Patchwork [3/7] Emit macro expansion related diagnostics

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Submitter Dodji Seketeli
Date Sept. 16, 2011, 7:55 a.m.
Message ID <m3mxe4ex9s.fsf@redhat.com>
Download mbox | patch
Permalink /patch/114898/
State New
Headers show

Comments

Dodji Seketeli - Sept. 16, 2011, 7:55 a.m.
> On 08/04/2011 11:32 AM, Dodji Seketeli wrote:
> > +++ b/gcc/diagnostic.c
> > @@ -30,6 +30,7 @@ along with GCC; see the file COPYING3.  If not see
> >  #include "input.h"
> >  #include "intl.h"
> >  #include "diagnostic.h"
> > +#include "vec.h"
> 
> Do you still need this?

Oops, no.  Removed and adjusted gcc/Makefile.in accordingly.

> 
> >  // Just discard errors pointing at header files
> >  // { dg-prune-output "include" }
> > +// { dg-prune-output "        from" }
> 
> These should be pruned by testsuite/lib/prune.exp.  I'm surprised they
> aren't already.

OK.  I have added that pruning to prune.exp and removed it from the
relevant test case files.

> 
> > +#define APPEND_LOC_TO_VEC(LOC)                                         \
> > +  if (num_locs >=3D loc_vec_capacity)                                  \
> > +    {                                                                  \
> > +      loc_vec_capacity +=3D 4;                                         \
> > +      loc_vec =3D XRESIZEVEC (loc_t, loc_vec, loc_vec_capacity);
> > \
> > +    }                                                                  \
> > +  loc_vec[num_locs++] =3D LOC;
> 
> Why not use VEC since we're in gcc/ here?

This is another leftover of when this code wasn't in gcc/.  I am using
VEC now in the amended patch.

> 
> > +/* Unwind the different macro expansions that lead to the token which
> > +   location is WHERE and emit diagnostics showing the resulting
> > +   unwound macro expansion stack.  If TOPMOST_EXP_POINT_MAP is
> > +   non-null, *TOPMOST_EXP_POINT_MAP is set to the map of the expansion
> > +   point of the top most macro of the stack.  This must be an ordinary
> > +   map.  */
> 
> I find the use of "top" here confusing.  You mean the place in the
> source that first triggered the macro expansion, right?

Yes.

>  Can we avoid talking about stacks here?

OK.  Sorry for the confusion.  I have removed the stack analogy added
hopefully more accurate comments.

[...]

> > +  while (unwind)
> > +    {
> ...
> > +      if (!linemap_macro_expansion_map_p (map))
> > +       unwind =3D false;
> > +    }
> 
> This seems like a job for do/while.

Updated accordingly.

I have fixed some other nits in the patch, bootstrapped and tested it on
x86_64-unknown-linux-gnu against a tree based on trunk and containing
the previous patches of the set.

Thanks.

From: Dodji Seketeli <dodji@redhat.com>
Date: Sat, 4 Dec 2010 16:31:35 +0100
Subject: [PATCH 3/7] Emit macro expansion related diagnostics

In this third instalment the diagnostic machinery -- when faced with
the virtual location of a token resulting from macro expansion -- uses
the new linemap APIs to unwind the stack of macro expansions that led
to that token and emits a [hopefully] more useful message than what we
have today.

diagnostic_report_current_module has been slightly changed to use the
location given by client code instead of the global input_location
variable.  This results in more precise diagnostic locations in
general but then the patch adjusts some C++ tests which output changed
as a result of this.

Three new regression tests have been added.

The mandatory screenshot goes like this:

[dodji@adjoa gcc]$ cat -n test.c
     1    #define OPERATE(OPRD1, OPRT, OPRD2) \
     2      OPRD1 OPRT OPRD2;
     3
     4    #define SHIFTL(A,B) \
     5      OPERATE (A,<<,B)
     6
     7    #define MULT(A) \
     8      SHIFTL (A,1)
     9
    10    void
    11    g ()
    12    {
    13      MULT (1.0);/* 1.0 << 1; <-- so this is an error.  */
    14    }

[dodji@adjoa gcc]$ ./cc1 -quiet -ftrack-macro-expansion test.c
test.c: In function ‘g’:
test.c:5:14: erreur: invalid operands to binary << (have ‘double’ and ‘int’)
test.c:2:9: note: in expansion of macro 'OPERATE'
test.c:5:3: note: expanded from here
test.c:5:14: note: in expansion of macro 'SHIFTL'
test.c:8:3: note: expanded from here
test.c:8:3: note: in expansion of macro 'MULT2'
test.c:13:3: note: expanded from here

