GCC Middle and Back End API Reference
lra_elim_table Class Reference
Collaboration diagram for lra_elim_table:

Data Fields

int from
int to
poly_int64 previous_offset
poly_int64 offset
bool can_eliminate
bool prev_can_eliminate
rtx from_rtx
rtx to_rtx

Detailed Description

Code for RTL register eliminations. Copyright (C) 2010-2025 Free Software Foundation, Inc. Contributed by Vladimir Makarov <vmakarov@redhat.com>. 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.

Field Documentation

◆ can_eliminate

◆ from

◆ from_rtx

◆ offset

◆ prev_can_eliminate

bool lra_elim_table::prev_can_eliminate

◆ previous_offset

◆ to

◆ to_rtx


The documentation for this class was generated from the following file: