Re: C/C++ PATCH to implement -Wmultiline-expansion (PR c/80116)

[Marek Polacek <polacek at redhat dot com>]

On Thu, Jun 01, 2017 at 02:08:21PM -0400, David Malcolm wrote:
> On Thu, 2017-06-01 at 18:45 +0200, Marek Polacek wrote:
> > A motivating example for this warning can be found e.g. in
> > 
> >   PRE10-C. Wrap multistatement macros in a do-while loop
> >   https://www.securecoding.cert.org/confluence/x/jgL7
> > 
> > i.e., 
> > 
> > #define SWAP(x, y) \
> >   tmp = x; \
> >   x = y; \
> >   y = tmp
> > 
> > used like this [1]
> > 
> > int x, y, z, tmp;
> > if (z == 0)
> >   SWAP(x, y);
> > 
> > expands to the following [2], which is certainly not what the
> > programmer intended:
> > 
> > int x, y, z, tmp;
> > if (z == 0)
> >   tmp = x;
> > x = y;
> > y = tmp;
> > 
> > This has also happened in our codebase, see PR80063.
> 
> The warning looks like a good idea.

Thanks, I hope it will help detect sneaky bugs.

> This reminds me a lot of -Wmisleading-indentation.  Does that fire for
> any of the cases?

Only when I used -ftrack-macro-expansion=0.
 
> The patch appears to only consider "if" and "else" clauses.  Shouldn't
> it also cover "for", "while" and "do/while"?
 
It should cover "for" and "while".  My new version implements this.

> > I tried to summarize the way I approached this problem in the
> > commentary in
> > warn_for_multiline_expansion, but I'll try to explain the crux of the
> > matter
> > here, too.
> > 
> > For code like [1], in the FEs we'll see [2], of course.  When parsing
> > the
> > then-branch we see that the body of the if isn't wrapped in { } so we
> > create a
> > compound statement with just the first statement "tmp = x;", and the
> > other two
> > will be executed unconditionally.
> > 
> > My idea was to look at the location info of the following token after
> > the body
> > of the if has been parsed and determine if they come from the same
> > macro expansion,
> > and if they do (and the if itself doesn't), warn (taking into account
> > various
> > corner cases, as usually).
> > 
> > For this I had to dive into line_maps, macro maps, etc., so CCing
> > David to check
> > if my understanding of that is reasonable (hadn't worked with them
> > before).
> 
> (am looking)
> 
> > I've included this warning in -Wall, because there should be no false
> > positives
> > (fingers crossed) and for most cases the warning should be pretty
> > cheap.
> > 
> > I probably should've added a fix-it hint for good measure, too ("you
> > better wrap
> > the damn macro in do {} while (0)"), but that can be done as a follow
> > -up.
> 
> That would be excellent, but might be fiddly.  The fix-it hint
> machinery currently "avoids" macros.
> 
> See rich_location::reject_impossible_fixit, where we currently reject
> source_location (aka location_t) values that are within macro maps,
> putting the rich_location into a "something awkward is going on" mode
> where it doesn't display fix-it hints.  It ought to work if you're sure
> to use locations for the fixit that are within the line_map_ordinary
> for the *definition* of the macro - so some care is required.

Ouch.  Let's let it be for now.