The combination of this patch and the previous ones boostrapped with
--enable-languages=all,ada and passed regression tests on
x86_64-unknown-linux-gnu.

gcc/
	* gcc/diagnostic.h (diagnostic_report_current_module): Add a
	location parameter.
	* diagnostic.c (diagnostic_report_current_module): Add a location
	parameter to the function definition.  Use it instead of
	input_location.  Resolve the virtual location rather than just
	looking up its map and risking to touch a resulting macro map.
	(default_diagnostic_starter): Pass the relevant diagnostic
	location to diagnostic_report_current_module.
	* tree-diagnostic.c (maybe_unwind_expanded_macro_loc): New.
	(virt_loc_aware_diagnostic_finalizer): Likewise.
	(diagnostic_report_current_function): Pass the
	relevant location to diagnostic_report_current_module.
	* tree-diagnostic.h (virt_loc_aware_diagnostic_finalizer): Declare
	new function.
	* toplev.c (general_init): By default, use the new
	virt_loc_aware_diagnostic_finalizer as diagnostic finalizer.
	* Makefile.in: Add vec.h dependency to tree-diagnostic.c.

gcc/cp/

	* error.c (cp_diagnostic_starter): Pass the relevant location to
	diagnostic_report_current_module.
	(cp_diagnostic_finalizer): Call virt_loc_aware_diagnostic_finalizer.

gcc/testsuite/

	* lib/prune.exp (prune_gcc_output):  Prune output referring to
	included files.
	* gcc.dg/cpp/macro-exp-tracking-1.c: New test.
	* gcc.dg/cpp/macro-exp-tracking-2.c: Likewise.
	* gcc.dg/cpp/macro-exp-tracking-3.c: Likewise.
	* gcc.dg/cpp/pragma-diagnostic-2.c: Likewise.
---
 gcc/Makefile.in                                 |    3 +-
 gcc/cp/error.c                                  |    5 +-
 gcc/diagnostic.c                                |   13 +-
 gcc/diagnostic.h                                |    2 +-
 gcc/testsuite/gcc.dg/cpp/macro-exp-tracking-1.c |   21 +++
 gcc/testsuite/gcc.dg/cpp/macro-exp-tracking-2.c |   21 +++
 gcc/testsuite/gcc.dg/cpp/macro-exp-tracking-3.c |   14 ++
 gcc/testsuite/gcc.dg/cpp/macro-exp-tracking-4.c |   14 ++
 gcc/testsuite/gcc.dg/cpp/pragma-diagnostic-2.c  |   34 ++++
 gcc/testsuite/lib/prune.exp                     |    1 +
 gcc/toplev.c                                    |    3 +
 gcc/tree-diagnostic.c                           |  208 ++++++++++++++++++++++-
 gcc/tree-diagnostic.h                           |    3 +-
 13 files changed, 331 insertions(+), 11 deletions(-)
 create mode 100644 gcc/testsuite/gcc.dg/cpp/macro-exp-tracking-1.c
 create mode 100644 gcc/testsuite/gcc.dg/cpp/macro-exp-tracking-2.c
 create mode 100644 gcc/testsuite/gcc.dg/cpp/macro-exp-tracking-3.c
 create mode 100644 gcc/testsuite/gcc.dg/cpp/macro-exp-tracking-4.c
 create mode 100644 gcc/testsuite/gcc.dg/cpp/pragma-diagnostic-2.c
Jason Merrill - Sept. 17, 2011, 2:11 p.m.
On 09/16/2011 03:55 AM, Dodji Seketeli wrote:
> +    test.c: In function ‘g’:
> +    test.c:5:14: error: invalid operands to binary << (have ‘double’ and ‘int’)
> +    test.c:2:9: note: in expansion of macro 'OPERATE'
> +    test.c:5:3: note: expanded from here
> +    test.c:5:14: note: in expansion of macro 'SHIFTL'
> +    test.c:8:3: note: expanded from here
> +    test.c:8:3: note: in expansion of macro 'MULT2'
> +    test.c:13:3: note: expanded from here
> +
> +   The part that goes from the third to the sixth line of this
> +   diagnostic (the lines containing the 'note:' string) is called the
> +   unwound macro expansion trace.  That's the part generated by this
> +   function.

