Article View: comp.lang.asm.x86
Article #167Re: Jump table without the table?
From: Vis Mike
Date: Wed, 12 Feb 2003 09:56
Date: Wed, 12 Feb 2003 09:56
51 lines
3726 bytes
3726 bytes
I found a nice feature in gcc - -malign-jumps=n > wcc -O3 -malign-jumps=3 vmtest2.cpp > a.exe Testing stack4... &&addi = 0x401180 +4 +8 +16 0x401180 is the address of the first label, followed by the offset of the rest of the labels. So if I can squeeze the code in, all I need is a "jmp (opcode*4)". This is sooo unportable :) I'll just use conditional compilation. I don't quite understand gcc's inline assembly syntax, any help for the jmp? Or am I just going overboard? Cheers! Mike "James Van Buskirk" <not_valid@attbi.com> wrote in message news:dCk2a.80786$HN5.307889@rwcrnsc51.ops.asp.att.net... > > "Matt Taylor" <para@tampabay.rr.com> wrote in message > news:h2j2a.61691$AQ3.981480@twister.tampabay.rr.com... > > > Any good compiler will convert switch/case to a jump table for you. VC > does > > this; I'm quite sure that GCC does too. Unless you're writing in assembly, > > don't try to pad things out. The compiler should do this for you. I don't > > know when specifically VC/GCC do and when they don't. > > I think I have a different interpretation of Mike's question than > you: my take on it is that he has code where all branches are just > little snippets no longer than some nice easy number of bytes to > multiply by, e.g. if all his branches were less than 32 bytes > long and he had 16 possible cases numbered 0:15 he could, assuming > the branch number were in EAX do something like: > > lea EAX, [4*EAX] > jmp branch_implementation_start[8*EAX] > > In this way he could avoid waiting for the address of a previous > write to memory to be computed before prefetching the code > following the branch. Maybe ALIGN directives or ORG > directives could tell the assembler where to insert padding > without having to maintain it by hand every time the code > is modified? > > >
Technical Details
Message-ID:
<b2d1sj$8j4$1@bob.news.rcn.net>Path:
arc1-ca.pugleaf.net!archiver1.google.com!news1.google.com!sn-xit-02!sn-xit-04!sn-xit-06!sn-post-01!supernews.com!corp.supernews.com!news.crayne.org!clax86!robomod!not-for-mailReferences:
<b2c5s4$p9p$1@bob.news.rcn.net> <h2j2a.61691$AQ3.981480@twister.tampabay.rr.com> <dCk2a.80786$HN5.307889@rwcrnsc51.ops.asp.att.net>Show Full Headers
Complete Article Headers
X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: 11318c,d7bccf0f48f83a38,start X-Google-Attributes: gid11318c,public X-Google-Thread: 114917,fa3250fb672bd972 X-Google-Attributes: gid114917,public X-Google-ArrivalTime: 2003-02-12 01:56:27 PST Path: arc1-ca.pugleaf.net!archiver1.google.com!news1.google.com!sn-xit-02!sn-xit-04!sn-xit-06!sn-post-01!supernews.com!corp.supernews.com!news.crayne.org!clax86!robomod!not-for-mail From: "Vis Mike" <visionary25@_nospam_hotmail.com> Newsgroups: comp.lang.asm.x86,gnu.gcc.help Subject: Re: Jump table without the table? Approved: CLAX86 Moderators <ccrayne@crayne.org> Date: Wed, 12 Feb 2003 09:56:10 +0000 (UTC) Organization: Crayne Consulting Message-ID: <b2d1sj$8j4$1@bob.news.rcn.net> Sender: clax86@wotan.asgard References: <b2c5s4$p9p$1@bob.news.rcn.net> <h2j2a.61691$AQ3.981480@twister.tampabay.rr.com> <dCk2a.80786$HN5.307889@rwcrnsc51.ops.asp.att.net> X-CLAX86-Policy: http://www.pacificsites.com/~ccrayne/clax86.html X-CLAX86-Faq: http://www2.dgsys.com/~raymoon/x86faqs.html X-CLAX86-Info-1: Send submissions to comp-lang-asm-x86@moderators.isc.org X-CLAX86-Info-2: Send technical complaints to ccrayne@crayne.org X-CLAX86-Info-3: Send complaints about policy to ccrayne@crayne.org X-Comment: moderators do not necessarily agree or disagree with this article. X-MimeOLE: Produced By Microsoft MimeOLE V5.50.4807.1700 X-Priority: 3 X-Newsreader: Microsoft Outlook Express 5.50.4807.1700 X-MSMail-Priority: Normal X-Spam-Flag: Spam YES X-Spam-Level: Level ****** X-MailScanner: Found to be clean X-MailScanner-SpamCheck: not spam (whitelisted), SpamAssassin (score=6, required 5, FORGED_HOTMAIL_RCVD, FROM_ENDS_IN_NUMS, NO_MX_FOR_FROM, PRIORITY_NO_NAME, QUOTED_EMAIL_TEXT, REFERENCES, SPAM_PHRASE_00_01) X-UIDL: :GQ!!@mA!!+?(#!"<e"! X-Complaints-To: abuse@supernews.com Lines: 54