linux-kernel - Re: [PATCH] x86/crypto: make constants readonly, allow linker to merge them

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Date:   Fri, 20 Jan 2017 00:09:13 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Denys Vlasenko <dvlasenk@...hat.com>
cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Xiaodong Liu <xiaodong.liu@...el.com>,
        Megha Dey <megha.dey@...el.com>, linux-crypto@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/crypto: make constants readonly, allow linker to
 merge them

On Thu, 19 Jan 2017, Denys Vlasenko wrote:

> A lot of asm-optimized routines in arch/x86/crypto/ keep its
> constants in .data. This is wrong, they should be on .rodata.
> 
> Mnay of these constants are the same in different modules.
> For example, 128-bit shuffle mask 0x000102030405060708090A0B0C0D0E0F
> exists in at least half a dozen places.
> 
> There is a way to let linker merge them and use just one copy.
> The rules are as follows: mergeable objects of different sizes
> should not share sections. You can't put them all in one .rodata
> section, they will lose "mergeability".
> 
> GCC puts its mergeable constants in ".rodata.cstSIZE" sections,
> or ".rodata.cstSIZE.<object_name>" if -fdata-sections is used.
> This patch does the same:
> 
> 	.section .rodata.cst16.SHUF_MASK, "aM", @progbits, 16
> 
> It is important that all data in such section consists of
> 16-byte elements, not larger ones, and there are no implicit
> use of one element from another.
> 
> When this is not the case, use non-mergeable section:
> 
> 	.section .rodata[.VAR_NAME], "a", @progbits
> 
> This reduces .data by ~15 kbytes:
> 
>     text    data     bss     dec      hex filename
> 11097415 2705840 2630712 16433967  fac32f vmlinux-prev.o
> 11112095 2690672 2630712 16433479  fac147 vmlinux.o

And at the same time it increases text size by ~15k. The overall change in
total size is 488 byte reduction. Weird.

Thanks,

	tglx