No UTF-8 in the code, please.

Do you mean eighth rather than sixth?

> +      /* If the token at RESOLVED_LOCATION [at macro definition point]
> +         is itself inside an expanded macro then we keep unwinding the
> +         trace by reaching the "parent macro" that got expanded inside
> +         the definition of the macro of MAP...  */

This doesn't make sense to me; a macro definition can't be inside an 
expanded macro.  Can you describe this situation better?

> +  if (first_exp_point_map)
> +    *first_exp_point_map = map;
> +
> +  /* Walk LOC_VEC and print the macro expansion trace, unless the
> +     first macro which expansion triggered this trace was expanded
> +     inside a system header.  */
> +  if (!LINEMAP_SYSP (resolved_map))

Should the SYSP check use "map" rather than "resolved_map"?

Jason

Patch

diff --git a/gcc/Makefile.in b/gcc/Makefile.in
index 92016f2..d26c682 100644
--- a/gcc/Makefile.in
+++ b/gcc/Makefile.in
@@ -2796,7 +2796,8 @@  tree-pretty-print.o : tree-pretty-print.c $(CONFIG_H) $(SYSTEM_H) \
    $(TM_H) coretypes.h tree-iterator.h $(SCEV_H) langhooks.h \
    $(TREE_PASS_H) value-prof.h output.h tree-pretty-print.h
 tree-diagnostic.o : tree-diagnostic.c $(CONFIG_H) $(SYSTEM_H) coretypes.h \
-   $(TREE_H) $(DIAGNOSTIC_H) tree-diagnostic.h langhooks.h $(LANGHOOKS_DEF_H)
+   $(TREE_H) $(DIAGNOSTIC_H) tree-diagnostic.h langhooks.h $(LANGHOOKS_DEF_H) \
+   $(VEC_H)
 fold-const.o : fold-const.c $(CONFIG_H) $(SYSTEM_H) coretypes.h $(TM_H) \
    $(TREE_H) $(FLAGS_H) $(DIAGNOSTIC_CORE_H) $(HASHTAB_H) $(EXPR_H) $(RTL_H) \
    $(GGC_H) $(TM_P_H) langhooks.h $(MD5_H) intl.h $(TARGET_H) \
diff --git a/gcc/cp/error.c b/gcc/cp/error.c
index 598ddf1..8fa163f 100644
--- a/gcc/cp/error.c
+++ b/gcc/cp/error.c
@@ -2767,7 +2767,7 @@  static void
 cp_diagnostic_starter (diagnostic_context *context,
 		       diagnostic_info *diagnostic)
 {
-  diagnostic_report_current_module (context);
+  diagnostic_report_current_module (context, diagnostic->location);
   cp_print_error_function (context, diagnostic);
   maybe_print_instantiation_context (context);
   maybe_print_constexpr_context (context);
@@ -2777,8 +2777,9 @@  cp_diagnostic_starter (diagnostic_context *context,
 
 static void
 cp_diagnostic_finalizer (diagnostic_context *context,
-			 diagnostic_info *diagnostic ATTRIBUTE_UNUSED)
+			 diagnostic_info *diagnostic)
 {
+  virt_loc_aware_diagnostic_finalizer (context, diagnostic);
   pp_base_destroy_prefix (context->printer);
 }
 
diff --git a/gcc/diagnostic.c b/gcc/diagnostic.c
index b46eb35..0344937 100644
--- a/gcc/diagnostic.c
+++ b/gcc/diagnostic.c
@@ -255,9 +255,9 @@  diagnostic_action_after_output (diagnostic_context *context,
 }
 
 void
-diagnostic_report_current_module (diagnostic_context *context)
+diagnostic_report_current_module (diagnostic_context *context, location_t where)
 {
-  const struct line_map *map;
+  const struct line_map *map = NULL;
 
   if (pp_needs_newline (context->printer))
     {
@@ -265,10 +265,13 @@  diagnostic_report_current_module (diagnostic_context *context)
       pp_needs_newline (context->printer) = false;
     }
 
-  if (input_location <= BUILTINS_LOCATION)
+  if (where <= BUILTINS_LOCATION)
     return;
 
