From: Richard Guenther <richard.guenther@gmail.com>
To: Zhenqiang Chen <zhenqiang.chen@arm.com>
Cc: gcc-patches@gcc.gnu.org
Subject: Re: [PATCH] Disable loop2_invariant for -Os
Date: Wed, 27 Jun 2012 09:09:00 -0000 [thread overview]
Message-ID: <CAFiYyc1qL32NoWXM=KY9xAnrVRzUsTb8i6uPAJrAXB_oOJ46ew@mail.gmail.com> (raw)
In-Reply-To: <4feac6f5.4abd440a.074d.ffffcfbaSMTPIN_ADDED@mx.google.com>
On Wed, Jun 27, 2012 at 10:40 AM, Zhenqiang Chen <zhenqiang.chen@arm.com> wrote:
> Hi,
>
> In general, invariant motion itself can not reduce code size.
It can expose CSE opportunities across loops though.
> But it will
> change the liverange of the invariant, which might lead to more spilling.
"might" - indeed. I wonder what the trade-off is here ... but given that you
leave tree loop invariant motion enabled it might not make much of a difference.
Still as this is mostly a spilling issue it looks odd to do that generally. In
fact you could improve things by only disabling motion when that increases
register lifetime - it can after all reduce overall register lifetime:
for (;;)
inv = inv1 + inv2;
... use inv;
to
inv = inv1 + inv2;
for (;;)
... use inv;
has register lifetime reduced.
Or at least like I suggest below.
> The patch disables loop2_invariant when optimizing for size.
>
> I measured the code size benefit for four targets based on CSiBE benchmark:
>
> ARM: 0.33%
> MIPS: 1.15%
> PPC: 0.24%
> X86: 0.45%
>
> Is it OK for trunk?
>
> Thanks!
> -Zhenqiang
>
> ChangeLog:
> 2012-06-27 Zhenqiang Chen <zhenqiang.chen@arm.com>
>
> * loop-init.c (gate_rtl_move_loop_invariants): Disable
> loop2_invariant
> when optimizing function for size.
>
> diff --git a/gcc/loop-init.c b/gcc/loop-init.c
> index 03f8f61..5d8cf73 100644
> --- a/gcc/loop-init.c
> +++ b/gcc/loop-init.c
> @@ -273,6 +273,12 @@ struct rtl_opt_pass pass_rtl_loop_done =
> static bool
> gate_rtl_move_loop_invariants (void)
> {
> + /* In general, invariant motion can not reduce code size. But it will
> + change the liverange of the invariant, which increases the register
> + pressure and might lead to more spilling. */
> + if (optimize_function_for_size_p (cfun))
> + return false;
> +
Can you do this per loop instead? Using optimize_loop_nest_for_size_p?
Thanks,
Richard.
> return flag_move_loop_invariants;
> }
>
>
>
next parent reply other threads:[~2012-06-27 8:53 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <4feac6f5.4abd440a.074d.ffffcfbaSMTPIN_ADDED@mx.google.com>
2012-06-27 9:09 ` Richard Guenther [this message]
2012-06-28 9:03 ` Zhenqiang Chen
[not found] ` <4fec169c.e908b40a.26d1.ffffc3f1SMTPIN_ADDED@mx.google.com>
2012-06-28 9:38 ` Richard Guenther
2012-07-03 8:29 ` Zhenqiang Chen
[not found] ` <4ff2ad57.8aa0d80a.5910.ffffb0d6SMTPIN_ADDED@mx.google.com>
2012-07-03 9:32 ` Richard Guenther
2012-07-09 8:40 ` Zhenqiang Chen
[not found] <4feac6fa.e288440a.73b3.ffffa5c7SMTPIN_ADDED@mx.google.com>
2012-06-27 9:53 ` Steven Bosscher
2012-06-28 9:05 ` Zhenqiang Chen
2012-06-27 8:48 Zhenqiang Chen
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