Hi,
This patch series rewrites parts of IVOPTs. The change consists of below
described parts:
A) New cost computation model. Currently, there are big amount code trying
to understand
tree expression and estimate its computation cost. The model is designed
long ago
for generic tree expressions. In order to process generic expression
(even address
expression of array/memory references), it has code for too many corner
cases. The
problem is it's somehow impossible to handle all complicated expressions,
even with
complicated logic in functions like get_computation_cost_at,
difference_cost,
ptr_difference_cost, get_address_cost and so on... The second problem is
it's hard
to keep cost model consistent among special cases. As special cases being
added
from time to time, the model is no long unified any more. There are cases
that right
cost results in bad code, or vice versa, wrong cost results in good code.
Finally,
it's also difficult to add code for new cases.
This patch introduces a new cost computation model by using tree affine.
Tree exprs
are lowered to aff_tree which is simple arithmetic operation usually.
Code handling
special cases is no longer necessary, which brings us quite simplicity.
It is also
easier to compute consistent costs among different expressions using tree
affine,
which gives us a unified cost model.
This change is implemented in [PATCH rewrite-cost-computation-*.txt].
B) In rewriting both nonlinear iv_use and address iv_use, current code does
bad association
by mixing computation of invariant and induction. This introduces
inconsistency
between cost computation and code generation because costs of invariant
and induction
are computed separately. This also prevents loop inv from being hoisted
out of loop.
This change fixes the issue by re-associating invariant and induction
parts separately
for both nonlinear and address iv_use.
This patch is implemented in two patches:
[PATCH nonlinear-iv_use-rewrite-*.txt]
[PATCH address-iv_use-rewrite-*.txt]
C) Current implementation shares the same register pressure computation with
RTL loop
inv pass. It has difficulty in handling (especially large) loop nest, and
quite
often generating too many candidates (especially for outer loops). This
change
introduces new register pressure estimation. The brief idea is to
differentiate
(hot) innermost loop and outer loop. for (possibly hot) innermost loop,
more registers
are allowed as long as overall register pressure is within the range of
number of
target available registers.
This change is implemented in below patches:
[PATCH record-newly-used-inv_var-*.txt]
[PATCH skip-non_int-phi-reg-pressure-*.txt]
[PATCH ivopt-reg_pressure-model-*.txt]
D) Other small refactors and improvements. These will be described in each
patch's review
message.
E) Patches allow better induction variable optimizations for vectorized
loops. These
patches are blocked at the moment because current IVOPTs implementation
can generate
worse code on targets with limited addressing mode support.
[PATCH range_info-for-vect_loop-niters-*.txt]
[PATCH pr69710-*.txt]
As a bonus, issues like PR53090/PR71361 are now fixed with better code
generation than what
the two PRs were expecting.
I collected spec2k6 data on my local AArch64 and X86_64 machines. Overall FP
is improved
+1% on both machines; while INT mainly remains neutral. I think part of
improvement comes
from IVOPTs itself, and rest of it comes from opportunities enabled as
described by E). Also It
would be great if other targets can run some benchmarks with this patch series
in case of any
performance breakage.
The patch series is bootstrap and test on X86_64 and AArch64, no real
regression found,
though some tests do need further adjustment.
As the start, this is the first patch of the series. It simply handles
TRUNCATE between
tieable modes in rtx_cost. Since we don't need additional instruction for such
truncate,
it simply return 0 cost.
Is it OK?
Thanks,
bin
2017-04-11 Bin Cheng <bin.ch...@arm.com>
* rtlanal.c (rtx_cost): Handle TRUNCATE between tieable modes.From d9b17e5d303d5fb1c75f489753b4578f8c907453 Mon Sep 17 00:00:00 2001
From: Bin Cheng <binch...@e108451-lin.cambridge.arm.com>
Date: Mon, 27 Feb 2017 14:51:56 +0000
Subject: [PATCH 01/33] no_cost-for-tieable-type-truncate-20170220.txt
---
gcc/rtlanal.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/gcc/rtlanal.c b/gcc/rtlanal.c
index acb4230..6019c3e 100644
--- a/gcc/rtlanal.c
+++ b/gcc/rtlanal.c
@@ -4146,6 +4146,14 @@ rtx_cost (rtx x, machine_mode mode, enum rtx_code
outer_code,
return COSTS_N_INSNS (2 + factor);
break;
+ case TRUNCATE:
+ /* If we can tie these modes, make this cheap. */
+ if (MODES_TIEABLE_P (mode, GET_MODE (SUBREG_REG (x))))
+ {
+ total = 0;
+ break;
+ }
+ /* FALLTHRU */
default:
if (targetm.rtx_costs (x, mode, outer_code, opno, &total, speed))
return total;
--
1.9.1
