<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML ><HEAD ><TITLE >How to NOT use Assembly</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK REL="HOME" TITLE="Linux Assembly HOWTO" HREF="index.html"><LINK REL="UP" TITLE="Do you need assembly?" HREF="doyouneed.html"><LINK REL="PREVIOUS" TITLE="Pros and Cons" HREF="x120.html"><LINK REL="NEXT" TITLE="Linux and assembly" HREF="landa.html"></HEAD ><BODY CLASS="SECTION" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#840084" ALINK="#0000FF" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="3" ALIGN="center" >Linux Assembly HOWTO</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="bottom" ><A HREF="x120.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" >Chapter 2. Do you need assembly?</TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="landa.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECTION" ><H1 CLASS="SECTION" ><A NAME="AEN190" ></A >2.2. How to NOT use Assembly</H1 ><DIV CLASS="SECTION" ><H2 CLASS="SECTION" ><A NAME="AEN192" ></A >2.2.1. General procedure to achieve efficient code</H2 ><P >As Charles Fiterman says on <A HREF="news:comp.compilers" TARGET="_top" >comp.compilers</A > about human vs computer-generated assembly code:</P ><A NAME="AEN196" ></A ><BLOCKQUOTE CLASS="BLOCKQUOTE" ><P CLASS="LITERALLAYOUT" >The human should always win and here is why.<br> <br> First the human writes the whole thing in a high level language.<br> Second he profiles it to find the hot spots where it spends its time.<br> Third he has the compiler produce assembly for those small sections of code.<br> Fourth he hand tunes them looking for tiny improvements over the machine generated code.<br> <br> The human wins because he can use the machine.</P ></BLOCKQUOTE ></DIV ><DIV CLASS="SECTION" ><H2 CLASS="SECTION" ><A NAME="AEN198" ></A >2.2.2. Languages with optimizing compilers</H2 ><P >Languages like ObjectiveCAML, SML, CommonLISP, Scheme, ADA, Pascal, C, C++, among others, all have free optimizing compilers that will optimize the bulk of your programs, and often do better than hand-coded assembly even for tight loops, while allowing you to focus on higher-level details, and without forbidding you to grab a few percent of extra performance in the above-mentioned way, once you've reached a stable design. Of course, there are also commercial optimizing compilers for most of these languages, too!</P ><P >Some languages have compilers that produce C code, which can be further optimized by a C compiler: LISP, Scheme, Perl, and many other. Speed is fairly good.</P ></DIV ><DIV CLASS="SECTION" ><H2 CLASS="SECTION" ><A NAME="AEN202" ></A >2.2.3. General procedure to speed your code up</H2 ><P >As for speeding code up, you should do it only for parts of a program that a profiling tool has consistently identified as being a performance bottleneck.</P ><P >Hence, if you identify some code portion as being too slow, you should <P ></P ><UL ><LI ><P >first try to use a better algorithm;</P ></LI ><LI ><P >then try to compile it rather than interpret it;</P ></LI ><LI ><P >then try to enable and tweak optimization from your compiler;</P ></LI ><LI ><P >then give the compiler hints about how to optimize (typing information in LISP; register usage with GCC; lots of options in most compilers, etc).</P ></LI ><LI ><P >then possibly fallback to assembly programming</P ></LI ></UL > </P ><P >Finally, before you end up writing assembly, you should inspect generated code, to check that the problem really is with bad code generation, as this might really not be the case: compiler-generated code might be better than what you'd have written, particularly on modern multi-pipelined architectures! Slow parts of a program might be intrinsically so. The biggest problems on modern architectures with fast processors are due to delays from memory access, cache-misses, TLB-misses, and page-faults; register optimization becomes useless, and you'll more profitably re-think data structures and threading to achieve better locality in memory access. Perhaps a completely different approach to the problem might help, then.</P ></DIV ><DIV CLASS="SECTION" ><H2 CLASS="SECTION" ><A NAME="AEN218" ></A >2.2.4. Inspecting compiler-generated code</H2 ><P >There are many reasons to inspect compiler-generated assembly code. Here is what you'll do with such code: <P ></P ><UL ><LI ><P >check whether generated code can be obviously enhanced with hand-coded assembly (or by tweaking compiler switches)</P ></LI ><LI ><P >when that's the case, start from generated code and modify it instead of starting from scratch</P ></LI ><LI ><P >more generally, use generated code as stubs to modify, which at least gets right the way your assembly routines interface to the external world</P ></LI ><LI ><P >track down bugs in your compiler (hopefully the rarer)</P ></LI ></UL > </P ><P >The standard way to have assembly code be generated is to invoke your compiler with the <TT CLASS="OPTION" >-S</TT > flag. This works with most Unix compilers, including the GNU C Compiler (GCC), but YMMV. As for GCC, it will produce more understandable assembly code with the <TT CLASS="OPTION" >-fverbose-asm</TT > command-line option. Of course, if you want to get good assembly code, don't forget your usual optimization options and hints!</P ></DIV ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="x120.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="landa.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Pros and Cons</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="doyouneed.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Linux and assembly</TD ></TR ></TABLE ></DIV ></BODY ></HTML >
