[PATCH][AArch64] Add legitimize_address_displacement hook

[Wilco Dijkstra]

This patch adds legitimize_address_displacement hook so that stack accesses
with large offsets are split into a more efficient sequence.  Byte and halfword
accesses use a 4KB range, wider accesses use a 16KB range to maximise the
available addressing range and increase opportunities to share the base address.

int f(int x)
{
  int arr[8192];
  arr[4096] = 0;
  arr[6000] = 0;
  arr[7000] = 0;
  arr[8191] = 0;
  return arr[x];
}

Now generates:

        sub     sp, sp, #32768
        add     x1, sp, 16384
        str     wzr, [x1]
        str     wzr, [x1, 7616]
        str     wzr, [x1, 11616]
        str     wzr, [x1, 16380]
        ldr     w0, [sp, w0, sxtw 2]
        add     sp, sp, 32768
        ret

instead of:

        sub     sp, sp, #32768
        mov     x2, 28000
        add     x1, sp, 16384
        mov     x3, 32764
        str     wzr, [x1]
        mov     x1, 24000
        add     x1, sp, x1
        str     wzr, [x1]
        add     x1, sp, x2
        str     wzr, [x1]
        add     x1, sp, x3
        str     wzr, [x1]
        ldr     w0, [sp, w0, sxtw 2]
        add     sp, sp, 32768
        ret

Bootstrap, GCC regression OK.

ChangeLog:
2016-08-04  Wilco Dijkstra  <wdijk...@arm.com>

    gcc/
        * config/aarch64/aarch64.c (aarch64_legitimize_address_displacement):
        New function.
        (TARGET_LEGITIMIZE_ADDRESS_DISPLACEMENT): Define.
--

diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index 
a0e7680ad0946a27d95a67a9892bb7e264a90451..7bf12475494fb004f5a92445ae31fdc52af43c3b
 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -4132,6 +4132,19 @@ aarch64_legitimate_address_p (machine_mode mode, rtx x,
   return aarch64_classify_address (&addr, x, mode, outer_code, strict_p);
 }
 
+/* Split an out-of-range address displacement into a base and offset.
+   Use 4KB range for 1- and 2-byte accesses and a 16KB range otherwise.  */
+
+static bool
+aarch64_legitimize_address_displacement (rtx *disp, rtx *off, machine_mode 
mode)
+{
+  HOST_WIDE_INT mask = GET_MODE_SIZE (mode) < 4 ? 0xfff : 0x3fff;
+
+  *off = GEN_INT (INTVAL (*disp) & ~mask);
+  *disp = GEN_INT (INTVAL (*disp) & mask);
+  return true;
+}
+
 /* Return TRUE if rtx X is immediate constant 0.0 */
 bool
 aarch64_float_const_zero_rtx_p (rtx x)
@@ -14096,6 +14109,10 @@ aarch64_optab_supported_p (int op, machine_mode mode1, 
machine_mode,
 #undef TARGET_LEGITIMATE_CONSTANT_P
 #define TARGET_LEGITIMATE_CONSTANT_P aarch64_legitimate_constant_p
 
+#undef TARGET_LEGITIMIZE_ADDRESS_DISPLACEMENT
+#define TARGET_LEGITIMIZE_ADDRESS_DISPLACEMENT \
+  aarch64_legitimize_address_displacement
+
 #undef TARGET_LIBGCC_CMP_RETURN_MODE
 #define TARGET_LIBGCC_CMP_RETURN_MODE aarch64_libgcc_cmp_return_mode