From: Xinliang David Li <davidxl@google.com>
To: Richard Guenther <richard.guenther@gmail.com>
Cc: Ian Lance Taylor <iant@google.com>,
"Paulo J. Matos" <pocmatos@gmail.com>,
gcc@gcc.gnu.org
Subject: Re: Clustering switch cases
Date: Sat, 28 Aug 2010 00:21:00 -0000 [thread overview]
Message-ID: <AANLkTinS5Whjy1mTmm6+YPX3fj7TURunh5oOOTuNKUDV@mail.gmail.com> (raw)
In-Reply-To: <AANLkTim3vfWL5BsfeGg+VV2DOjnFRxkas2LT3waUQwQj@mail.gmail.com>
Another main thing missing is to consider profile information (if
available) so that most frequent cases can be peeled out.
David
On Fri, Aug 27, 2010 at 8:03 AM, Richard Guenther
<richard.guenther@gmail.com> wrote:
> On Fri, Aug 27, 2010 at 4:47 PM, Ian Lance Taylor <iant@google.com> wrote:
>> "Paulo J. Matos" <pocmatos@gmail.com> writes:
>>
>>> In the first case, it generates a binary tree, and in the second two
>>> jump tables. The jump tables solution is much more elegant (at least
>>> in our situation), generating less code and being faster.
>>> Now, what I am wondering is the reason why GCC doesn't try to cluster
>>> the cases trying to find for clusters of contiguous values in the
>>> switch.
>>>
>>> If there is no specific reason then I would implement such pass, which
>>> would before expansion split switches according to value clustering,
>>> since I find it would be a good code improvement.
>>>
>>> Currently GCC seems to only use jump table is the range of the switch
>>> is not much bigger than its count, which works well in most cases
>>> except when you have big switches with clusters of contiguous values
>>> (like the first example I sent).
>>
>> I don't know of any specific reason not to look for clusters of switch
>> cases. The main issue would be the affect on compilation time. If you
>> can do it with an algorithm which is linear in the number of cases, then
>> I think it would be an acceptable optimization.
>
> In fact we might want to move switch optimization up to the tree level
> (just because it's way easier to deal with there). Thus, lower switch
> to a mixture of binary tree & jump-tables (possibly using perfect
> hashing).
>
> Richard.
>
next prev parent reply other threads:[~2010-08-27 21:56 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-08-27 13:49 Paulo J. Matos
2010-08-27 15:03 ` Ian Lance Taylor
2010-08-27 15:09 ` Paulo J. Matos
2010-08-27 15:27 ` Richard Guenther
2010-08-28 0:21 ` Xinliang David Li [this message]
2010-08-28 11:08 ` Paulo J. Matos
2010-08-31 15:59 ` Rahul Kharche
[not found] ` <201009010034.18924.paul@codesourcery.com>
2010-09-01 8:39 ` Richard Guenther
2010-09-01 9:15 ` Paulo J. Matos
2010-09-01 9:33 ` Richard Guenther
2010-09-01 12:37 ` Paulo J. Matos
[not found] ` <D84AB18A31AC674EBE9B5300634A800601BF0B0932@EXCHANGEVS.IceraSemi.local>
[not found] ` <AANLkTinFhrgJw3Wz8aLhBU6PMYpJzBFTRxLUH3eoz4M0@mail.gmail.com>
[not found] ` <4D60B0700D1DB54A8C0C6E9BE691637008965A28@EXCHANGEVS.IceraSemi.local>
[not found] ` <AANLkTinkjc4Y9baHXXK7v3ibQNwMtwGhA_oGmaFbykkA@mail.gmail.com>
2010-09-03 10:29 ` Rahul Kharche
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