[PATCH 3/4] Use SOURCE_PREFIX_MAP envvar to transform __FILE__

[Ximin Luo <infinity0 at pwned dot gg>]

Honour the SOURCE_PREFIX_MAP environment variable when expanding the __FILE__
macro, in the same way that debug-prefix-map works for debugging symbol paths.

This patch follows similar lines to the earlier patch for SOURCE_DATE_EPOCH.
Specifically, we read the environment variable not in libcpp but via a hook
which has an implementation defined in gcc/c-family. However, to achieve this
is more complex than the earlier patch: we need to share the prefix_map data
structure and associated functions between libcpp and c-family. Therefore, we
need to move these to libiberty. (For comparison, the SOURCE_DATE_EPOCH patch
did not need this because time_t et. al. are in the standard C library.)

Acknowledgements
----------------

Dhole <dhole@openmailbox.org> who wrote the earlier patch for SOURCE_DATE_EPOCH
which saved me a lot of time on figuring out what to edit.

ChangeLogs
----------

include/ChangeLog:

2016-11-01  Ximin Luo  <infinity0@pwned.gg>

	* prefix-map.h: New file, mostly derived from /gcc/final.c.

libiberty/ChangeLog:

2016-11-01  Ximin Luo  <infinity0@pwned.gg>

	* prefix-map.c: New file, mostly derived from /gcc/final.c.
	* Makefile.in: Update for new files.

gcc/ChangeLog:

2016-11-01  Ximin Luo  <infinity0@pwned.gg>

	* final.c: Generalise and refactor code related to debug_prefix_map.
	Move some of it to /libiberty/prefix-map.c, /include/prefix-map.h
	and refactor the remaining code to use the moved-out things.
	* doc/invoke.texi (Environment Variables): Update SOURCE_PREFIX_MAP to
	describe how it affects __FILE__ expansion.

gcc/c-family/ChangeLog:

2016-11-01  Ximin Luo  <infinity0@pwned.gg>

	* c-common.c (cb_get_source_prefix_map): Define new call target.
	* c-common.h (cb_get_source_prefix_map): Declare call target.
	* c-lex.c (init_c_lex): Set the get_source_prefix_map callback.

libcpp/ChangeLog:

2016-11-01  Ximin Luo  <infinity0@pwned.gg>

	* include/cpplib.h (cpp_callbacks): Add get_source_prefix_map
	callback.
	* init.c (cpp_create_reader): Initialise source_prefix_map field.
	* internal.h (cpp_reader): Add new field source_prefix_map.
	* macro.c (_cpp_builtin_macro_text): Set the source_prefix_map field
	if unset and apply it to the __FILE__ macro.

gcc/testsuite/ChangeLog:

2016-11-01  Ximin Luo  <infinity0@pwned.gg>

	* gcc.dg/cpp/source_prefix_map-1.c: New test.
	* gcc.dg/cpp/source_prefix_map-2.c: New test.

