (x, y); /* { dg-message "in expansion of macro .SWAP." } */
+  goto L;
+}
+
+void
+fn1 (void)
+{
+  int i;
+  M1; /* { dg-message "in expansion of macro .M1." } */
+  M2;
+  M3;
+  M4; /* { dg-message "in expansion of macro .M4." } */
+  goto L1;
+}
+
+void
+fn2 (void)
+{
+  for (int i = 0; i < 1; ++i)
+    INC
+
+  for (int i = 0; i < 1; ++i)
+    ({ x = 10; x++; });
+}
diff --git gcc/testsuite/c-c++-common/Wmultistatement-macros-9.c gcc/testsuite/c-c++-common/Wmultistatement-macros-9.c
index e69de29..350c4f9f 100644
--- gcc/testsuite/c-c++-common/Wmultistatement-macros-9.c
+++ gcc/testsuite/c-c++-common/Wmultistatement-macros-9.c
@@ -0,0 +1,62 @@
+/* PR c/80116 */
+/* { dg-options "-Wmultistatement-macros" } */
+/* { dg-do compile } */
+
+#define SWAP(x, y) \
+  tmp = x; /* { dg-warning "macro expands to multiple statements" } */ \
+  x = y; \
+  y = tmp
+
+#define M1	\
+  while (x) /* { dg-message "not guarded by this 'while' claus" } */ \
+    SWAP (x, y) /* { dg-message "in expansion of macro .SWAP." } */
+
+#define M2	\
+  while (x)	\
+    x++
+
+#define M3	\
+  while (x)	\
+    x++;;
+
+#define M4	\
+  while (x) /* { dg-message "not guarded by this 'while' claus" } */ \
+L1:		\
+    SWAP (x, y) /* { dg-message "in expansion of macro .SWAP." } */
+
+#define INC	\
+  x++;;
+
+int x, y, tmp;
+
+void
+fn0 (void)
+{
+  while (x) /* { dg-message "not guarded by this 'while' claus" } */
+    SWAP (x, y); /* { dg-message "in expansion of macro .SWAP." } */
+
+  while (x) /* { dg-message "not guarded by this 'while' claus" } */
+L:
+    SWAP (x, y); /* { dg-message "in expansion of macro .SWAP." } */
+  goto L;
+}
+
+void
+fn1 (void)
+{
+  M1; /* { dg-message "in expansion of macro .M1." } */
+  M2;
+  M3;
+  M4; /* { dg-message "in expansion of macro .M4." } */
+  goto L1;
+}
+
+void
+fn2 (void)
+{
+  while (x)
+    INC
+
+  while (x)
+    ({ x = 10; x++; });
+}

	Marek