From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 11561 invoked by alias); 19 Feb 2014 15:15:37 -0000 Mailing-List: contact gcc-help-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Id: List-Archive: List-Post: List-Help: Sender: gcc-help-owner@gcc.gnu.org Received: (qmail 11488 invoked by uid 89); 19 Feb 2014 15:15:36 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-2.4 required=5.0 tests=AWL,BAYES_00,FREEMAIL_FROM,RCVD_IN_DNSWL_LOW,SPF_PASS autolearn=ham version=3.3.2 X-HELO: mail-qa0-f53.google.com Received: from mail-qa0-f53.google.com (HELO mail-qa0-f53.google.com) (209.85.216.53) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with (AES128-SHA encrypted) ESMTPS; Wed, 19 Feb 2014 15:15:36 +0000 Received: by mail-qa0-f53.google.com with SMTP id cm18so606223qab.12 for ; Wed, 19 Feb 2014 07:15:34 -0800 (PST) MIME-Version: 1.0 X-Received: by 10.236.129.198 with SMTP id h46mr43941675yhi.17.1392822933977; Wed, 19 Feb 2014 07:15:33 -0800 (PST) Received: by 10.170.83.7 with HTTP; Wed, 19 Feb 2014 07:15:33 -0800 (PST) In-Reply-To: References: Date: Wed, 19 Feb 2014 15:15:00 -0000 Message-ID: Subject: Re: Moving C++ code to a different ELF section From: Saul Tamari To: Ian Lance Taylor Cc: "gcc-help@gcc.gnu.org" Content-Type: text/plain; charset=ISO-8859-1 X-SW-Source: 2014-02/txt/msg00109.txt.bz2 Hi, I am trying to see if moving mostly unused code (e.g. conditional debug print statements) to a different section (and to different pages) would impact performance in a large application. Thanks, Saul On Wed, Feb 19, 2014 at 9:11 AM, Ian Lance Taylor wrote: > On Wed, Feb 19, 2014 at 5:57 AM, Saul Tamari wrote: >> >> I'm trying to move some C++ code to a different ELF section and am >> facing some errors which I don't understand. I'm using g++ v4.8.1 on >> x86. >> >> When compiling the following code I'm getting these errors: >> /tmp/ccpp2AkE.s: Assembler messages: >> /tmp/ccpp2AkE.s:63: Error: CFI instruction used without previous .cfi_startproc >> /tmp/ccpp2AkE.s:64: Error: CFI instruction used without previous .cfi_startproc >> /tmp/ccpp2AkE.s:66: Error: .cfi_endproc without corresponding .cfi_startproc >> /tmp/ccpp2AkE.s: Error: open CFI at the end of file; missing >> .cfi_endproc directive >> >> >> The source is: >> #include >> #include >> >> int qqq; >> >> int main(int argc, char* argv[]) >> { >> std::cout << "hey " << std::endl; >> >> qqq = rand(); >> if (qqq > 0x1000000) { >> asm volatile ("jmp 1f \n\t .pushsection >> __kuku,\"ax\",@progbits \n\t 1:"); >> std::cout << "0x123456" << std::endl; >> throw 12345; >> asm volatile("jmp 3f \n\t .popsection \n\t 3:"); >> } >> >> return 0; >> } >> >> >> What do these errors mean? Is there a way to fix them? Is there an >> alternate method to move similar code to a different section? > > The assembler errors occur because GCC emits debug info in the > assembler stream using CFI pseudo-ops, and you are moving the > pseudo-ops to a different section in a way that GCC does not > understand. The assembler is seeing CFI pseudo-ops that make no > sense, so it is giving errors about them. > > The approach you are using can not work. The compiler is not an > assembler. It does not issue instructions in precise sequence. It > copies and duplicates and rearranges instructions as it sees fit. > This is so even though you are using asm volatile. All the asm > volatile promises is that the string will appear at the right point in > execution sequence. Your strings can only work if they appear at the > right point in the assembler output. That is a different matter that > the compiler does not guarantee. > > You didn't see what you are trying to do, but at a guess you should > look at the -freorder-blocks-and-partition option. > > Ian