[PATCH][AArch64] Mark symbols as constant

[Wilco Dijkstra]

Aarch64_legitimate_constant_p currently returns false for symbols,
eventhough they are always valid constants.  This means LOSYM isn't
CSEd correctly.  If we return true CSE works better, resulting in
smaller/faster code (0.3% smaller code on SPEC2006).

int x0 = 1, x1 = 2, x2 = 3;

int 
f (int x, int y)
{
  x += x1;
  if (x > 100)
    y += x2;
  x += x0;
  return x + y;
}

Before:
        adrp    x3, .LANCHOR0
        add     x4, x3, :lo12:.LANCHOR0
        ldr     w2, [x3, #:lo12:.LANCHOR0]
        add     w0, w0, w2
        cmp     w0, 100
        ble     .L5
        ldr     w2, [x4, 8]
        add     w1, w1, w2
.L5:
        add     x3, x3, :lo12:.LANCHOR0
        ldr     w2, [x3, 4]
        add     w0, w0, w2
        add     w0, w0, w1
        ret

After:
        adrp    x2, .LANCHOR0
        add     x3, x2, :lo12:.LANCHOR0
        ldr     w2, [x2, #:lo12:.LANCHOR0]
        add     w0, w0, w2
        cmp     w0, 100
        ble     .L5
        ldr     w2, [x3, 8]
        add     w1, w1, w2
.L5:
        ldr     w2, [x3, 4]
        add     w0, w0, w2
        add     w0, w0, w1
        ret

Passes regress and bootstrap, OK for commit?

ChangeLog:
2017-06-19  Wilco Dijkstra  <wdijk...@arm.com>

        * config/aarch64/aarch64.c (aarch64_legitimate_constant_p):
        Return true for symbols.
--
diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index 
5ec6bbfcf484baa4005b8a88cb98d0d04f710877..4b7d961102e41ce927d89d458fc89ddfc2adcd6f
 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -10111,6 +10111,9 @@ aarch64_legitimate_constant_p (machine_mode mode, rtx x)
       && aarch64_valid_symref (XEXP (x, 0), GET_MODE (XEXP (x, 0))))
     return true;
 
+  if (SYMBOL_REF_P (x))
+    return true;
+
   return aarch64_constant_address_p (x);
 }