Remove colour attributes from body and strip most of the font tags out
This commit is contained in:
James Gregory
parent
7e957c0bb4
commit
c1f88ddb41
362 changed files with 1632 additions and 1706 deletions
16
08-02.html
16
08-02.html
|
|
@ -24,7 +24,7 @@
|
|||
<!--CHAPTER=08//-->
|
||||
<!--PAGES=153-156//-->
|
||||
<!--UNASSIGNED1//-->
|
||||
<!--UNASSIGNED2//--></HEAD><BODY LINK=#0000FF ALINK=#000099 VLINK=#0000FF BGCOLOR=#FFFFFF>
|
||||
<!--UNASSIGNED2//--></HEAD><body>
|
||||
|
||||
<CENTER>
|
||||
<TABLE BORDER>
|
||||
|
|
@ -36,18 +36,18 @@
|
|||
</TABLE>
|
||||
</CENTER>
|
||||
<P><BR></P>
|
||||
<H3><A NAME="Heading5"></A><FONT COLOR="#000077">Stack Frames Slow So Much</FONT></H3>
|
||||
<H3><A NAME="Heading5"></A>Stack Frames Slow So Much</H3>
|
||||
<P>C compilers work within the stack frame model, whereby variables reside in a block of stack memory and are accessed via offsets from BP. Compilers may store a couple of variables in registers and may briefly keep other variables in registers when they’re used repeatedly, but the stack frame is the underlying architecture. It’s a nice architecture; it’s flexible, convenient, easy to program, and makes for fairly compact code. However, stack frames have a few drawbacks. They must be constructed and destroyed, which takes both time and code. They are so easy to use that they tend to bias the assembly language programmer in favor of accessing memory variables more often than might be necessary. Finally, you cannot use BP as a general-purpose register if you intend to access a stack frame, and having that seventh register available is sometimes useful indeed.
|
||||
</P>
|
||||
<P>That doesn’t mean you shouldn’t use stack frames, which are useful and often necessary. Just don’t fall victim to their undeniable charms.</P>
|
||||
<H3><A NAME="Heading6"></A><FONT COLOR="#000077">Torn Between Two Segments</FONT></H3>
|
||||
<H3><A NAME="Heading6"></A>Torn Between Two Segments</H3>
|
||||
<P>C compilers are not terrific at handling segments. Some compilers can efficiently handle a single far pointer used in a loop by leaving ES set for the duration of the loop. But two far pointers used in the same loop confuse every compiler I’ve seen, causing the full segment:offset address to be reloaded each time either pointer is used.
|
||||
</P>
|
||||
<TABLE WIDTH="100%"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="5%"><IMG SRC="images/i.jpg"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="95%"><SMALL><I>This particularly affects performance in 286 protected mode (under OS/2 1.X or the Rational DOS Extender, for example) because segment loads in protected mode take a minimum of 17 cycles, versus a mere 2 cycles in real mode. </I></SMALL>
|
||||
</TABLE>
|
||||
<P>In assembly language you have full control over segments. Use it, and, if necessary, reorganize your code to minimize segment loading.
|
||||
</P>
|
||||
<H4 ALIGN="LEFT"><A NAME="Heading7"></A><FONT COLOR="#000077">Why Speeding Up Is Hard to Do</FONT></H4>
|
||||
<H4 ALIGN="LEFT"><A NAME="Heading7"></A>Why Speeding Up Is Hard to Do</H4>
|
||||
<P>You might think that the most obvious advantage assembly language has over C is that it allows the use of all forms of instructions and all registers in all ways, whereas C compilers tend to use a subset of registers and instructions in a limited number of ways. Yes and no. It’s true that C compilers typically don’t generate instructions such as <B>XLAT,</B> rotates, or the string instructions. On the other hand, <B>XLAT</B> and rotates are useful in a limited set of circumstances, and string instructions <I>are</I> used in the C library functions. In fact, C library code is likely to be carefully optimized by experts, and may be much better than equivalent code you’d produce yourself.</P>
|
||||
<P>Am I saying that C compilers produce better code than you do? No, I’m saying that they <I>can,</I> unless you use assembly language properly. Writing code in assembly language rather than C guarantees nothing.</P>
|
||||
<TABLE WIDTH="100%"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="5%"><IMG SRC="images/i.jpg"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="95%"><SMALL><I>You can write good assembly, bad assembly, or assembly that is virtually indistinguishable from compiled code; you are more likely than not to write the latter if you think that optimization consists of tweaking compiled C code. </I></SMALL>
|
||||
|
|
@ -56,7 +56,7 @@
|
|||
</P>
|
||||
<P>True optimization requires rethinking your code to take advantage of assembly language. A C loop that searches through an integer array for matches might compile</P>
|
||||
<P><A NAME="Fig1"><!-- </A><A HREF="javascript:displayWindow('images/08-01.jpg',413,198 )"> --><IMG SRC="images/08-01.jpg"><BR><!-- </A>
|
||||
<BR><A HREF="javascript:displayWindow('images/08-01.jpg',413,198)"> --><FONT COLOR="#000077"><B>Figure 8.1</B></FONT></A> <I>Tweaked compiler output for a loop.</I>
|
||||
<BR><A HREF="javascript:displayWindow('images/08-01.jpg',413,198)"> --><B>Figure 8.1</B></A> <I>Tweaked compiler output for a loop.</I>
|
||||
</P>
|
||||
<P>to something like Figure 8.1A. You might look at that and tweak it to the code shown in Figure 8.1B.
|
||||
</P>
|
||||
|
|
@ -69,7 +69,7 @@ jz Match
|
|||
<!-- END CODE SNIP //-->
|
||||
<P>It’s a simple example—but, I hope, a convincing one. Stretch your brain when you optimize.
|
||||
</P>
|
||||
<H3><A NAME="Heading8"></A><FONT COLOR="#000077">Taking It to the Limit</FONT></H3>
|
||||
<H3><A NAME="Heading8"></A>Taking It to the Limit</H3>
|
||||
<P>The ultimate in assembly language optimization comes when you change the rules; that is, when you reorganize the entire program to allow the use of better assembly language code in the small section of code that most affects overall performance. For example, consider that the data searched in the last example is stored in an array of structures, with each structure in the array containing other information as well. In this situation, <B>REP SCASW</B> couldn’t be used because the data searched through wouldn’t be contiguous.</P>
|
||||
<P>However, if the need for performance in searching the array is urgent enough, there’s no reason why you can’t reorganize the data. This might mean removing the array elements from the structures and storing them in their own array so that <B>REP SCASW</B> <I>could</I> be used.</P>
|
||||
<TABLE WIDTH="100%"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="5%"><IMG SRC="images/i.jpg"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="95%"><SMALL><I>Organizing a program’s data so that the performance of the critical sections can be optimized is a key part of design, and one that’s easily shortchanged unless, during the design stage, you thoroughly understand and work to bring together your data needs, the critical sections of your program, and potential assembly language optimizations.</I></SMALL>
|
||||
|
|
@ -86,7 +86,7 @@ jz Match
|
|||
</ul>
|
||||
<P>That said, let me show some of these precepts in action.
|
||||
</P>
|
||||
<H4 ALIGN="LEFT"><A NAME="Heading9"></A><FONT COLOR="#000077">A C-to-Assembly Case Study</FONT></H4>
|
||||
<H4 ALIGN="LEFT"><A NAME="Heading9"></A>A C-to-Assembly Case Study</H4>
|
||||
<P>Listing 8.1 is the sample C application I’m going to use to examine optimization in action. Listing 8.1 isn’t really complete—it doesn’t handle the “no-matches” case well, and it assumes that the sum of all matches will fit into an <B>int—</B>but it will do just fine as an optimization example.</P><P><BR></P>
|
||||
<CENTER>
|
||||
<TABLE BORDER>
|
||||
|
|
@ -100,7 +100,7 @@ jz Match
|
|||
|
||||
<hr width="90%" size="1" noshade>
|
||||
<div align="center">
|
||||
<font face="Verdana,sans-serif" size="1">Graphics Programming Black Book © 2001 Michael Abrash</font>
|
||||
Graphics Programming Black Book © 2001 Michael Abrash
|
||||
</div>
|
||||
<!-- all of the reference materials (books) have the footer and subfoot reveresed -->
|
||||
<!-- reference_subfoot = footer -->
|
||||
|
|
|
|||
Loading…
Reference in a new issue