273 lines
11 KiB
HTML
273 lines
11 KiB
HTML
<HTML>
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<HEAD>
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<META name=vsisbn content="1576101746">
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<META name=vstitle content="Michael Abrash's Graphics Programming Black Book, Special Edition">
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<META name=vsauthor content="Michael Abrash">
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<META name=vspublisher content="The Coriolis Group">
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<META name=vspubdate content="07/01/97">
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<META name=vscategory content="Web and Software Development: Game Development,Web and Software Development: Graphics and Multimedia Development">
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<TITLE>Michael Abrash's Graphics Programming Black Book Special Edition: Speeding Up C with Assembly Language</TITLE>
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<!-- HEADER -->
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<!-- Empty Reference Subhead -->
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<!--ISBN=1576101746//-->
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<!--TITLE=Michael Abrash's Graphics Programming Black Book Special Edition//-->
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<!--AUTHOR=Michael Abrash//-->
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<!--PUBLISHER=The Coriolis Group, Inc.//-->
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<!--CHAPTER=08//-->
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<!--PAGES=160-163//-->
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<!--UNASSIGNED1//-->
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<!--UNASSIGNED2//--></HEAD><BODY LINK=#0000FF ALINK=#000099 VLINK=#0000FF BGCOLOR=#FFFFFF>
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<CENTER>
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<TABLE BORDER>
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<TR>
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<TD><A HREF="08-03.html">Previous</A></TD>
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<TD><A HREF="index.html">Table of Contents</A></TD>
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<TD><A HREF="08-05.html">Next</A></TD>
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</CENTER>
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<P><BR></P>
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<P>It’s hard to squeeze much more performance from this code by tweaking it, as exemplified by Listing 8.3, a fine-tuned assembly version of <B>FindIDAverage</B> that was produced by looking at the assembly output of MS C/C<SMALL>++</SMALL> and tightening it. Listing 8.3 eliminates all stack frame access in the inner loop, but that’s about all the tightening there is to do. The result, as shown in Table 8.1, is that Listing 8.3 runs a modest 11 percent faster than Listing 8.1 on a 386. The results could vary considerably, depending on the nature of the data set searched through (average block size and frequency of matches). But, then, understanding the typical and worst case conditions is part of optimization, isn’t it?</P>
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<P><B>LISTING 8.3 L8-3.ASM</B></P>
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<!-- CODE //-->
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<PRE>
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; Typically optimized assembly language version of FindIDAverage.
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SearchedForID equ 4 ;Passed parameter offsets in the
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BlockPointer equ 6 ; stack frame (skip over pushed BP
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; and the return address)
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NextBlock equ 0 ;Field offsets in struct BlockHeader
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BlockCount equ 2
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BLOCK_HEADER_SIZE equ 4 ;Number of bytes in struct BlockHeader
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ID equ 0 ;struct DataElement field offsets
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Value equ 2
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DATA_ELEMENT_SIZE equ 4 ;Number of bytes in struct DataElement
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.model small
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.code
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public _FindIDAverage
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</PRE>
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<!-- END CODE //-->
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<TABLE WIDTH="100%">
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<TR>
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<TD ALIGN="LEFT" VALIGN="TOP" COLSPAN="3"><HR>
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<TR>
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<TH ALIGN="LEFT" VALIGN="TOP">
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<TH ALIGN="LEFT" VALIGN="TOP">On 20 MHz 386
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<TH ALIGN="LEFT" VALIGN="TOP">On 10 MHz 286
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<TR>
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<TD ALIGN="LEFT" VALIGN="TOP" COLSPAN="3"><HR>
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<TR>
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<TD ALIGN="LEFT" VALIGN="TOP"><B>Listing 8.1</B>
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<TD ALIGN="LEFT" VALIGN="TOP">294 microseconds
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<TD ALIGN="LEFT" VALIGN="TOP">768 microseconds
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<TR>
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<TD ALIGN="LEFT" VALIGN="TOP">(MSC with maximum optimization)
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<TD ALIGN="LEFT" VALIGN="TOP">
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<TD ALIGN="LEFT" VALIGN="TOP">
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<TR>
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<TD ALIGN="LEFT" VALIGN="TOP"><B>Listing 8.3</B>
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<TD ALIGN="LEFT" VALIGN="TOP">265