Index: gcc-7-20161030/include/prefix-map.h
===================================================================
--- /dev/null
+++ gcc-7-20161030/include/prefix-map.h
@@ -0,0 +1,71 @@
+/* Declarations for manipulating filename prefixes.
+
+   Copyright (C) 2016 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software Foundation,
+   Inc., 51 Franklin Street - Fifth Floor, Boston, MA 02110-1301, USA.  */
+
+#ifndef _PREFIX_MAP_H
+#define _PREFIX_MAP_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifdef HAVE_STDLIB_H
+#include <stdlib.h>
+#endif
+
+/* Linked-list of mappings from old prefixes to new prefixes.  */
+
+struct prefix_map
+{
+  const char *old_prefix;
+  const char *new_prefix;
+  size_t old_len;
+  size_t new_len;
+  struct prefix_map *next;
+};
+
+/* Parse a single prefix-map.
+
+   The string `arg' is split at the final '=' character. The part before
+   it is used to set `map->old_prefix' and `map->old_len', and the part
+   after it is used to set `map->new_prefix' and `map->new_len'.
+
+   If `arg' does not contain a '=' then 0 is returned. Otherwise, a
+   non-zero value is returned.
+   */
+
+extern int parse_prefix_map (const char *arg, struct prefix_map *map);
+
+/* Perform mapping of filename prefixes.
+
+   Return the filename corresponding to `old_name'. The return value is
+   equal to `old_name' if no transformation occurred, else it is equal
+   to `new_name' where the new filename is stored.
+
+   On entry into this function, `new_name' must be able to hold at least
+   `(old_name - map->old_len + map->old_len + 1)' characters, where
+   `map' is the mapping that will be selected and performed.
+   */
+
+extern const char *apply_prefix_map (const char *old_name, char *new_name,
+				     struct prefix_map *map_head);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _PREFIX_MAP_H */
Index: gcc-7-20161030/libiberty/prefix-map.c
===================================================================
--- /dev/null
+++ gcc-7-20161030/libiberty/prefix-map.c
@@ -0,0 +1,88 @@
+/* Definitions for manipulating filename prefixes.
+
+   Copyright (C) 2016 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software Foundation,
+   Inc., 51 Franklin Street - Fifth Floor, Boston, MA 02110-1301, USA.  */
+
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
+
+#ifdef HAVE_STRING_H
+#include <string.h>
+#endif
+
+#ifdef HAVE_STDLIB_H
+#include <stdlib.h>
+#endif
+
+#include "filenames.h"
+#include "libiberty.h"
+#include "prefix-map.h"
+
+/* Parse a single prefix-map.
+
+   The string `arg' is split at the final '=' character. The part before
+   it is used to set `map->old_prefix' and `map->old_len', and the part
+   after it is used to set `map->new_prefix' and `map->new_len'.
+
+   If `arg' does not contain a '=' then 0 is returned. Otherwise, a
+   non-zero value is returned.
+   */
+int
+parse_prefix_map (const char *arg, struct prefix_map *map)
+{
+  const char *p;
+  p = strrchr (arg, '=');
+  if (!p)
+    {
+      return 0;
+    }
+  map->old_prefix = xstrndup (arg, p - arg);
+  map->old_len = p - arg;
+  p++;
+  map->new_prefix = xstrdup (p);
+  map->new_len = strlen (p);
+  return 1;
+}
+
+/* Perform mapping of filename prefixes.
+
+   Return the filename corresponding to `old_name'. The return value is
+   equal to `old_name' if no transformation occurred, else it is equal
+   to `new_name' where the new filename is stored.
+
+   On entry into this function, `new_name' must be able to hold at least
+   `(old_name - map->old_len + map->old_len + 1)' characters, where
+   `map' is the mapping that will be selected and performed.
+   */
+const char *
+apply_prefix_map (const char *old_name, char *new_name,
+		  struct prefix_map *map_head)
+{
+  struct prefix_map *map;
+  const char *name;
+
+  for (map = map_head; map; map = map->next)
+    if (filename_ncmp (old_name, map->old_prefix, map->old_len) == 0)
+      break;
+  if (!map)
+    return old_name;
+
+  name = old_name + map->old_len;
+  memcpy (new_name, map->new_prefix, map->new_len);
+  memcpy (new_name + map->new_len, name, strlen (name) + 1);
+  return new_name;
+}
Index: gcc-7-20161030/libiberty/Makefile.in
===================================================================
--- gcc-7-20161030.orig/libiberty/Makefile.in
+++ gcc-7-20161030/libiberty/Makefile.in
@@ -145,6 +145,7 @@ CFILES = alloca.c argv.c asprintf.c atex
 	 pex-common.c pex-djgpp.c pex-msdos.c pex-one.c			\
 	 pex-unix.c pex-win32.c						\
          physmem.c putenv.c						\
+	prefix-map.c \
 	random.c regex.c rename.c rindex.c				\
 	safe-ctype.c setenv.c setproctitle.c sha1.c sigsetmask.c        \
 	 simple-object.c simple-object-coff.c simple-object-elf.c	\
@@ -183,6 +184,7 @@ REQUIRED_OFILES =							\
 	./partition.$(objext) ./pexecute.$(objext) ./physmem.$(objext)	\
 	./pex-common.$(objext) ./pex-one.$(objext)			\
 	./@pexecute@.$(objext) ./vprintf-support.$(objext)		\
+	./prefix-map.$(objext) \
 	./safe-ctype.$(objext)						\
 	./simple-object.$(objext) ./simple-object-coff.$(objext)	\
 	./simple-object-elf.$(objext) ./simple-object-mach-o.$(objext)	\
@@ -756,7 +758,7 @@ $(CONFIGURED_OFILES): stamp-picdir stamp
 	$(COMPILE.c) $(srcdir)/fibheap.c $(OUTPUT_OPTION)
 