> > Bootstrapped/regtested on x86_64-linux, ok for trunk?
> > 
> > 2017-06-01  Marek Polacek  <polacek@redhat.com>
> > 
> > 	PR c/80116
> > 	* c-common.h (warn_for_multiline_expansion): Declare.
> > 	* c-warn.c (warn_for_multiline_expansion): New function.
> > 	* c.opt (Wmultiline-expansion): New option.
> > 
> > 	* c-parser.c (c_parser_if_body): Set the location of the
> > 	body of the conditional after parsing all the labels.  Call
> > 	warn_for_multiline_expansion.
> > 	(c_parser_else_body): 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_multiline_expansion.
> > 
> > 	* doc/invoke.texi: Document -Wmultiline-expansion.
> > 
> > 	* c-c++-common/Wmultiline-expansion-1.c: New test.
> > 	* c-c++-common/Wmultiline-expansion-2.c: New test.
> > 	* c-c++-common/Wmultiline-expansion-3.c: New test.
> > 	* c-c++-common/Wmultiline-expansion-4.c: New test.
> > 	* c-c++-common/Wmultiline-expansion-5.c: New test.
> > 	* c-c++-common/Wmultiline-expansion-6.c: New test.
> > 
> > diff --git gcc/c-family/c-common.h gcc/c-family/c-common.h
> > index 79072e6..6efbebc 100644
> > --- gcc/c-family/c-common.h
> > +++ gcc/c-family/c-common.h
> > @@ -1539,6 +1539,7 @@ 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_multiline_expansion (location_t, location_t,
> > location_t);
> >  
> >  /* In c-attribs.c.  */
> >  extern bool attribute_takes_identifier_p (const_tree);
> > diff --git gcc/c-family/c-warn.c gcc/c-family/c-warn.c
> > index 012675b..16c6fc3 100644
> > --- gcc/c-family/c-warn.c
> > +++ gcc/c-family/c-warn.c
> > @@ -2392,3 +2392,105 @@ do_warn_duplicated_branches_r (tree *tp, int
> > *, void *)
> >      do_warn_duplicated_branches (*tp);
> >    return NULL_TREE;
> >  }
> > +
> > +/* Implementation of -Wmultiline-expansion.  This warning warns
> > about
> 
> Is the name of the warning correct?  Shouldn't the warning be about
> multiple *statement* macros, rather than multi-line macros?  The user
> could have written:
> 
> #define SWAP(x, y) tmp = x; x = y; y = tmp
> 
> which is all on one line.  (and is terrible code, yes).
 
Yeah, by "multiline" I really mean "multistatement".  But I hate the
name "-Wmultistatement-expansion".  Can anybode come up with something better?

> > +   cases when a macro expands to multiple statements not wrapped in
> > +   do {} while (0) or ({ }) and is used as a then branch or as an
> > else
> > +   branch.  For example,
> > +
> > +   #define DOIT x++; y++
> > +
> > +   if (c)
> > +     DOIT;
> > +
> > +   will increment y unconditionally.
> > +
> > +   BODY_LOC is the location of the if/else body, NEXT_LOC is the
> > location
> > +   of the next token after the if/else body has been parsed, and
> > IF_LOC
> > +   is the location of the if condition or of the "else" keyword.  */
> > +
> > +void
> > +warn_for_multiline_expansion (location_t body_loc, location_t
> > next_loc,
> > +			      location_t if_loc)
> 
> Should "if_loc" be "guard_loc"?
> (and cover while, for, do)
 
Sure, changed.

> > +{
> > +  if (!warn_multiline_expansion)
> > +    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 if_loc_exp
> > +    = linemap_resolve_location (line_table, if_loc,
> > +				LRK_MACRO_DEFINITION_LOCATION,
> > NULL);
> > +  /* These are some funky cases we don't want to warn about.  */
> > +  if (body_loc_exp == if_loc_exp
> 
> What if they are all in one macro expansion, e.g.:
> 
> #define BODY_AND_IF(COND, X, Y)   if (COND) SWAP (X, Y);
> 
> Then
> 
>    BODY_AND_IF(some_flag, x, y)
> 
> would (I believe) expand to:
> 
>   if (some_flag)
>     tmp = x;
>   x = y;
>   y = tmp;
> 
> which merits a warning.

Yea, we warn for this case, as we should.  I've added a new test, although
it should've been covered by the previous tests, too.
 
> > +      || next_loc_exp == if_loc_exp
> > +      || body_loc_exp == next_loc_exp)
> > +    return;
> 
> 
> > +  /* Find the macro map for the macro expansion BODY_LOC.  Every
> > +     macro expansion gets its own macro map and we're looking for
> > +     the macro map containing the expansion of BODY_LOC.  We can't
> > +     simply check if BODY_LOC == MAP_START_LOCATION, because the
> > +     macro can start with a label, and then the BODY_LOC is a bit
> > +     offset farther into the map.  */
> > +  const line_map_macro *map = NULL;
> 
> Suggest renaming "map" to "macro_map" to emphasize that it's a macro
> expansion.

