xtensa: add support for SSP

gcc/
2017-05-08  Max Filippov  <jcmvbkbc@gmail.com>

	* config/xtensa/xtensa-protos.h
       	(xtensa_initial_elimination_offset): New declaration.
	* config/xtensa/xtensa.c (xtensa_initial_elimination_offset):
	New function. Move its body from the INITIAL_ELIMINATION_OFFSET
	macro definition, add case for FRAME_POINTER_REGNUM when
	FRAME_GROWS_DOWNWARD.
	* config/xtensa/xtensa.h (FRAME_GROWS_DOWNWARD): New macro
	definition.
	(INITIAL_ELIMINATION_OFFSET): Replace body with call to
	xtensa_initial_elimination_offset.

From-SVN: r247771

gcc/ChangeLog

+2017-05-08  Max Filippov  <jcmvbkbc@gmail.com>
+
+	* config/xtensa/xtensa-protos.h
+       	(xtensa_initial_elimination_offset): New declaration.
+	* config/xtensa/xtensa.c (xtensa_initial_elimination_offset):
+	New function. Move its body from the INITIAL_ELIMINATION_OFFSET
+	macro definition, add case for FRAME_POINTER_REGNUM when
+	FRAME_GROWS_DOWNWARD.
+	* config/xtensa/xtensa.h (FRAME_GROWS_DOWNWARD): New macro
+	definition.
+	(INITIAL_ELIMINATION_OFFSET): Replace body with call to
+	xtensa_initial_elimination_offset.
+
 2017-05-08  Nathan Sidwell  <nathan@acm.org>
 
 	* doc/invoke.texi: Alphabetize -fdump options.

gcc/config/xtensa/xtensa-protos.h

@@ -73,5 +73,6 @@ extern void xtensa_expand_prologue (void);
 extern void xtensa_expand_epilogue (void);
 extern void order_regs_for_local_alloc (void);
 extern enum reg_class xtensa_regno_to_class (int regno);
+extern HOST_WIDE_INT xtensa_initial_elimination_offset (int from, int to);
 
 #endif /* !__XTENSA_PROTOS_H__ */

gcc/config/xtensa/xtensa.c

@@ -2676,6 +2676,30 @@ xtensa_frame_pointer_required (void)
   return false;
 }
 
+HOST_WIDE_INT
+xtensa_initial_elimination_offset (int from, int to)
+{
+  long frame_size = compute_frame_size (get_frame_size ());
+  HOST_WIDE_INT offset;
+
+  switch (from)
+    {
+    case FRAME_POINTER_REGNUM:
+      if (FRAME_GROWS_DOWNWARD)
+	offset = frame_size - (WINDOW_SIZE * UNITS_PER_WORD)
+	  - cfun->machine->callee_save_size;
+      else
+	offset = 0;
+      break;
+    case ARG_POINTER_REGNUM:
+      offset = frame_size;
+      break;
+    default:
+      gcc_unreachable ();
+    }
+
+  return offset;
+}
 
 /* minimum frame = reg save area (4 words) plus static chain (1 word)
    and the total number of words must be a multiple of 128 bits.  */

gcc/config/xtensa/xtensa.h

@@ -460,9 +460,11 @@ enum reg_class
 
 #define STACK_GROWS_DOWNWARD 1
 
+#define FRAME_GROWS_DOWNWARD flag_stack_protect
+
 /* Offset within stack frame to start allocating local variables at.  */
 #define STARTING_FRAME_OFFSET						\
-  crtl->outgoing_args_size
+  (FRAME_GROWS_DOWNWARD ? 0 : crtl->outgoing_args_size)
 
 /* The ARG_POINTER and FRAME_POINTER are not real Xtensa registers, so
    they are eliminated to either the stack pointer or hard frame pointer.  */
@@ -474,20 +476,7 @@ enum reg_class
 
 /* Specify the initial difference between the specified pair of registers.  */
 #define INITIAL_ELIMINATION_OFFSET(FROM, TO, OFFSET)			\
-  do {									\
-    long frame_size = compute_frame_size (get_frame_size ());		\
-    switch (FROM)							\
-      {									\
-      case FRAME_POINTER_REGNUM:					\
-        (OFFSET) = 0;							\
-	break;								\
-      case ARG_POINTER_REGNUM:						\
-        (OFFSET) = frame_size;						\
-	break;								\
-      default:								\
-	gcc_unreachable ();						\
-      }									\
-  } while (0)
+  (OFFSET) = xtensa_initial_elimination_offset ((FROM), (TO))
 
 /* If defined, the maximum amount of space required for outgoing
    arguments will be computed and placed into the variable