linux-kernel - Re: [PATCH] lib: zstd: make const array rtbTable static, reduces object code size

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Message-ID: <6C490CCD-32E1-4AC3-BCCF-5EB3A56EA887@fb.com>
Date:   Fri, 22 Sep 2017 19:14:05 +0000
From:   Nick Terrell <terrelln@...com>
To:     Colin King <colin.king@...onical.com>, Chris Mason <clm@...com>
CC:     "kernel-janitors@...r.kernel.org" <kernel-janitors@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lib: zstd: make const array rtbTable static, reduces
 object code size

On 9/22/17, 8:00 AM, "linux-kernel-owner@...r.kernel.org on behalf of Colin King" <linux-kernel-owner@...r.kernel.org on behalf of colin.king@...onical.com> wrote:
> From: Colin Ian King <colin.king@...onical.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@...onical.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@...com>

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