-  map = linemap_lookup (line_table, input_location);
+  linemap_resolve_location (line_table, where,
+			    LRK_MACRO_PARM_REPLACEMENT_POINT,
+			    &map);
+
   if (map && diagnostic_last_module_changed (context, map))
     {
       diagnostic_set_last_module (context, map);
@@ -301,7 +304,7 @@  void
 default_diagnostic_starter (diagnostic_context *context,
 			    diagnostic_info *diagnostic)
 {
-  diagnostic_report_current_module (context);
+  diagnostic_report_current_module (context, diagnostic->location);
   pp_set_prefix (context->printer, diagnostic_build_prefix (context,
 							    diagnostic));
 }
diff --git a/gcc/diagnostic.h b/gcc/diagnostic.h
index 8074354..4b1265b 100644
--- a/gcc/diagnostic.h
+++ b/gcc/diagnostic.h
@@ -253,7 +253,7 @@  extern diagnostic_context *global_dc;
 /* Diagnostic related functions.  */
 extern void diagnostic_initialize (diagnostic_context *, int);
 extern void diagnostic_finish (diagnostic_context *);
-extern void diagnostic_report_current_module (diagnostic_context *);
+extern void diagnostic_report_current_module (diagnostic_context *, location_t);
 
 /* Force diagnostics controlled by OPTIDX to be kind KIND.  */
 extern diagnostic_t diagnostic_classify_diagnostic (diagnostic_context *,
diff --git a/gcc/testsuite/gcc.dg/cpp/macro-exp-tracking-1.c b/gcc/testsuite/gcc.dg/cpp/macro-exp-tracking-1.c
new file mode 100644
index 0000000..d975c8c
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/cpp/macro-exp-tracking-1.c
@@ -0,0 +1,21 @@ 
+/*
+   { dg-options "-ftrack-macro-expansion=1" }
+   { dg-do compile }
+*/
+
+#define OPERATE(OPRD1, OPRT, OPRD2) \
+do \
+{ \
+  OPRD1 OPRT OPRD2; /* { dg-message "expansion" }*/ 	   \
+} while (0)
+
+#define SHIFTL(A,B) \
+  OPERATE (A,<<,B) /* { dg-message "expanded|expansion" } */
+
+void
+foo ()
+{
+  SHIFTL (0.1,0.2); /* { dg-message "expanded" } */
+}
+
+/* { dg-error "invalid operands" "" { target *-*-* } 13 } */
diff --git a/gcc/testsuite/gcc.dg/cpp/macro-exp-tracking-2.c b/gcc/testsuite/gcc.dg/cpp/macro-exp-tracking-2.c
new file mode 100644
index 0000000..684af4c
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/cpp/macro-exp-tracking-2.c
@@ -0,0 +1,21 @@ 
+/* 
+   { dg-options "-ftrack-macro-expansion=1" }
+   { dg-do compile }
+*/
+
+#define OPERATE(OPRD1, OPRT, OPRD2) \
+ OPRD1 OPRT OPRD2;		/* { dg-message "expansion" } */
+
+#define SHIFTL(A,B) \
+  OPERATE (A,<<,B) /* { dg-message "expanded|expansion" } */
+
+#define MULT(A) \
+  SHIFTL (A,1)			/* { dg-message "expanded|expansion" } */
+
+void
+foo ()
+{
+  MULT (1.0);			/* { dg-message "expanded" } */
+}
+
+/* { dg-error "invalid operands to binary <<" "" { target *-*-* } { 10 } } */
diff --git a/gcc/testsuite/gcc.dg/cpp/macro-exp-tracking-3.c b/gcc/testsuite/gcc.dg/cpp/macro-exp-tracking-3.c
new file mode 100644
index 0000000..119053e
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/cpp/macro-exp-tracking-3.c
@@ -0,0 +1,14 @@ 
+/*
+  { dg-options "-fshow-column -ftrack-macro-expansion=1" }
+  { dg-do compile }
+ */
+
+#define SQUARE(A) A * A		/* { dg-message "expansion" } */
+
+void
+foo()
+{
+  SQUARE (1 << 0.1);		/* { dg-message "expanded" } */
+}
+
+/* { dg-error "16:invalid operands to binary <<" "" {target *-*-* } { 11 } } */
diff --git a/gcc/testsuite/gcc.dg/cpp/macro-exp-tracking-4.c b/gcc/testsuite/gcc.dg/cpp/macro-exp-tracking-4.c
new file mode 100644
index 0000000..1f9fe6a
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/cpp/macro-exp-tracking-4.c
@@ -0,0 +1,14 @@ 
+/*
+  { dg-options "-fshow-column -ftrack-macro-expansion=2" }
+  { dg-do compile }
+ */
+
+#define SQUARE(A) A * A		/* { dg-message "expansion" } */
+
+void
+foo()
+{
+  SQUARE (1 << 0.1);		/* { dg-message "expanded" } */
+}
+
+/* { dg-error "13:invalid operands to binary <<" "" { target *-*-* } { 11 } } */
diff --git a/gcc/testsuite/gcc.dg/cpp/pragma-diagnostic-2.c b/gcc/testsuite/gcc.dg/cpp/pragma-diagnostic-2.c
new file mode 100644
index 0000000..7ab95b0
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/cpp/pragma-diagnostic-2.c
@@ -0,0 +1,34 @@ 
+/*
+  { dg-options "-Wuninitialized -ftrack-macro-expansion=2" }
+  { dg-do compile }
+*/
+
+void f (unsigned);
+
+#define CODE_WITH_WARNING \
+  int a; /* { dg-message "expansion|declared here" } */  \
+  f (a)	 /* { dg-message "expansion" } */
+
+#pragma GCC diagnostic ignored "-Wuninitialized"
+
+void
+g (void)
+{
+  CODE_WITH_WARNING;
+}
+
+#pragma GCC diagnostic push
+
+#pragma GCC diagnostic error "-Wuninitialized"
+
+void
+h (void)
+{
+  CODE_WITH_WARNING;		/* { dg-message "expanded" } */
+}
+
+/*
+  { dg-message "some warnings being treated as errors" "" {target *-*-*} 0 }
+*/
+
+/* { dg-error "uninitialized" "" { target *-*-* } { 10 } } */
diff --git a/gcc/testsuite/lib/prune.exp b/gcc/testsuite/lib/prune.exp
index 4683f93..09d2581 100644
--- a/gcc/testsuite/lib/prune.exp
+++ b/gcc/testsuite/lib/prune.exp
@@ -29,6 +29,7 @@  proc prune_gcc_output { text } {
     regsub -all "(^|\n)collect: re(compiling|linking)\[^\n\]*" $text "" text
     regsub -all "(^|\n)Please submit.*instructions\[^\n\]*" $text "" text
     regsub -all "(^|\n)\[0-9\]\[0-9\]* errors\." $text "" text
+    regsub -all "(^|\n)(In file included|\[ \]+from)\[^\n\]*" $text "" text
 
