From mboxrd@z Thu Jan 1 00:00:00 1970 From: Josh Triplett Subject: Re: [RFC 1/8] x86: objtool: use asm macro for better compiler decisions Date: Tue, 15 May 2018 14:55:41 -0700 Message-ID: <20180515215540.GB7298@localhost> References: <20180515141124.84254-1-namit@vmware.com> <20180515141124.84254-2-namit@vmware.com> <20180515213713.GA7298@localhost> <4E72C06C-7A4B-4E18-83DA-8E0C9F701854@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <4E72C06C-7A4B-4E18-83DA-8E0C9F701854@gmail.com> Sender: linux-kernel-owner@vger.kernel.org To: Nadav Amit Cc: linux-kernel@vger.kernel.org, Christopher Li , linux-sparse@vger.kernel.org List-Id: linux-sparse@vger.kernel.org On Tue, May 15, 2018 at 02:53:52PM -0700, Nadav Amit wrote: > Josh Triplett wrote: > > > On Tue, May 15, 2018 at 07:11:08AM -0700, Nadav Amit wrote: > >> GCC considers the number of statements in inlined assembly blocks, > >> according to new-lines and semicolons, as an indication to the cost of > >> the block in time and space. This data is distorted by the kernel code, > >> which puts information in alternative sections. As a result, the > >> compiler may perform incorrect inlining and branch optimizations. > >> > >> In the case of objtool, this distortion is extreme, since anyhow the > >> annotations of objtool are discarded during linkage. > >> > >> The solution is to set an assembly macro and call it from the inlinedv > >> assembly block. As a result GCC considers the inline assembly block as > >> a single instruction. > >> > >> This patch slightly increases the kernel size. > >> > >> text data bss dec hex filename > >> 18126699 10066728 2936832 31130259 1db0293 ./vmlinux before > >> 18126824 10067268 2936832 31130924 1db052c ./vmlinux after (+665) > > > > With what kernel config? What's the text size change for an allnoconfig? > > I use my custom config and to include the drivers in the image. Other > configs were not too appropriate: defconfig does not enable paravirt and > allyesconfig is too heavy. > > There is no effect with allnoconfig on size: > > text data bss dec hex filename > 1018441 230148 1215604 2464193 2599c1 ./vmlinux > 1018441 230148 1215604 2464193 2599c1 ./vmlinux > > I think it is expected since CONFIG_STACK_VALIDATION is off. No? Thanks for checking; fine by me then. :)