Hi,
GIMPLE IVO needs to call backend interface to calculate costs for addr
expressions like below:
   FORM1: "r73 + r74 + 16380"
   FORM2: "r73 << 2 + r74 + 16380"

They are invalid address expression on AArch64, so will be legitimized by
aarch64_legitimize_address.  Below are what we got from that function:

For FORM1, the address expression is legitimized into below insn sequence
and rtx:
   r84:DI=r73:DI+r74:DI
   r85:DI=r84:DI+0x3000
   r83:DI=r85:DI
   "r83 + 4092"

For FORM2, the address expression is legitimized into below insn sequence
and rtx:
   r108:DI=r73:DI<<0x2
   r109:DI=r108:DI+r74:DI
   r110:DI=r109:DI+0x3000
   r107:DI=r110:DI
   "r107 + 4092"

So the costs computed are 12/16 respectively.  The high cost prevents IVO
from choosing right candidates.  Besides cost computation, I also think the
legitmization is bad in terms of code generation.
The root cause in aarch64_legitimize_address can be described by it's
comment:
   /* Try to split X+CONST into Y=X+(CONST & ~mask), Y+(CONST&mask),
      where mask is selected by alignment and size of the offset.
      We try to pick as large a range for the offset as possible to
      maximize the chance of a CSE.  However, for aligned addresses
      we limit the range to 4k so that structures with different sized
      elements are likely to use the same base.  */
I think the split of CONST is intended for REG+CONST where the const offset
is not in the range of AArch64's addressing modes.  Unfortunately, it
doesn't explicitly handle/reject "REG+REG+CONST" and "REG+REG<<SCALE+CONST"
when the CONST are in the range of addressing modes.  As a result, these two
cases fallthrough this logic, resulting in sub-optimal results.

It's obvious we can do below legitimization:
FORM1:
   r83:DI=r73:DI+r74:DI
   "r83 + 16380"
FORM2:
   r107:DI=0x3ffc
   r106:DI=r74:DI+r107:DI
      REG_EQUAL r74:DI+0x3ffc
   "r106 + r73 << 2"

This patch handles these two cases as described.
Bootstrap & test on AArch64 along with other patch.  Is it OK?

2015-11-04  Bin Cheng  <bin.cheng@arm.com>
	    Jiong Wang  <jiong.wang@arm.com>

	* config/aarch64/aarch64.c (aarch64_legitimize_address): Handle
	address expressions like REG+REG+CONST and REG+NON_REG+CONST.