Done.

> > +  for (const line_map_macro *cur_map = LINEMAPS_MACRO_MAPS
> > (line_table);
> > +       cur_map && cur_map <= LINEMAPS_LAST_MACRO_MAP (line_table);
> > +       ++cur_map)
> > +    {
> > +      linemap_assert (linemap_macro_expansion_map_p (cur_map));
> > +      if (body_loc >= MAP_START_LOCATION (cur_map)
> > +	  && body_loc < (MAP_START_LOCATION (cur_map)
> > +			 + MACRO_MAP_NUM_MACRO_TOKENS (cur_map)))
> > +	map = cur_map;
> > +    }
> 
> I believe you can use linemap_macro_map_lookup for this, which does a
> binary search through the macro maps, with caching, rather than a
> linear one.

Well, I can't use linemap_macro_map_lookup (that is a static function), but
what I can do, is to use linemap_lookup + linemap_check_macro.

> > +  /* We'd better find it.  */
> > +  gcc_assert (map != NULL);
> 
> I was wondering if this needs to be defensively coded, rather than an
> assert but given the
>   from_macro_expansion_at (body_loc)
> above, I don't see a way for map to be non-NULL.

I think given the above, no need to bother here.

> > +  /* 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're only looking at yN (i.e., the spelling locations) in
> > +     line_map_macro->macro_locations.  */
> 
> "and hence we only look at odd-numbered indexes within the
>  MACRO_MAP_LOCATIONS array" or somesuch.

Ok, I've adjusted the comment here.

> AIUI, this is looking at the spelling locations of the tokens within
> the *definition* of the macro, so e.g. for 
> 
> #define SWAP(x, y) \
>    tmp = x; \
>    x = y; \
>    y = tmp 
> 
> int a, b, c, tmp;
> if (c == 0)
>    SWAP(a, b);
> 
> expanded to:
> 
> int a, b, c, tmp;
> if (c == 0)  <== IF_LOC
>    tmp = a;  <== BODY_LOC, within macro map
>                            x = loc of "a" in SWAP usage
>                            y = loc of "tmp = x" in SWAP defn
>    a = b;    <== NEXT_LOC, within macro map
>                            x = loc of "b" in SWAP usage
>                            y = loc of "x = y" in SWAP defn
>    b = tmp;
> 
> though I'd have to check.

I think you are correct.  Matches my thinking.

> > +  bool found_if = false;
> > +  bool found_next = false;
> > +  for (unsigned int i = 1;
> > +       i < 2 * MACRO_MAP_NUM_MACRO_TOKENS (map);
> > +       i += 2)
> > +    {
> > +      if (MACRO_MAP_LOCATIONS (map)[i] == next_loc_exp)
> > +	found_next = true;
> > +      if (MACRO_MAP_LOCATIONS (map)[i] == if_loc_exp)
> > +	found_if = true;
> > +    }
> 
> So it looks like you're checking if NEXT_LOC_EXP and IF_LOC_EXP come
> from the definition.
> 
> Is there any chance that we're not dealing with tokens here?  (I was
> worried that we might be dealing with a location from an expression). 

I'm not sure I follow.  But...

>  The leading comment says "NEXT_LOC is the location of the next token";
> should the description of the rest of the arguments clarify that they
> too are locations of tokens?

...I clarified the comment, I hope.  So far it's been working as expected.

> > +  /* The if/else 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_if
> > +      && warning_at (body_loc, OPT_Wmultiline_expansion,
> > +		     "multiline macro expansion"))
> > +    inform (if_loc, "statement not guarded by this conditional");
> 
> IMHO this would be clearer if you restructure this final set of
> conditionals into a rejection conditional, and then the warning as a
> separate conditional; something like:
> 
> if (!found_next)
>   return;
> if (found_if)
>   return;
> 
> /* Passed all tests.  */
> 
> if (warning_at (etc))
>   inform (etc);

Ok, I've changed this, although in this case I could go either way.

