Add a prepare_pch_save target hook

Index: gcc/doc/tm.texi.in
===================================================================
--- gcc/doc/tm.texi.in	2011-12-04 08:52:33.000000000 +0000
+++ gcc/doc/tm.texi.in	2011-12-04 11:30:12.000000000 +0000
@@ -9974,6 +9974,8 @@  of @code{target_flags}.  @var{pch_flags}
 value is the same as for @code{TARGET_PCH_VALID_P}.
 @end deftypefn
 
+@hook TARGET_PREPARE_PCH_SAVE
+
 @node C++ ABI
 @section C++ ABI parameters
 @cindex parameters, c++ abi
Index: gcc/doc/tm.texi
===================================================================
--- gcc/doc/tm.texi	2011-12-04 08:52:33.000000000 +0000
+++ gcc/doc/tm.texi	2011-12-04 11:30:12.000000000 +0000
@@ -10079,6 +10079,13 @@  of @code{target_flags}.  @var{pch_flags}
 value is the same as for @code{TARGET_PCH_VALID_P}.
 @end deftypefn
 
+@deftypefn {Target Hook} void TARGET_PREPARE_PCH_SAVE (void)
+Called before writing out a PCH file.  If the target has some
+garbage-collected data that needs to be in a particular state on PCH loads,
+it can use this hook to enforce that state.  Very few targets need
+to do anything here.
+@end deftypefn
+
 @node C++ ABI
 @section C++ ABI parameters
 @cindex parameters, c++ abi
Index: gcc/target.def
===================================================================
--- gcc/target.def	2011-12-04 08:52:33.000000000 +0000
+++ gcc/target.def	2011-12-04 11:30:12.000000000 +0000
@@ -1819,6 +1819,15 @@  DEFHOOK
  const char *, (const void *data, size_t sz),
  default_pch_valid_p)
 
+DEFHOOK
+(prepare_pch_save,
+ "Called before writing out a PCH file.  If the target has some\n\
+garbage-collected data that needs to be in a particular state on PCH loads,\n\
+it can use this hook to enforce that state.  Very few targets need\n\
+to do anything here.",
+ void, (void),
+ hook_void_void)
+
 /* If nonnull, this function checks whether a PCH file with the
    given set of target flags can be used.  It returns NULL if so,
    otherwise it returns an error message.  */
Index: gcc/c-family/c-pch.c
===================================================================
--- gcc/c-family/c-pch.c	2011-12-04 08:52:33.000000000 +0000
+++ gcc/c-family/c-pch.c	2011-12-04 11:30:12.000000000 +0000
@@ -180,6 +180,8 @@  c_common_write_pch (void)
 
   timevar_push (TV_PCH_SAVE);
 
+  targetm.prepare_pch_save ();
+
   (*debug_hooks->handle_pch) (1);
 
   cpp_write_pch_deps (parse_in, pch_outfile);
Index: gcc/config/mips/mips.c
===================================================================
--- gcc/config/mips/mips.c	2011-12-04 08:52:32.000000000 +0000
+++ gcc/config/mips/mips.c	2011-12-04 12:01:44.000000000 +0000
@@ -15197,14 +15197,8 @@  mips_output_mi_thunk (FILE *file, tree t
 }
 
 /* The last argument passed to mips_set_mips16_mode, or negative if the
-   function hasn't been called yet.
-
-   There are two copies of this information.  One is saved and restored
-   by the PCH process while the other is specific to this compiler
-   invocation.  The information calculated by mips_set_mips16_mode
-   is invalid unless the two variables are the same.  */
+   function hasn't been called yet.  */
 static int was_mips16_p = -1;
-static GTY(()) int was_mips16_pch_p = -1;
 
 /* Set up the target-dependent global state so that it matches the
    current function's ISA mode.  */
@@ -15212,8 +15206,7 @@  static GTY(()) int was_mips16_pch_p = -1
 static void
 mips_set_mips16_mode (int mips16_p)
 {
-  if (mips16_p == was_mips16_p
-      && mips16_p == was_mips16_pch_p)
+  if (mips16_p == was_mips16_p)
     return;
 
   /* Restore base settings of various flags.  */
@@ -15310,7 +15303,6 @@  mips_set_mips16_mode (int mips16_p)
     restore_target_globals (&default_target_globals);
 
   was_mips16_p = mips16_p;
-  was_mips16_pch_p = mips16_p;
 }
 
 /* Implement TARGET_SET_CURRENT_FUNCTION.  Decide whether the current
@@ -16325,6 +16317,34 @@  mips_shift_truncation_mask (enum machine
   return GET_MODE_BITSIZE (mode) - 1;
 }
 
+/* Implement TARGET_PREPARE_PCH_SAVE.  */
+
+static void
+mips_prepare_pch_save (void)
+{
+  /* We are called in a context where the current MIPS16 vs. non-MIPS16
+     setting should be irrelevant.  The question then is: which setting
+     makes most sense at load time?
+
+     The PCH is loaded before the first token is read.  We should never
+     have switched into MIPS16 mode by that point, and thus should not
+     have populated mips16_globals.  Nor can we load the entire contents
+     of mips16_globals from the PCH file, because mips16_globals contains
+     a combination of GGC and non-GGC data.
+
+     There is therefore no point in trying save the GGC part of
+     mips16_globals to the PCH file, or to preserve MIPS16ness across
+     the PCH save and load.  The loading compiler would not have access
+     to the non-GGC parts of mips16_globals (either from the PCH file,
+     or from a copy that the loading compiler generated itself) and would
+     have to call target_reinit anyway.
+
+     It therefore seems best to switch back to non-MIPS16 mode at
+     save time, and to ensure that mips16_globals remains null after
+     a PCH load.  */
+  mips_set_mips16_mode (false);
+  mips16_globals = 0;
+}
 
 /* Initialize the GCC target structure.  */
 #undef TARGET_ASM_ALIGNED_HI_OP
@@ -16544,6 +16564,9 @@  #define TARGET_ASM_OUTPUT_SOURCE_FILENAM
 #undef TARGET_SHIFT_TRUNCATION_MASK
 #define TARGET_SHIFT_TRUNCATION_MASK mips_shift_truncation_mask
 
+#undef TARGET_PREPARE_PCH_SAVE
+#define TARGET_PREPARE_PCH_SAVE mips_prepare_pch_save
+
 struct gcc_target targetm = TARGET_INITIALIZER;
 
 #include "gt-mips.h"