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: Wed, 30 Nov 2016 23:29:00 -0000 [thread overview]
Message-ID: <be510e59-10a1-bd46-c0fa-7d2ecefba176@redhat.com> (raw)
In-Reply-To: <CAFiYyc1nWAHfiZoBi2bOCxrEyOPXASi9nk58j1Kdj-aWzFMR2A@mail.gmail.com>
On 11/30/2016 01:38 AM, Richard Biener wrote:
> On Tue, Nov 29, 2016 at 5:07 PM, Jeff Law <law@redhat.com> wrote:
>> 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
>
> Ok.
>
>> 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.
>
> Thinking about this again maybe targets w/o insn-length should simply
> always use the 'simple' algorithm instead of the STV one? At least that
> might be what your change effectively does in some way?
From reading the comments I don't think STC will collapse down into the
simple algorithm if block copying is disabled. But Segher would know
for sure.
WRT the choice of simple vs STC, I doubt it matters much for the
processors in question.
JEff
next prev parent reply other threads:[~2016-11-30 23:29 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
2016-11-30 8:38 ` Richard Biener
2016-11-30 23:29 ` Jeff Law [this message]
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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