     # Ignore informational notes.
     regsub -all "(^|\n)\[^\n\]*: note: \[^\n\]*" $text "" text
diff --git a/gcc/toplev.c b/gcc/toplev.c
index de0a58a..5f63b69 100644
--- a/gcc/toplev.c
+++ b/gcc/toplev.c
@@ -1132,6 +1132,9 @@  general_init (const char *argv0)
      can give warnings and errors.  */
   diagnostic_initialize (global_dc, N_OPTS);
   diagnostic_starter (global_dc) = default_tree_diagnostic_starter;
+  /* By default print macro expansion contexts in the diagnostic
+     finalizer -- for tokens resulting from macro macro expansion.  */
+  diagnostic_finalizer (global_dc) = virt_loc_aware_diagnostic_finalizer;
   /* Set a default printer.  Language specific initializations will
      override it later.  */
   pp_format_decoder (global_dc->printer) = &default_tree_printer;
diff --git a/gcc/tree-diagnostic.c b/gcc/tree-diagnostic.c
index b456a2a..ff7a749 100644
--- a/gcc/tree-diagnostic.c
+++ b/gcc/tree-diagnostic.c
@@ -28,6 +28,7 @@  along with GCC; see the file COPYING3.  If not see
 #include "tree-diagnostic.h"
 #include "langhooks.h"
 #include "langhooks-def.h"
+#include "vec.h"
 
 /* Prints out, if necessary, the name of the current function
    that caused an error.  Called from all error and warning functions.  */
@@ -35,7 +36,7 @@  void
 diagnostic_report_current_function (diagnostic_context *context,
 				    diagnostic_info *diagnostic)
 {
-  diagnostic_report_current_module (context);
+  diagnostic_report_current_module (context, diagnostic->location);
   lang_hooks.print_error_function (context, input_filename, diagnostic);
 }
 