> 
> 
> FWIW I strongly prefer the approach of
>   if (!test_1)
>     return
>   if (!test_2)
>     return;
>   ...
>   if (!test_N)
>     return;
> 
>   do_something ();
> 
> over:
> 
>   if (test_1)
>     if (test_2)
>       ...
>         if (test_N)
>            do_something ();
> 
> since the latter coding style tends to push things over to the right
> margin.

Yea, me too.
 
> Nitpick about the messages; should it be something like:
> 
> warning_at (..., "macro expands to multiple statements without %<do {}
> while (0)%> guard");

I've used this, but without the do while part.

> inform (..., "some parts of macro expansion are not guarded by this
> <%if%>")

And this too, but I think it should read "conditional" instead of "if".

> > +Wmultiline-expansion
> > +C ObjC C++ ObjC++ Var(warn_multiline_expansion) Warning
> > LangEnabledBy(C ObjC C++ ObjC++,Wall)
> > +Warn about macros expanding to multiple statements in a body of a
> > conditional.
> 
> (earlier nitpick applies here).

I've expanded a bit on the description.  Better name for the warning still
solicited ;).

> > +@item -Wmultiline-expansion
> > +@opindex Wmultiline-expansion
> > +@opindex Wno-multiline-expansion
> > +Warn about macros expanding to multiple statements in a body of a
> > conditional.
> > +For example:
> > +
> > +@smallexample
> > +#define DOIT x++; y++
> > +if (c)
> > +  DOIT;
> > +@end smallexample
> > +
> > +will increment @code{y} unconditionally, not just when @code{c}
> > holds.
> > +
> > +This warning is enabled by @option{-Wall} in C and C++.
> 
> Might be nice to mention the do-while workaround here.

Definitely.  Done along with a small example.

> Should the testcases have a version of PR 80063 within them?

I think not; I believe that one is well-covered by the other tests.

> Thanks; hope this was constructive.

Thanks for the review, it definitely made the patch much better!

This is the revised patch.

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

2017-06-02  Marek Polacek  <polacek@redhat.com>

	PR c/80116
	* c-common.h (warn_for_multiline_expansion): Declare.
	* c-warn.c (warn_for_multiline_expansion): New function.
	* c.opt (Wmultiline-expansion): New option.

	* c-parser.c (c_parser_if_body): Set the location of the
	body of the conditional after parsing all the labels.  Call
	warn_for_multiline_expansion.
	(c_parser_else_body): 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_multiline_expansion.
	(cp_parser_already_scoped_statement): Likewise.

	* doc/invoke.texi: Document -Wmultiline-expansion.

	* c-c++-common/Wmultiline-expansion-1.c: New test.
	* c-c++-common/Wmultiline-expansion-2.c: New test.
	* c-c++-common/Wmultiline-expansion-3.c: New test.
	* c-c++-common/Wmultiline-expansion-4.c: New test.
	* c-c++-common/Wmultiline-expansion-5.c: New test.
	* c-c++-common/Wmultiline-expansion-6.c: New test.
	* c-c++-common/Wmultiline-expansion-7.c: New test.
	* c-c++-common/Wmultiline-expansion-8.c: New test.
	* c-c++-common/Wmultiline-expansion-9.c: New test.

diff --git gcc/c-family/c-common.h gcc/c-family/c-common.h
index 79072e6..6efbebc 100644
--- gcc/c-family/c-common.h
+++ gcc/c-family/c-common.h
@@ -1539,6 +1539,7 @@ 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_multiline_expansion (location_t, location_t, location_t);
 
 /* In c-attribs.c.  */
 extern bool attribute_takes_identifier_p (const_tree);
diff --git gcc/c-family/c-warn.c gcc/c-family/c-warn.c
index 012675b..3e807be 100644
--- gcc/c-family/c-warn.c
+++ gcc/c-family/c-warn.c
@@ -2392,3 +2392,91 @@ do_warn_duplicated_branches_r (tree *tp, int *, void *)
     do_warn_duplicated_branches (*tp);
   return NULL_TREE;
 }
+
+/* Implementation of -Wmultiline-expansion.  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_multiline_expansion (location_t body_loc, location_t next_loc,
+			      location_t guard_loc)
+{
+  if (!warn_multiline_expansion)
+    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_Wmultiline_expansion,
+		  "macro expands to multiple statements"))
+    inform (guard_loc, "some parts of macro expansion are not guarded by "
+	    "this conditional");
+}
diff --git gcc/c-family/c.opt gcc/c-family/c.opt
index 37bb236..1043615 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.
 
