[ARC] Prevent moving stores to the frame before the stack adjustment.

If the stack pointer is needed, emit a special barrier that will prevent
the scheduler from moving stores to the frame before the stack adjustment.

For example:

[snip]
	mov_s	fp,sp 	; frame pointer is set here
[snip]
	st	r1,[fp,-24]	; frame pointer is used here
[snip]
	sub_s	sp,sp,0x20	; stack pointer adjusted

So we can easily see that any interrupt between the `st` and `sub`
instruction will lead to faulty code as the interrupt routine will use
a faulty sp register, and, potentially, overwriting the value stored
by 'st' instruction. Thus, adding a scheduler barrier will force the
compiler to emit the `sub` instruction before the store one.

2017-06-01  Claudiu Zissulescu  <claziss@synopsys.com>

	* config/arc/arc.c (arc_expand_prologue): Emit a special barrier
	to prevent store reordering.
	* config/arc/arc.md (UNSPEC_ARC_STKTIE): Define.
	(type): Add block type.
	(stack_tie): Define special instruction to be used in
	expand_prologue.

From-SVN: r248781

gcc/ChangeLog

@@ -1,3 +1,12 @@
+2017-06-01  Claudiu Zissulescu  <claziss@synopsys.com>
+
+	* config/arc/arc.c (arc_expand_prologue): Emit a special barrier
+	to prevent store reordering.
+	* config/arc/arc.md (UNSPEC_ARC_STKTIE): Define.
+	(type): Add block type.
+	(stack_tie): Define special instruction to be used in
+	expand_prologue.
+
 2017-06-01  Claudiu Zissulescu  <claziss@synopsys.com>
 
 	* config/arc/arc.md (commutative_binary_comparison): Remove 'I'

gcc/config/arc/arc.c

@@ -2968,7 +2968,15 @@ arc_expand_prologue (void)
   frame_size_to_allocate -= first_offset;
   /* Allocate the stack frame.  */
   if (frame_size_to_allocate > 0)
-    frame_stack_add ((HOST_WIDE_INT) 0 - frame_size_to_allocate);
+    {
+      frame_stack_add ((HOST_WIDE_INT) 0 - frame_size_to_allocate);
+      /* If the frame pointer is needed, emit a special barrier that
+	 will prevent the scheduler from moving stores to the frame
+	 before the stack adjustment.  */
+      if (arc_frame_pointer_needed ())
+	emit_insn (gen_stack_tie (stack_pointer_rtx,
+				  hard_frame_pointer_rtx));
+    }
 
   /* Setup the gp register, if needed.  */
   if (crtl->uses_pic_offset_table)

gcc/config/arc/arc.md

@@ -135,6 +135,7 @@
   UNSPEC_ARC_VMAC2HU
   UNSPEC_ARC_VMPY2H
   UNSPEC_ARC_VMPY2HU
+  UNSPEC_ARC_STKTIE
   ])
 
 (define_c_enum "vunspec" [
@@ -205,7 +206,7 @@
    simd_vcompare, simd_vpermute, simd_vpack, simd_vpack_with_acc,
    simd_valign, simd_valign_with_acc, simd_vcontrol,
    simd_vspecial_3cycle, simd_vspecial_4cycle, simd_dma, mul16_em, div_rem,
-   fpu"
+   fpu, block"
   (cond [(eq_attr "is_sfunc" "yes")
 	 (cond [(match_test "!TARGET_LONG_CALLS_SET && (!TARGET_MEDIUM_CALLS || GET_CODE (PATTERN (insn)) != COND_EXEC)") (const_string "call")
 		(match_test "flag_pic") (const_string "sfunc")]
@@ -6606,6 +6607,18 @@
   (set_attr "predicable" "yes,no,no,yes,no")
   (set_attr "cond" "canuse,nocond,nocond,canuse_limm,nocond")])
 
+(define_insn "stack_tie"
+  [(set (mem:BLK (scratch))
+	(unspec:BLK [(match_operand:SI 0 "register_operand" "rb")
+		     (match_operand:SI 1 "register_operand" "rb")]
+		    UNSPEC_ARC_STKTIE))]
+  ""
+  ""
+  [(set_attr "length" "0")
+   (set_attr "iscompact" "false")
+   (set_attr "type" "block")]
+  )
+
 ;; include the arc-FPX instructions
 (include "fpx.md")