@@ -47,3 +48,208 @@  default_tree_diagnostic_starter (diagnostic_context *context,
   pp_set_prefix (context->printer, diagnostic_build_prefix (context,
 							    diagnostic));
 }
+
+/* This is a pair made of a location and the line map it originated
+   from.  It's used in the maybe_unwind_expanded_macro_loc function
+   below.  */
+typedef struct
+{
+  const struct line_map *map;
+  source_location where;
+} loc_t;
+
+DEF_VEC_O (loc_t);
+DEF_VEC_ALLOC_O (loc_t, heap);
+
+/* Unwind the different macro expansions that lead to the token which
+   location is WHERE and emit diagnostics showing the resulting
+   unwound macro expansion trace.  Let's look at an example to see how
+   the trace looks like.  Suppose we have this piece of code,
+   artificially annotated with the line numbers to increase
+   legibility:
+
+    $ cat -n test.c
+      1    #define OPERATE(OPRD1, OPRT, OPRD2) \
+      2      OPRD1 OPRT OPRD2;
+      3
+      4    #define SHIFTL(A,B) \
+      5      OPERATE (A,<<,B)
+      6
+      7    #define MULT(A) \
+      8      SHIFTL (A,1)
+      9
+     10    void
+     11    g ()
+     12    {
+     13      MULT (1.0);// 1.0 << 1; <-- so this is an error.
+     14    }
+
+   Here is the diagnostic that we want the compiler to generate:
+
+    test.c: In function ‘g’:
+    test.c:5:14: error: invalid operands to binary << (have ‘double’ and ‘int’)
+    test.c:2:9: note: in expansion of macro 'OPERATE'
+    test.c:5:3: note: expanded from here
+    test.c:5:14: note: in expansion of macro 'SHIFTL'
+    test.c:8:3: note: expanded from here
+    test.c:8:3: note: in expansion of macro 'MULT2'
+    test.c:13:3: note: expanded from here
+
+   The part that goes from the third to the sixth line of this
+   diagnostic (the lines containing the 'note:' string) is called the
+   unwound macro expansion trace.  That's the part generated by this
+   function.
+
+   If FIRST_EXP_POINT_MAP is non-null, *FIRST_EXP_POINT_MAP is set to
+   the map of the location in the source that first triggered the
+   macro expansion.  This must be an ordinary map.  */
+
+static void
+maybe_unwind_expanded_macro_loc (diagnostic_context *context,
+                                 diagnostic_info *diagnostic,
+                                 source_location where,
+                                 const struct line_map **first_exp_point_map)
+{
+  const struct line_map *map, *resolved_map;
+  source_location resolved_location;
+  VEC(loc_t,heap) *loc_vec = NULL;
+  unsigned ix;
+  loc_t loc, *iter;
+
+  map = linemap_lookup (line_table, where);
+  if (!linemap_macro_expansion_map_p (map))
+    return;
+
+  /* Let's unwind the macros that got expanded and led to the token
+     which location is WHERE.  We are going to store these macros into
+     LOC_VEC, so that we can later walk it at our convenience to
+     display a somewhat meaningful trace of the macro expansion
+     history to the user.  Note that the first macro of the trace
+     (which is OPERATE in the example above) is going to be stored at
+     the beginning of LOC_VEC.  */
+
+  do
+    {
+      loc.where = where;
+      loc.map = map;
+
+      VEC_safe_push (loc_t, heap, loc_vec, &loc);
+
+      /* WHERE is the location of a token inside the expansion of a
+         macro.  MAP is the map holding the locations of that macro
+         expansion.  Let's get the location of the token inside the
+         *definition* of the macro of MAP, that got expanded at WHERE.
+         This is basically how we go "down" in the trace of macro
+         expansions that led to WHERE.  */
+      resolved_location =
+        linemap_macro_map_loc_to_def_point (map, where, false);
+      resolved_map = linemap_lookup (line_table, resolved_location);
+
+      /* If the token at RESOLVED_LOCATION [at macro definition point]
+         is itself inside an expanded macro then we keep unwinding the
+         trace by reaching the "parent macro" that got expanded inside
+         the definition of the macro of MAP...  */
+      if (linemap_macro_expansion_map_p (resolved_map))
+        {
+          where = resolved_location;
+          map = resolved_map;
+        }
+      else
+        {
+          /* Otherwise, let's consider the location of the expansion
+             point of the macro of MAP.  Keep in mind that MAP is a
+             macro expansion map.  To get a "normal map" (i.e a non
+             macro expansion map) and be done with the unwinding, we