 ./filename_cmp.$(objext): $(srcdir)/filename_cmp.c config.h $(INCDIR)/ansidecl.h \
-	$(INCDIR)/filenames.h $(INCDIR)/hashtab.h \
+	$(INCDIR)/filenames.h $(INCDIR)/hashtab.h $(INCDIR)/libiberty.h \
 	$(INCDIR)/safe-ctype.h
 	if [ x"$(PICFLAG)" != x ]; then \
 	  $(COMPILE.c) $(PICFLAG) $(srcdir)/filename_cmp.c -o pic/$@; \
@@ -1103,7 +1105,8 @@ $(CONFIGURED_OFILES): stamp-picdir stamp
 	$(COMPILE.c) $(srcdir)/pex-one.c $(OUTPUT_OPTION)
 
 ./pex-unix.$(objext): $(srcdir)/pex-unix.c config.h $(INCDIR)/ansidecl.h \
-	$(INCDIR)/libiberty.h $(srcdir)/pex-common.h
+	$(INCDIR)/environ.h $(INCDIR)/libiberty.h \
+	$(srcdir)/pex-common.h
 	if [ x"$(PICFLAG)" != x ]; then \
 	  $(COMPILE.c) $(PICFLAG) $(srcdir)/pex-unix.c -o pic/$@; \
 	else true; fi
@@ -1142,6 +1145,15 @@ $(CONFIGURED_OFILES): stamp-picdir stamp
 	else true; fi
 	$(COMPILE.c) $(srcdir)/physmem.c $(OUTPUT_OPTION)
 
+./prefix-map.$(objext): $(srcdir)/prefix-map.c config.h $(INCDIR)/prefix-map.h
+	if [ x"$(PICFLAG)" != x ]; then \
+	  $(COMPILE.c) $(PICFLAG) $(srcdir)/prefix-map.c -o pic/$@; \
+	else true; fi
+	if [ x"$(NOASANFLAG)" != x ]; then \
+	  $(COMPILE.c) $(PICFLAG) $(NOASANFLAG) $(srcdir)/prefix-map.c -o noasan/$@; \
+	else true; fi
+	$(COMPILE.c) $(srcdir)/prefix-map.c $(OUTPUT_OPTION)
+
 ./putenv.$(objext): $(srcdir)/putenv.c config.h $(INCDIR)/ansidecl.h
 	if [ x"$(PICFLAG)" != x ]; then \
 	  $(COMPILE.c) $(PICFLAG) $(srcdir)/putenv.c -o pic/$@; \
@@ -1198,7 +1210,8 @@ $(CONFIGURED_OFILES): stamp-picdir stamp
 	else true; fi
 	$(COMPILE.c) $(srcdir)/safe-ctype.c $(OUTPUT_OPTION)
 
-./setenv.$(objext): $(srcdir)/setenv.c config.h $(INCDIR)/ansidecl.h
+./setenv.$(objext): $(srcdir)/setenv.c config.h $(INCDIR)/ansidecl.h \
+	$(INCDIR)/environ.h
 	if [ x"$(PICFLAG)" != x ]; then \
 	  $(COMPILE.c) $(PICFLAG) $(srcdir)/setenv.c -o pic/$@; \
 	else true; fi
@@ -1649,7 +1662,7 @@ $(CONFIGURED_OFILES): stamp-picdir stamp
 	$(COMPILE.c) $(srcdir)/xexit.c $(OUTPUT_OPTION)
 
 ./xmalloc.$(objext): $(srcdir)/xmalloc.c config.h $(INCDIR)/ansidecl.h \
-	$(INCDIR)/libiberty.h
+	$(INCDIR)/environ.h $(INCDIR)/libiberty.h
 	if [ x"$(PICFLAG)" != x ]; then \
 	  $(COMPILE.c) $(PICFLAG) $(srcdir)/xmalloc.c -o pic/$@; \
 	else true; fi
@@ -1707,3 +1720,4 @@ $(CONFIGURED_OFILES): stamp-picdir stamp
 	  $(COMPILE.c) $(PICFLAG) $(NOASANFLAG) $(srcdir)/xvasprintf.c -o noasan/$@; \
 	else true; fi
 	$(COMPILE.c) $(srcdir)/xvasprintf.c $(OUTPUT_OPTION)
+
Index: gcc-7-20161030/gcc/final.c
===================================================================
--- gcc-7-20161030.orig/gcc/final.c
+++ gcc-7-20161030/gcc/final.c
@@ -46,6 +46,7 @@ along with GCC; see the file COPYING3.
 #define INCLUDE_ALGORITHM /* reverse */
 #include "system.h"
 #include "coretypes.h"
+#include "prefix-map.h"
 #include "backend.h"
 #include "target.h"
 #include "rtl.h"
@@ -1502,22 +1503,11 @@ asm_str_count (const char *templ)
   return count;
 }
 
