[PATCH][AArch64] Simplify aarch64_can_eliminate

[Wilco Dijkstra]

Simplify aarch64_can_eliminate - if we need a frame pointer, we must
eliminate to HARD_FRAME_POINTER_REGNUM.  Rather than hardcoding all
combinations from the ELIMINABLE_REGS list, just do the correct check.

ChangeLog:
2017-08-07  Wilco Dijkstra  <wdijk...@arm.com>

    gcc/
        * config/aarch64/aarch64.c (aarch64_can_eliminate): Simplify logic.
--

diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index 
9463edf63ffe770062b4120fada090e20c93a269..fbb4244cb58d2a25203aa27342c2f7988c30bd98
 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -5858,26 +5858,12 @@ aarch64_secondary_reload (bool in_p ATTRIBUTE_UNUSED, 
rtx x,
 }
 
 static bool
-aarch64_can_eliminate (const int from, const int to)
+aarch64_can_eliminate (const int from ATTRIBUTE_UNUSED, const int to)
 {
-  /* If we need a frame pointer, we must eliminate FRAME_POINTER_REGNUM into
-     HARD_FRAME_POINTER_REGNUM and not into STACK_POINTER_REGNUM.  */
-
+  /* If we need a frame pointer, ARG_POINTER_REGNUM and FRAME_POINTER_REGNUM
+     can only eliminate to HARD_FRAME_POINTER_REGNUM.  */
   if (frame_pointer_needed)
-    {
-      if (from == ARG_POINTER_REGNUM && to == HARD_FRAME_POINTER_REGNUM)
-       return true;
-      if (from == ARG_POINTER_REGNUM && to == STACK_POINTER_REGNUM)
-       return false;
-      if (from == FRAME_POINTER_REGNUM && to == STACK_POINTER_REGNUM
-         && !cfun->calls_alloca)
-       return true;
-      if (from == FRAME_POINTER_REGNUM && to == HARD_FRAME_POINTER_REGNUM)
-       return true;
-
-      return false;
-    }
-
+    return to == HARD_FRAME_POINTER_REGNUM;
   return true;
 }