From: Jeff Law <law@redhat.com>
To: Richard Biener <richard.guenther@gmail.com>
Cc: gcc-patches <gcc-patches@gcc.gnu.org>
Subject: Re: [RFA] Handle target with no length attributes sanely in bb-reorder.c
Date: Tue, 29 Nov 2016 16:07:00 -0000 [thread overview]
Message-ID: <deefa649-b81f-d3b8-632c-dd5eee2cc98c@redhat.com> (raw)
In-Reply-To: <CAFiYyc04nne8VH0dpJVWELvp4gxBSTWYtFkG2CtmrGKMCmTPyA@mail.gmail.com>
On 11/29/2016 03:23 AM, Richard Biener wrote:
> On Mon, Nov 28, 2016 at 10:23 PM, Jeff Law <law@redhat.com> wrote:
>>
>>
>> I was digging into issues around the patches for 78120 when I stumbled upon
>> undesirable bb copying in bb-reorder.c on the m68k.
>>
>> The core issue is that the m68k does not define a length attribute and
>> therefore generic code assumes that the length of all insns is 0 bytes.
>
> What other targets behave like this?
ft32, nvptx, mmix, mn10300, m68k, c6x, rl78, vax, ia64, m32c
cris has a hack to define a length, even though no attempt is made to
make it accurate. The hack specifically calls out that it's to make
bb-reorder happy.
>
>> That in turn makes bb-reorder think it is infinitely cheap to copy basic
>> blocks. In the two codebases I looked at (GCC's runtime libraries and
>> newlib) this leads to a 10% and 15% undesirable increase in code size.
>>
>> I've taken a slight variant of this patch and bootstrapped/regression tested
>> it on x86_64-linux-gnu to verify sanity as well as built the m68k target
>> libraries noted above.
>>
>> OK for the trunk?
>
> I wonder if it isn't better to default to a length of 1 instead of zero when
> there is no length attribute. There are more users of the length attribute
> in bb-reorder.c (and elsewhere as well I suppose).
I pondered that as well, but felt it was riskier given we've had a
default length of 0 for ports that don't define lengths since the early
90s. It's certainly easy enough to change that default if you'd prefer.
I don't have a strong preference either way.
Jeff
next prev parent reply other threads:[~2016-11-29 16:07 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-11-28 21:23 Jeff Law
2016-11-29 10:23 ` Richard Biener
2016-11-29 16:07 ` Jeff Law [this message]
2016-11-30 8:38 ` Richard Biener
2016-11-30 23:29 ` Jeff Law
2016-12-01 9:19 ` Richard Biener
2016-12-01 12:04 ` Segher Boessenkool
2016-12-01 17:28 ` Jeff Law
2016-12-02 8:47 ` Richard Biener
2016-12-02 22:22 ` Segher Boessenkool
2016-12-03 2:20 ` Jeff Law
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