-/* ??? This is probably the wrong place for these.  */
-/* Structure recording the mapping from source file and directory
-   names at compile time to those to be embedded in debug
-   information.  */
-struct debug_prefix_map
-{
-  const char *old_prefix;
-  const char *new_prefix;
-  size_t old_len;
-  size_t new_len;
-  struct debug_prefix_map *next;
-};
 
-/* Linked list of such structures.  */
-static debug_prefix_map *debug_prefix_maps;
+/* Linked list of `struct prefix_map'.  */
+static prefix_map *debug_prefix_maps;
 
+static size_t max_prefix_replace = 0;
 
 /* Record a debug file prefix mapping.  ARG is the argument to
    -fdebug-prefix-map and must be of the form OLD=NEW.  */
@@ -1525,23 +1515,19 @@ static debug_prefix_map *debug_prefix_ma
 void
 add_debug_prefix_map (const char *arg)
 {
-  debug_prefix_map *map;
-  const char *p;
-
-  p = strrchr (arg, '=');
-  if (!p)
+  prefix_map *map = XNEW (prefix_map);
+  if (!parse_prefix_map (arg, map))
     {
       error ("invalid value %qs for debug-prefix-map", arg);
+      free (map);
       return;
     }
-  map = XNEW (debug_prefix_map);
-  map->old_prefix = xstrndup (arg, p - arg);
-  map->old_len = p - arg;
-  p++;
-  map->new_prefix = xstrdup (p);
-  map->new_len = strlen (p);
+
   map->next = debug_prefix_maps;
   debug_prefix_maps = map;
+
+  if (map->new_len > max_prefix_replace)
+    max_prefix_replace = map->new_len;
 }
 
 /* Perform user-specified mapping of debug filename prefixes.  Return
@@ -1550,22 +1536,13 @@ add_debug_prefix_map (const char *arg)
 const char *
 remap_debug_filename (const char *filename)
 {
-  debug_prefix_map *map;
-  char *s;
-  const char *name;
-  size_t name_len;
-
-  for (map = debug_prefix_maps; map; map = map->next)
-    if (filename_ncmp (filename, map->old_prefix, map->old_len) == 0)
-      break;
-  if (!map)
+  char *newname = (char *) alloca (strlen (filename) + max_prefix_replace + 1);
+  const char *name = apply_prefix_map (filename, newname, debug_prefix_maps);
+
+  if (name == filename)
     return filename;
-  name = filename + map->old_len;
-  name_len = strlen (name) + 1;
-  s = (char *) alloca (name_len + map->new_len);
-  memcpy (s, map->new_prefix, map->new_len);
-  memcpy (s + map->new_len, name, name_len);
-  return ggc_strdup (s);
+
+  return ggc_strdup (newname);
 }
 
 /* Return true if DWARF2 debug info can be emitted for DECL.  */
Index: gcc-7-20161030/gcc/doc/invoke.texi
===================================================================
--- gcc-7-20161030.orig/gcc/doc/invoke.texi
+++ gcc-7-20161030/gcc/doc/invoke.texi
@@ -26224,7 +26224,8 @@ compiler uses @code{mblen} and @code{mbt
 recognize and translate multibyte characters.
 
 @item SOURCE_PREFIX_MAP If this variable is set, it specifies a mapping
-that is used to transform filepaths that are output in debug symbols.
+that is used to transform filepaths that are output in debug symbols,
+as well as filepaths emitted during expansions of the __FILE__ macro.
 This helps the embedded paths become reproducible, without having the
 unreproducible value be visible in other input sources - such as GCC
 command-line flags or standardised build-time environment variables like
Index: gcc-7-20161030/gcc/c-family/c-common.c
===================================================================
--- gcc-7-20161030.orig/gcc/c-family/c-common.c
+++ gcc-7-20161030/gcc/c-family/c-common.c
@@ -21,6 +21,7 @@ along with GCC; see the file COPYING3.
 
