[PATCH] c-family: Stabilize the location for macros restored after PCH load [PR105608]

libcpp currently lacks the infrastructure to assign correct locations to
macros that were defined prior to loading a PCH and then restored
afterwards. While I plan to address that fully for GCC 15, this patch
improves things by using at least a valid location, even if it's not the
best one. Without this change, libcpp uses pfile->directive_line as the
location for the restored macros, but this location_t applies to the old
line map, not the one that was just restored from the PCH, so the resulting
location is unpredictable and depends on what was stored in the line maps
before. With this change, all restored macros get assigned locations at the
line of the #include that triggered the PCH restore. A future patch will
store the actual file name and line number of each definition and then
synthesize locations in the new line map pointing to the right place.

gcc/c-family/ChangeLog:

	PR preprocessor/105608
	* c-pch.cc (c_common_read_pch): Adjust line map so that libcpp
	assigns a location to restored macros which is the same location
	that triggered the PCH include.

libcpp/ChangeLog:

	PR preprocessor/105608
	* pch.cc (cpp_read_state): Set a valid location for restored
	macros.

diff --git a/gcc/c-family/c-pch.cc b/gcc/c-family/c-pch.cc
index 79b4f88fe4d..971af90be0d 100644
--- a/gcc/c-family/c-pch.cc
+++ b/gcc/c-family/c-pch.cc
@@ -318,6 +318,7 @@ c_common_read_pch (cpp_reader *pfile, const char *name,
   struct save_macro_data *smd;
   expanded_location saved_loc;
   bool saved_trace_includes;
+  int cpp_result;
 
   timevar_push (TV_PCH_RESTORE);
 
@@ -343,20 +344,26 @@ c_common_read_pch (cpp_reader *pfile, const char *name,
   cpp_set_line_map (pfile, line_table);
   rebuild_location_adhoc_htab (line_table);
   line_table->trace_includes = saved_trace_includes;
-  linemap_add (line_table, LC_ENTER, 0, saved_loc.file, saved_loc.line);
+
+  /* Set the current location to the line containing the #include (or the
+     #pragma GCC_preprocess) for the purpose of assigning locations to any
+     macros that are about to be restored.  */
+  linemap_add (line_table, LC_ENTER, 0, saved_loc.file,
+	       saved_loc.line > 1 ? saved_loc.line - 1 : saved_loc.line);
 
   timevar_push (TV_PCH_CPP_RESTORE);
-  if (cpp_read_state (pfile, name, f, smd) != 0)
-    {
-      fclose (f);
-      timevar_pop (TV_PCH_CPP_RESTORE);
-      goto end;
-    }
-  timevar_pop (TV_PCH_CPP_RESTORE);
+  cpp_result = cpp_read_state (pfile, name, f, smd);
 
+  /* Set the current location to the line following the #include, where we
+     were prior to processing the PCH.  */
+  linemap_line_start (line_table, saved_loc.line, 0);
 
+  timevar_pop (TV_PCH_CPP_RESTORE);
   fclose (f);
 
+  if (cpp_result != 0)
+    goto end;
+
   /* Give the front end a chance to take action after a PCH file has
      been loaded.  */
   if (lang_post_pch_load)
diff --git a/libcpp/pch.cc b/libcpp/pch.cc
index e156fe257b3..f2f74ed6ea9 100644
--- a/libcpp/pch.cc
+++ b/libcpp/pch.cc
@@ -838,7 +838,14 @@ cpp_read_state (cpp_reader *r, const char *name, FILE *f,
 	      != NULL)
 	    {
 	      _cpp_clean_line (r);
-	      if (!_cpp_create_definition (r, h, 0))
+
+	      /* ??? Using r->line_table->highest_line is not ideal here, but we
+		 do need to use some location that is relative to the new line
+		 map just loaded, not the old one that was in effect when these
+		 macros were lexed.  The proper fix is to remember the file name
+		 and line number where each macro was defined, and then add
+		 these locations into the new line map.  See PR105608.  */
+	      if (!_cpp_create_definition (r, h, r->line_table->highest_line))
 		abort ();
 	      _cpp_pop_buffer (r);
 	    }