+Wmultiline-expansion
+C ObjC C++ ObjC++ Var(warn_multiline_expansion) 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.
+
 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..83d2c86b 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_multiline_expansion (body_loc_after_labels, next_tinfo.location,
+				  if_tinfo.location);
 
   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_multiline_expansion (body_loc_after_labels, next_tinfo.location,
+				  else_tinfo.location);
 
   return c_end_compound_stmt (body_loc, block, flag_isoc99);
 }
@@ -5783,7 +5804,8 @@ 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 +5814,10 @@ 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_multiline_expansion (loc_after_labels, next_tinfo.location,
+				  while_tinfo.location);
+
   c_break_label = save_break;
   c_cont_label = save_cont;
 }
@@ -6076,7 +6102,8 @@ 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 +6116,10 @@ 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_multiline_expansion (loc_after_labels, next_tinfo.location,
+				  for_tinfo.location);
+
   c_break_label = save_break;
   c_cont_label = save_cont;
 }
diff --git gcc/cp/parser.c gcc/cp/parser.c
index 313eebb..60dfc48 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
@@ -10531,7 +10531,8 @@ 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;
@@ -10724,6 +10725,10 @@ 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);
 
@@ -12264,6 +12269,7 @@ 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));
 
@@ -12293,7 +12299,8 @@ 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);
     }
@@ -12302,6 +12309,11 @@ 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_multiline_expansion (body_loc_after_labels, next_tinfo.location,
+				guard_tinfo.location);
+
   /* Return the statement.  */
   return statement;
 }
@@ -12320,11 +12332,17 @@ 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_multiline_expansion (loc_after_labels, next_tinfo.location,
+				      guard_tinfo.location);
     }
   else
     {
diff --git gcc/doc/invoke.texi gcc/doc/invoke.texi
index 819e800..4688058 100644
--- gcc/doc/invoke.texi
+++ gcc/doc/invoke.texi
@@ -292,7 +292,7 @@ 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  -Wmultiline-expansion  -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
@@ -3822,6 +3822,7 @@ 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
+-Wmultiline-expansion  @gol
 -Wnarrowing @r{(only for C++)}  @gol
 -Wnonnull  @gol
 -Wnonnull-compare  @gol
@@ -4494,6 +4495,29 @@ This warning is enabled by @option{-Wall}.
 @opindex Wno-missing-include-dirs
 Warn if a user-supplied include directory does not exist.
 