 #include "config.h"
 #include "system.h"
+#include "prefix-map.h"
 #include "coretypes.h"
 #include "target.h"
 #include "function.h"
@@ -7928,6 +7929,31 @@ cb_get_source_date_epoch (cpp_reader *pf
   return (time_t) epoch;
 }
 
+/* Read SOURCE_PREFIX_MAP from environment to have deterministic relative
+   paths to replace embedded absolute paths to get reproducible results.
+   Returns NULL if SOURCE_PREFIX_MAP is not defined or badly formed.  */
+
+prefix_map *
+cb_get_source_prefix_map (cpp_reader *pfile ATTRIBUTE_UNUSED)
+{
+  const char *arg;
+  prefix_map *map;
+
+  arg = getenv ("SOURCE_PREFIX_MAP");
+  if (!arg)
+    return NULL;
+
+  map = XNEW (prefix_map);
+  if (parse_prefix_map (arg, map))
+    return map;
+
+  free (map);
+  error_at (input_location, "environment variable SOURCE_PREFIX_MAP must "
+	    "be of the form ${old}=${new}");
+
+  return NULL;
+}
+
 /* Callback for libcpp for offering spelling suggestions for misspelled
    directives.  GOAL is an unrecognized string; CANDIDATES is a
    NULL-terminated array of candidate strings.  Return the closest
Index: gcc-7-20161030/gcc/c-family/c-common.h
===================================================================
--- gcc-7-20161030.orig/gcc/c-family/c-common.h
+++ gcc-7-20161030/gcc/c-family/c-common.h
@@ -1075,6 +1075,11 @@ extern time_t cb_get_source_date_epoch (
    __TIME__ can store.  */
 #define MAX_SOURCE_DATE_EPOCH HOST_WIDE_INT_C (253402300799)
 
+/* Read SOURCE_PREFIX_MAP from environment to have deterministic relative
+   paths to replace embedded absolute paths to get reproducible results.
+   Returns NULL if SOURCE_PREFIX_MAP is not defined or badly formed.  */
+extern prefix_map *cb_get_source_prefix_map (cpp_reader *pfile);
+
 /* Callback for libcpp for offering spelling suggestions for misspelled
    directives.  */
 extern const char *cb_get_suggestion (cpp_reader *, const char *,
Index: gcc-7-20161030/gcc/c-family/c-lex.c
===================================================================
--- gcc-7-20161030.orig/gcc/c-family/c-lex.c
+++ gcc-7-20161030/gcc/c-family/c-lex.c
@@ -81,6 +81,7 @@ init_c_lex (void)
   cb->read_pch = c_common_read_pch;
   cb->has_attribute = c_common_has_attribute;
   cb->get_source_date_epoch = cb_get_source_date_epoch;
+  cb->get_source_prefix_map = cb_get_source_prefix_map;
   cb->get_suggestion = cb_get_suggestion;
 
   /* Set the debug callbacks if we can use them.  */
Index: gcc-7-20161030/libcpp/include/cpplib.h
===================================================================
--- gcc-7-20161030.orig/libcpp/include/cpplib.h
+++ gcc-7-20161030/libcpp/include/cpplib.h
@@ -603,6 +603,9 @@ struct cpp_callbacks
   /* Callback to parse SOURCE_DATE_EPOCH from environment.  */
   time_t (*get_source_date_epoch) (cpp_reader *);
 
+  /* Callback to parse SOURCE_PREFIX_MAP from environment.  */
+  struct prefix_map *(*get_source_prefix_map) (cpp_reader *);
+
   /* Callback for providing suggestions for misspelled directives.  */
   const char *(*get_suggestion) (cpp_reader *, const char *, const char *const *);
 };
Index: gcc-7-20161030/libcpp/init.c
===================================================================
--- gcc-7-20161030.orig/libcpp/init.c
+++ gcc-7-20161030/libcpp/init.c
@@ -261,6 +261,9 @@ cpp_create_reader (enum c_lang lang, cpp
   /* Initialize source_date_epoch to -2 (not yet set).  */
   pfile->source_date_epoch = (time_t) -2;
 
