Add a configure option to disable system header canonicalizations (issue6495088)

[simonb at google dot com (Simon Baldwin)]

Add a configure option to disable system header canonicalizations.

Libcpp may canonicalize system header paths with lrealpath() for diagnostics,
dependency output, and similar.  If gcc is held in a symlink farm the
canonicalized paths may be meaningless to users, and will also conflict with
build frameworks that (for example) disallow absolute paths to header files.

This change adds --[en/dis]able-canonical-system-headers, allowing configure
to select whether or not to implement r186991.  See also PR c++/52974.

Tested for regressions with bootstrap builds of C and C++, both with and
without configure --disable-canonical-system-headers.

Okay for trunk?

libcpp/ChangeLog.google-integration
2012-09-05  Simon Baldwin  <simonb@google.com>

	* files.c (maybe_shorter_path): Suppress function definition if
	ENABLE_CANONICAL_SYSTEM_HEADERS is not defined.
	* (find_file_in_dir): Call maybe_shorter_path() only if
	ENABLE_CANONICAL_SYSTEM_HEADERS is defined.
	* configure.ac: Add new --enable-canonical-system-headers.
	* configure: Regenerate.
	* config.in: Regenerate.

gcc/ChangeLog.google-integration
2012-09-05  Simon Baldwin  <simonb@google.com>

	* doc/install.texi: Document --enable-canonical-system-headers.


Index: gcc/doc/install.texi
===================================================================
--- gcc/doc/install.texi	(revision 190968)
+++ gcc/doc/install.texi	(working copy)
@@ -1710,6 +1710,14 @@ link time when @option{-fuse-linker-plug
 This linker should have plugin support such as gold starting with
 version 2.20 or GNU ld starting with version 2.21.
 See @option{-fuse-linker-plugin} for details.
+
+@item --enable-canonical-system-headers
+@itemx --disable-canonical-system-headers
+Enable system header path canonicalization for @file{libcpp}.  This can
+produce shorter header file paths in diagnostics and dependency output
+files, but these changed header paths may conflict with some compilation
+environments.  Enabled by default, and may be disabled using
+@option{--disable-canonical-system-headers}.
 @end table
 
 @subheading Cross-Compiler-Specific Options
Index: libcpp/configure
===================================================================
--- libcpp/configure	(revision 190968)
+++ libcpp/configure	(working copy)
@@ -700,6 +700,7 @@ enable_rpath
 with_libiconv_prefix
 enable_maintainer_mode
 enable_checking
+enable_canonical_system_headers
 '
       ac_precious_vars='build_alias
 host_alias
@@ -1333,6 +1334,8 @@ Optional Features:
   --disable-rpath         do not hardcode runtime library paths
   --enable-maintainer-mode enable rules only needed by maintainers
   --enable-checking      enable expensive run-time checks
+  --enable-canonical-system-headers
+                          enable or disable system headers canonicalization
 
 Optional Packages:
   --with-PACKAGE[=ARG]    use PACKAGE [ARG=yes]
@@ -7094,6 +7097,19 @@ $as_echo "#define ENABLE_CHECKING 1" >>c
 
 fi
 
+# Check whether --enable-canonical-system-headers was given.
+if test "${enable_canonical_system_headers+set}" = set; then :
+  enableval=$enable_canonical_system_headers;
+else
+  enable_canonical_system_headers=yes
+fi
+
+if test $enable_canonical_system_headers != no; then
+
+$as_echo "#define ENABLE_CANONICAL_SYSTEM_HEADERS 1" >>confdefs.h
+
+fi
+
 
 case $target in
 	alpha*-*-* | \
Index: libcpp/files.c
===================================================================
--- libcpp/files.c	(revision 190968)
+++ libcpp/files.c	(working copy)
@@ -345,6 +345,7 @@ pch_open_file (cpp_reader *pfile, _cpp_f
    shorter, otherwise return NULL.  This function does NOT free the
    memory pointed by FILE.  */
 
+#ifdef ENABLE_CANONICAL_SYSTEM_HEADERS
 static char *
 maybe_shorter_path (const char * file)
 {
@@ -359,6 +360,7 @@ maybe_shorter_path (const char * file)
       return NULL;
     }
 }
+#endif
 
 /* Try to open the path FILE->name appended to FILE->dir.  This is
    where remap and PCH intercept the file lookup process.  Return true
@@ -384,6 +386,7 @@ find_file_in_dir (cpp_reader *pfile, _cp
       char *copy;
       void **pp;
 
+#ifdef ENABLE_CANONICAL_SYSTEM_HEADERS
       /* We try to canonicalize system headers.  */
       if (file->dir->sysp)
 	{
@@ -396,6 +399,7 @@ find_file_in_dir (cpp_reader *pfile, _cp
 	      path = canonical_path;
 	    }
 	}
+#endif
 
       hv = htab_hash_string (path);
       if (htab_find_with_hash (pfile->nonexistent_file_hash, path, hv) != NULL)
Index: libcpp/configure.ac
===================================================================
--- libcpp/configure.ac	(revision 190968)
+++ libcpp/configure.ac	(working copy)
@@ -132,6 +132,16 @@ if test $enable_checking != no ; then
 [Define if you want more run-time sanity checks.])
 fi
 
+AC_ARG_ENABLE(canonical-system-headers,
+[  --enable-canonical-system-headers
+                          enable or disable system headers canonicalization],
+[],
+enable_canonical_system_headers=yes)
+if test $enable_canonical_system_headers != no; then
+  AC_DEFINE(ENABLE_CANONICAL_SYSTEM_HEADERS,
+            1, [Define to enable system headers canonicalization.])
+fi
+
 m4_changequote(,)
 case $target in
 	alpha*-*-* | \
Index: libcpp/config.in
===================================================================
--- libcpp/config.in	(revision 190968)
+++ libcpp/config.in	(working copy)
@@ -11,6 +11,9 @@
 /* Define to 1 if using `alloca.c'. */
 #undef C_ALLOCA
 
+/* Define to enable system headers canonicalization. */
+#undef ENABLE_CANONICAL_SYSTEM_HEADERS
+
 /* Define if you want more run-time sanity checks. */
 #undef ENABLE_CHECKING
 

--
This patch is available for review at http://codereview.appspot.com/6495088