+@item -Wmultiline-expansion
+@opindex Wmultiline-expansion
+@opindex Wno-multiline-expansion
+Warn about macros expanding to multiple statements in a body of a conditional,
+such as @code{if}, @code{else}, @code{for}, or @code{while}.
+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/Wmultiline-expansion-1.c gcc/testsuite/c-c++-common/Wmultiline-expansion-1.c
index e69de29..119d119 100644
--- gcc/testsuite/c-c++-common/Wmultiline-expansion-1.c
+++ gcc/testsuite/c-c++-common/Wmultiline-expansion-1.c
@@ -0,0 +1,118 @@
+/* PR c/80116 */
+/* { dg-options "-Wmultiline-expansion" } */
+/* { 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" } */ \
+    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" } */
+    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" } */
+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/Wmultiline-expansion-2.c gcc/testsuite/c-c++-common/Wmultiline-expansion-2.c
index e69de29..b39d253 100644
--- gcc/testsuite/c-c++-common/Wmultiline-expansion-2.c
+++ gcc/testsuite/c-c++-common/Wmultiline-expansion-2.c
@@ -0,0 +1,137 @@
+/* PR c/80116 */
+/* { dg-options "-Wmultiline-expansion" } */
+/* { 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" } */ \
+    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" } */
+    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" } */
+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/Wmultiline-expansion-3.c gcc/testsuite/c-c++-common/Wmultiline-expansion-3.c
index e69de29..c2e83af 100644
--- gcc/testsuite/c-c++-common/Wmultiline-expansion-3.c
+++ gcc/testsuite/c-c++-common/Wmultiline-expansion-3.c
@@ -0,0 +1,12 @@
+/* PR c/80116 */
+/* { dg-options "-Wmultiline-expansion" } */
+/* { 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/Wmultiline-expansion-4.c gcc/testsuite/c-c++-common/Wmultiline-expansion-4.c
index e69de29..3ab76a7 100644
--- gcc/testsuite/c-c++-common/Wmultiline-expansion-4.c
+++ gcc/testsuite/c-c++-common/Wmultiline-expansion-4.c
@@ -0,0 +1,14 @@
+/* PR c/80116 */
+/* { dg-options "-Wmultiline-expansion" } */
+/* { dg-do compile } */
+
+#define FN(C)		\
+  void			\
+  fn (void)		\
+  {			\
+    C;			\
+  }
+
+int i;
+
+FN (if (i) ++i)
diff --git gcc/testsuite/c-c++-common/Wmultiline-expansion-5.c gcc/testsuite/c-c++-common/Wmultiline-expansion-5.c
index e69de29..01ba0de 100644
--- gcc/testsuite/c-c++-common/Wmultiline-expansion-5.c
+++ gcc/testsuite/c-c++-common/Wmultiline-expansion-5.c
@@ -0,0 +1,18 @@
+/* PR c/80116 */
+/* { dg-options "-Wmultiline-expansion" } */
+/* { 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" } */
+   M (0); /* { dg-message "in expansion of macro .M." } */
+  if (x) /* { dg-message "not guarded" } */
+   M (1); /* { dg-message "in expansion of macro .M." } */
+}
diff --git gcc/testsuite/c-c++-common/Wmultiline-expansion-6.c gcc/testsuite/c-c++-common/Wmultiline-expansion-6.c
index e69de29..9f799e1 100644
--- gcc/testsuite/c-c++-common/Wmultiline-expansion-6.c
+++ gcc/testsuite/c-c++-common/Wmultiline-expansion-6.c
@@ -0,0 +1,22 @@
+/* PR c/80116 */
+/* { dg-options "-Wmultiline-expansion" } */
+/* { 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/Wmultiline-expansion-7.c gcc/testsuite/c-c++-common/Wmultiline-expansion-7.c
index e69de29..b561d43 100644
--- gcc/testsuite/c-c++-common/Wmultiline-expansion-7.c
+++ gcc/testsuite/c-c++-common/Wmultiline-expansion-7.c
@@ -0,0 +1,18 @@
+/* PR c/80116 */
+/* { dg-options "-Wmultiline-expansion" } */
+/* { 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/Wmultiline-expansion-8.c gcc/testsuite/c-c++-common/Wmultiline-expansion-8.c
index e69de29..c5d8205 100644
--- gcc/testsuite/c-c++-common/Wmultiline-expansion-8.c
+++ gcc/testsuite/c-c++-common/Wmultiline-expansion-8.c
@@ -0,0 +1,64 @@
+/* PR c/80116 */
+/* { dg-options "-Wmultiline-expansion" } */
+/* { 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" } */ \
+    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" } */ \
+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" } */
+    SWAP (x, y); /* { dg-message "in expansion of macro .SWAP." } */
+
+  for (i = 0; i < 1; ++i) /* { dg-message "not guarded" } */
+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/Wmultiline-expansion-9.c gcc/testsuite/c-c++-common/Wmultiline-expansion-9.c
index e69de29..c503ade 100644
--- gcc/testsuite/c-c++-common/Wmultiline-expansion-9.c
+++ gcc/testsuite/c-c++-common/Wmultiline-expansion-9.c
@@ -0,0 +1,62 @@
+/* PR c/80116 */
+/* { dg-options "-Wmultiline-expansion" } */
+/* { 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" } */ \
+    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" } */ \
+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" } */
+    SWAP (x, y); /* { dg-message "in expansion of macro .SWAP." } */
+
+  while (x) /* { dg-message "not guarded" } */
+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