+  /* Initialize source_prefix_map to NULL (not yet set).  */
+  pfile->source_prefix_map = NULL;
+
   /* The expression parser stack.  */
   _cpp_expand_op_stack (pfile);
 
Index: gcc-7-20161030/libcpp/internal.h
===================================================================
--- gcc-7-20161030.orig/libcpp/internal.h
+++ gcc-7-20161030/libcpp/internal.h
@@ -507,6 +507,11 @@ struct cpp_reader
      set to -1 to disable it or to a non-negative value to enable it.  */
   time_t source_date_epoch;
 
+  /* Externally set prefix-map to transform absolute paths, useful for
+     reproducibility.  It should be initialized to NULL (not yet set or
+     disabled) or to a `struct prefix_map` value to enable it.  */
+  struct prefix_map *source_prefix_map;
+
   /* EOF token, and a token forcing paste avoidance.  */
   cpp_token avoid_paste;
   cpp_token eof;
Index: gcc-7-20161030/libcpp/macro.c
===================================================================
--- gcc-7-20161030.orig/libcpp/macro.c
+++ gcc-7-20161030/libcpp/macro.c
@@ -26,6 +26,7 @@ along with this program; see the file CO
 #include "system.h"
 #include "cpplib.h"
 #include "internal.h"
+#include "prefix-map.h"
 
 typedef struct macro_arg macro_arg;
 /* This structure represents the tokens of a macro argument.  These
@@ -290,8 +291,22 @@ _cpp_builtin_macro_text (cpp_reader *pfi
       {
 	unsigned int len;
 	const char *name;
+	char *newname;
 	uchar *buf;
+	prefix_map *map = pfile->source_prefix_map;
 	
+	/* Set a prefix-map for __FILE__ if SOURCE_PREFIX_MAP is defined.  */
+	if (map == NULL && pfile->cb.get_source_prefix_map != NULL)
+	  {
+	    map = pfile->cb.get_source_prefix_map (pfile);
+	    if (map == NULL)
+	      {
+		/* Set a dummy value to avoid doing the check again.  */
+		map = XNEW (prefix_map);
+		memset (map, 0, sizeof (prefix_map));
+	      }
+	  }
+
 	if (node->value.builtin == BT_FILE)
 	  name = linemap_get_expansion_filename (pfile->line_table,
 						 pfile->line_table->highest_line);
@@ -301,6 +316,17 @@ _cpp_builtin_macro_text (cpp_reader *pfi
 	    if (!name)
 	      abort ();
 	  }
+
+	/* Strip source_prefix_map from name, if it's a prefix.  */
+	if (map != NULL && map->old_prefix != NULL)
+	  {
+	    newname = (char *) alloca (strlen (name)
+				       - map->old_len
+				       + map->new_len
+				       + 1);
+	    name = apply_prefix_map (name, newname, map);
+	  }
+
 	len = strlen (name);
 	buf = _cpp_unaligned_alloc (pfile, len * 2 + 3);
 	result = buf;
Index: gcc-7-20161030/gcc/testsuite/gcc.dg/cpp/source_prefix_map-1.c
===================================================================
--- /dev/null
+++ gcc-7-20161030/gcc/testsuite/gcc.dg/cpp/source_prefix_map-1.c
@@ -0,0 +1,11 @@
+/* __FILE__ should strip SOURCE_PREFIX_MAP if the latter is a prefix. */
+/* { dg-do run } */
+/* { dg-set-compiler-env-var SOURCE_PREFIX_MAP "$srcdir=MACROTEST" } */
+
+int
+main ()
+{
+  if (__builtin_strcmp (__FILE__, "MACROTEST/gcc.dg/cpp/source_prefix_map-1.c") != 0)
+    __builtin_abort ();
+  return 0;
+}
Index: gcc-7-20161030/gcc/testsuite/gcc.dg/cpp/source_prefix_map-2.c
===================================================================
--- /dev/null
+++ gcc-7-20161030/gcc/testsuite/gcc.dg/cpp/source_prefix_map-2.c
@@ -0,0 +1,11 @@
+/* __FILE__ should not be relative if SOURCE_PREFIX_MAP is not set, and gcc is
+   asked to compile an absolute filename as is the case with this test. */
+/* { dg-do run } */
+
+int
+main ()
+{
+  if (__builtin_strcmp (__FILE__, "./gcc.dg/cpp/source_prefix_map-1.c") == 0)
+    __builtin_abort ();
+  return 0;
+}