+             must either consider the location of the expansion point
+             of the macro or the location of the token inside the
+             macro definition that got expanded to WHERE.  */
+          where =
+            linemap_macro_map_loc_to_exp_point (map, where);
+          map = linemap_lookup (line_table, where);
+        }
+    } while (linemap_macro_expansion_map_p (map));
+
+  if (first_exp_point_map)
+    *first_exp_point_map = map;
+
+  /* Walk LOC_VEC and print the macro expansion trace, unless the
+     first macro which expansion triggered this trace was expanded
+     inside a system header.  */
+  if (!LINEMAP_SYSP (resolved_map))
+    FOR_EACH_VEC_ELT (loc_t, loc_vec, ix, iter)
+      {
+        source_location resolved_def_loc = 0, resolved_exp_loc = 0;
+        diagnostic_t saved_kind;
+        const char *saved_prefix;
+        source_location saved_location;
+
+        /* Okay, now here is what we want.  For each token resulting
+           from macro expansion we want to show: 1/ where in the
+           definition of the macro the token comes from; 2/ where the
+           macro got expanded.  */
+
+        /* Resolve the location iter->where into the locus 1/ of the
+           comment above.  */
+        resolved_def_loc =
+          linemap_resolve_location (line_table, iter->where,
+                                    LRK_MACRO_PARM_REPLACEMENT_POINT, NULL);
+
+        /* Resolve the location of the expansion point of the macro
+           which expansion gave the token represented by def_loc.
+           This is the locus 2/ of the earlier comment.  */
+        resolved_exp_loc =
+          linemap_resolve_location (line_table,
+                                    MACRO_MAP_EXPANSION_POINT_LOCATION (iter->map),
+                                    LRK_MACRO_PARM_REPLACEMENT_POINT, NULL);
+
+        saved_kind = diagnostic->kind;
+        saved_prefix = context->printer->prefix;
+        saved_location = diagnostic->location;
+
+        diagnostic->kind = DK_NOTE;
+        diagnostic->location = resolved_def_loc;
+        pp_base_set_prefix (context->printer,
+                            diagnostic_build_prefix (context,
+                                                     diagnostic));
+        pp_newline (context->printer);
+        pp_printf (context->printer, "in expansion of macro '%s'",
+                   linemap_map_get_macro_name (iter->map));
+        pp_destroy_prefix (context->printer);
+
+        diagnostic->location = resolved_exp_loc;
+        pp_base_set_prefix (context->printer,
+                            diagnostic_build_prefix (context,
+                                                     diagnostic));
+        pp_newline (context->printer);
+        pp_printf (context->printer, "expanded from here");
+        pp_destroy_prefix (context->printer);
+
+        diagnostic->kind = saved_kind;
+        diagnostic->location = saved_location;
+        context->printer->prefix = saved_prefix;
+      }
+
+  VEC_free (loc_t, heap, loc_vec);
+}
+
+/*  This is a diagnostic finalizer implementation that is aware of
+    virtual locations produced by libcpp.
+
+    It has to be called by the diagnostic finalizer of front ends that
+    uses libcpp and wish to get diagnostics involving tokens resulting
+    from macro expansion.
+
+    For a given location, if said location belongs to a token
+    resulting from a macro expansion, this starter prints the context
+    of the token.  E.g, for multiply nested macro expansion, it
+    unwinds the nested macro expansions and prints them in a manner
+    that is similar to what is done for function call stacks, or
+    template instantiation contexts.  */
+void
+virt_loc_aware_diagnostic_finalizer (diagnostic_context *context,
+				     diagnostic_info *diagnostic)
+{
+  maybe_unwind_expanded_macro_loc (context, diagnostic,
+				   diagnostic->location,
+				   NULL);
+}
diff --git a/gcc/tree-diagnostic.h b/gcc/tree-diagnostic.h
index 7d88089..6b8e8e6 100644
--- a/gcc/tree-diagnostic.h
+++ b/gcc/tree-diagnostic.h
@@ -52,5 +52,6 @@  along with GCC; see the file COPYING3.  If not see
 void default_tree_diagnostic_starter (diagnostic_context *, diagnostic_info *);
 extern void diagnostic_report_current_function (diagnostic_context *,
 						diagnostic_info *);
-
+void virt_loc_aware_diagnostic_finalizer (diagnostic_context *,
+					  diagnostic_info *);
 #endif /* ! GCC_TREE_DIAGNOSTIC_H */