From mboxrd@z Thu Jan 1 00:00:00 1970 From: Thomas Gleixner Subject: Re: [PATCH] x86/crypto: make constants readonly, allow linker to merge them Date: Fri, 20 Jan 2017 00:09:13 +0100 (CET) Message-ID: References: <20170119213304.18140-1-dvlasenk@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Cc: Herbert Xu , Josh Poimboeuf , Xiaodong Liu , Megha Dey , linux-crypto@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org To: Denys Vlasenko Return-path: In-Reply-To: <20170119213304.18140-1-dvlasenk@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-crypto.vger.kernel.org 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." 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