Data Fields
int	from
int	to
HOST_WIDE_INT	previous_offset
HOST_WIDE_INT	offset
bool	can_eliminate
bool	prev_can_eliminate
rtx	from_rtx
rtx	to_rtx
HOST_WIDE_INT	initial_offset
int	can_eliminate
int	can_eliminate_previous
int	ref_outside_mem

Detailed Description

@verbatim Code for RTL register eliminations.

This file is part of GCC.

GCC is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3, or (at your option) any later version.

GCC is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with GCC; see the file COPYING3. If not see http://www.gnu.org/licenses/.

Eliminable registers (like a soft argument or frame pointer) are
   widely used in RTL.  These eliminable registers should be replaced
   by real hard registers (like the stack pointer or hard frame
   pointer) plus some offset.  The offsets usually change whenever the
   stack is expanded.  We know the final offsets only at the very end
   of LRA.

   Within LRA, we usually keep the RTL in such a state that the
   eliminable registers can be replaced by just the corresponding hard
   register (without any offset).  To achieve this we should add the
   initial elimination offset at the beginning of LRA and update the
   offsets whenever the stack is expanded.  We need to do this before
   every constraint pass because the choice of offset often affects
   whether a particular address or memory constraint is satisfied.

   We keep RTL code at most time in such state that the virtual
   registers can be changed by just the corresponding hard registers
   (with zero offsets) and we have the right RTL code.  To achieve this
   we should add initial offset at the beginning of LRA work and update
   offsets after each stack expanding.  But actually we update virtual
   registers to the same virtual registers + corresponding offsets
   before every constraint pass because it affects constraint
   satisfaction (e.g. an address displacement became too big for some
   target).

   The final change of eliminable registers to the corresponding hard
   registers are done at the very end of LRA when there were no change
   in offsets anymore:

                     fp + 42     =>     sp + 42

This structure is used to record information about hard register
   eliminations.

This structure is used to record information about register eliminations.
   Each array entry describes one possible way of eliminating a register
   in favor of another.   If there is more than one way of eliminating a
   particular register, the most preferred should be specified first.