From: Nick Terrell <terrelln@fb.com>
To: Colin King <colin.king@canonical.com>, Chris Mason <clm@fb.com>
CC: "kernel-janitors@vger.kernel.org" <kernel-janitors@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] lib: zstd: make const array rtbTable static, reduces
 object code size
Thread-Topic: [PATCH] lib: zstd: make const array rtbTable static, reduces
 object code size
Thread-Index: AQHTM7N9KhSfulA6+kKHmmoJXouq9aLA0SkA
Date: Fri, 22 Sep 2017 19:14:05 +0000
Message-ID: <6C490CCD-32E1-4AC3-BCCF-5EB3A56EA887@fb.com>
References: <20170922145946.14316-1-colin.king@canonical.com>
In-Reply-To: <20170922145946.14316-1-colin.king@canonical.com>
Accept-Language: en-US
Content-Language: en-US
spamdiagnosticoutput: 1:99
spamdiagnosticmetadata: NSPM
Content-Type: text/plain; charset="utf-8"
Content-ID: <A15D010BE620F3429D37F3F0E333CF8A@namprd15.prod.outlook.com>
MIME-Version: 1.0
X-MS-Exchange-CrossTenant-originalarrivaltime: 22 Sep 2017 19:14:05.8748
 (UTC)
X-MS-Exchange-CrossTenant-fromentityheader: Hosted
X-MS-Exchange-CrossTenant-id: 8ae927fe-1255-47a7-a2af-5f3a069daaa2
X-MS-Exchange-Transport-CrossTenantHeadersStamped: CY4PR15MB1655
X-OriginatorOrg: fb.com
X-Proofpoint-Spam-Reason: safe
X-FB-Internal: Safe
X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-09-22_08:,,
 signatures=0
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Transfer-Encoding: 8bit
X-MIME-Autoconverted: from base64 to 8bit by nfs id v8MJEJVD001596
Content-Length: 1437
Lines: 36

On 9/22/17, 8:00 AM, "linux-kernel-owner@vger.kernel.org on behalf of Colin King" <linux-kernel-owner@vger.kernel.org on behalf of colin.king@canonical.com> wrote:
> From: Colin Ian King <colin.king@canonical.com>
> 
> Don't populate const array rtbTable on the stack, instead make it
> static. Also split overly long line to clean a chechkpach warning.
> Makes the object code smaller by nearly 500 bytes:
> 
> Before:
>    text	   data	    bss	    dec	    hex	filename
>   13297	    104	      0	  13401	   3459	lib/zstd/fse_compress.o
> 
> After:
>    text	   data	    bss	    dec	    hex	filename
>   12742	    160	      0	  12902	   3266	lib/zstd/fse_compress.o
> 
> (gcc 6.3.0, x86-64)
> 
> Signed-off-by: Colin Ian King <colin.king@canonical.com>

I tested your patch with gcc-7.1 on x86, and benchmarked the speed on
upstream zstd. There isn't a noticeable speed difference, since it isn't a
particularly hot piece of code. Would you be able to submit the same patch
upstream [1], or would you be okay with me porting it back upstream, so it
doesn't get lost on an update?

I didn't expect gcc to leave constant arrays on the stack, that seems
silly. Clang makes it static, but gcc loads it onto the stack, and in 6.3+
it saves the data statically, and then uses vector instructions to load it
onto the stack [2].

Tested-by: Nick Terrell <terrelln@fb.com>

[1] https://github.com/facebook/zstd
[2] https://godbolt.org/g/fvTcED