[PATCH v3] Ability to remap file names in __FILE__, etc (PR other/70268)

[Ximin Luo]

Boris Kolpackov:
> Thanks for the review. Third revision of the patch attached.
> 
> [..]
Here is a follow-up patch, meant to be applied on top of Boris' patch, to 
address some issues we found based on discussions and experiments at the 
Reproducible Builds project.

Most of this patch is re-implementing the patch from [1] originally by Daniel 
Kahn Gillmor. Back then it was rejected in favour of a simpler approach because 
its necessity was not realised. However, more experiments since then have 
revealed that its basic approach is useful in more general scenarios:

Higher-level build scripts sometimes like to save CFLAGS etc into the build 
output, making the overall build output unreproducible even if GCC is playing 
nicely. Rather than add logic to strip -f{file,debug,macro,...}-prefix-map, 
into all possible higher-level programs that might want to save CFLAGS, it is
simpler if GCC could read it from a user-specified environment variable. The
fact that the name of the environment variable is still present on the
command-line, should make it obvious what is being done, and avoid confusion
that might arise if an implicit "magic" environment variable is used.

In the patch thread (see [1] and follow-ups) there was concern that the prefix 
'$' would cause awkward escaping behaviour, so the prefix 'ENV:' was chosen. A 
concern about this conflicting with AmigaOS paths was voiced but not explored; 
of course it can be changed to something else if needed.

(NetBSD is already carrying this patch in their system GCC [2], presumably for
the same reason mentioned above.)

[1] https://gcc.gnu.org/ml/gcc-patches/2015-12/msg01168.html
[2] http://cvsweb.netbsd.org/bsdweb.cgi/src/external/gpl3/gcc/dist/gcc/final.c?rev=1.2&content-type=text/x-cvsweb-markup

The other thing that this patch does, is to split the old and new prefixs by
the last '=' sign rather than the first. This allows arbitrary old prefixes to
be mapped, which would be more flexible than mapping to arbitrary new prefixes.
The rust compiler is adopting this strategy too: [3].

[3] https://github.com/rust-lang/rust/issues/41555#issuecomment-321078174

ChangeLogs
----------

gcc/ChangeLog:

2017-12-19  Ximin Luo  <infinity0@pwned.gg>

	PR other/70268
	* file-prefix-map.c (add_prefix_map): Support reading old prefix from the
	environment. Split old and new parts by the last '=' rather than the first.
	* file-prefix-map.h: Define macros for the ENV: prefix.

gcc/testsuite/ChangeLog:

2017-12-19  Ximin Luo  <infinity0@pwned.gg>

	PR other/70268
	* c-c++-common/ffile-prefix-map-env.c: New test.

Index: gcc-8-20171217/gcc/file-prefix-map.c
===================================================================
--- gcc-8-20171217.orig/gcc/file-prefix-map.c
+++ gcc-8-20171217/gcc/file-prefix-map.c
@@ -41,16 +41,40 @@ add_prefix_map (file_prefix_map *&maps,
 {
   file_prefix_map *map;
   const char *p;
+  char *env;
+  const char *old;
+  size_t oldlen;
 
-  p = strchr (arg, '=');
+  p = strrchr (arg, '=');
   if (!p)
     {
       error ("invalid argument %qs to %qs", arg, opt);
       return;
     }
+  if (0 == strncmp(FILE_PREFIX_MAP_ENV_PREFIX, arg, FILE_PREFIX_MAP_ENV_PREFIX_OFFSET))
+    {
+      const char *arg_offset = arg + FILE_PREFIX_MAP_ENV_PREFIX_OFFSET;
+      env = xstrndup (arg_offset, p - arg_offset);
+      old = getenv (env);
+      if (!old)
+	{
+	  warning (0, "environment variable %qs not set in argument to %s",
+		   env, opt);
+	  free (env);
+	  return;
+	}
+      oldlen = strlen (old);
+      free (env);
+    }
+  else
+    {
+      old = xstrndup (arg, p - arg);
+      oldlen = p - arg;
+    }
+
   map = XNEW (file_prefix_map);
-  map->old_prefix = xstrndup (arg, p - arg);
-  map->old_len = p - arg;
+  map->old_prefix = old;
+  map->old_len = oldlen;
   p++;
   map->new_prefix = xstrdup (p);
   map->new_len = strlen (p);
Index: gcc-8-20171217/gcc/file-prefix-map.h
===================================================================
--- gcc-8-20171217.orig/gcc/file-prefix-map.h
+++ gcc-8-20171217/gcc/file-prefix-map.h
@@ -18,6 +18,9 @@
 #ifndef GCC_FILE_PREFIX_MAP_H
 #define GCC_FILE_PREFIX_MAP_H
 
+#define FILE_PREFIX_MAP_ENV_PREFIX "ENV:"
+#define FILE_PREFIX_MAP_ENV_PREFIX_OFFSET (sizeof(FILE_PREFIX_MAP_ENV_PREFIX) - 1)
+
 void add_macro_prefix_map (const char *);
 void add_debug_prefix_map (const char *);
 void add_file_prefix_map (const char *);
Index: gcc-8-20171217/gcc/testsuite/c-c++-common/ffile-prefix-map-env.c
===================================================================
--- /dev/null
+++ gcc-8-20171217/gcc/testsuite/c-c++-common/ffile-prefix-map-env.c
@@ -0,0 +1,13 @@
+/* Test __builtin_FILE() with ENV: prefix-map. */
+/* { dg-do run } */
+/* { dg-set-compiler-env-var SRCDIR "$srcdir" } */
+/* { dg-options "-ffile-prefix-map=ENV:SRCDIR=xxx" } */
+
+#include <stdio.h>
+
+int main ()
+{
+  printf ("__builtin_FILE starts with %s\n", __builtin_FILE ());
+}
+
+/* { dg-output "__builtin_FILE starts with xxx" } */