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<TD ALIGN="LEFT" VALIGN="TOP">644
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<TR>
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<TD ALIGN="LEFT" VALIGN="TOP">(Assembly)
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<TD ALIGN="LEFT" VALIGN="TOP">
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<TD ALIGN="LEFT" VALIGN="TOP">
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<TR>
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<TD ALIGN="LEFT" VALIGN="TOP"><B>Listing 8.4</B>
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<TD ALIGN="LEFT" VALIGN="TOP">212
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<TD ALIGN="LEFT" VALIGN="TOP">486
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<TR>
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<TD ALIGN="LEFT" VALIGN="TOP">(Optimized assembly)
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<TD ALIGN="LEFT" VALIGN="TOP">
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<TD ALIGN="LEFT" VALIGN="TOP">
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<TR>
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<TD ALIGN="LEFT" VALIGN="TOP"><B>Listing 8.6</B>
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<TD ALIGN="LEFT" VALIGN="TOP">100
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<TD ALIGN="LEFT" VALIGN="TOP">207
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<TR>
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<TD ALIGN="LEFT" VALIGN="TOP">(Optimized assembly with reorganized data)
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<TD ALIGN="LEFT" VALIGN="TOP">
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<TD ALIGN="LEFT" VALIGN="TOP">
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<TR>
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<TD ALIGN="LEFT" VALIGN="TOP" COLSPAN="3"><HR>
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<TR>
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<TH ALIGN="LEFT" VALIGN="TOP">Table 8.1 Execution Times of FindIDAverage.
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<TR>
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<TD ALIGN="LEFT" VALIGN="TOP" COLSPAN="3"><HR>
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<TR>
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</TABLE>
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<!-- CODE //-->
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<PRE>
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_FindIDAverage proc near
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push bp ;Save caller’s stack frame
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mov bp,sp ;Point to our stack frame
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push di ;Preserve C register variables
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push si
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sub dx,dx ;IDMatchSum = 0
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mov bx,dx ;IDMatchCount = 0
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mov si,[bp+BlockPointer] ;Pointer to first block
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mov ax,[bp+SearchedForID] ;ID we’re looking for
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; Search through all the linked blocks until the last block
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; (marked with a NULL pointer to the next block) has been searched.
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BlockLoop:
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; Point to the first DataElement entry within this block.
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lea di,[si+BLOCK_HEADER_SIZE]
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; Search through all the DataElement entries within this block
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; and accumulate data from all that match the desired ID.
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mov cx,[si+BlockCount]
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jcxz DoNextBlock ;No data in this block
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IntraBlockLoop:
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cmp [di+ID],ax ;Do we have an ID match?
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jnz NoMatch ;No match
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inc bx ;We have a match; IDMatchCount++;
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add dx,[di+Value] ;IDMatchSum += DataPointer->Value;
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NoMatch:
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add di,DATA_ELEMENT_SIZE ;point to the next element
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loop IntraBlockLoop
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; Point to the next block and continue if that pointer isn’t NULL.
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DoNextBlock:
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mov si,[si+NextBlock] ;Get pointer to the next block
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and si,si ;Is it a NULL pointer?
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jnz BlockLoop ;No, continue
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; Calculate the average of all matches.
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sub ax,ax ;Assume we found no matches
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and bx,bx
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jz Done ;We didn’t find any matches, return 0
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xchg ax,dx ;Prepare for division
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div bx ;Return IDMatchSum / IDMatchCount
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Done: pop si ;Restore C register variables
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pop di
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pop bp ;Restore caller’s stack frame
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ret
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_FindIDAverage ENDP
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end
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</PRE>
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<!-- END CODE //-->
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<P>Listing 8.4 tosses some sophisticated optimization techniques into the mix. The loop is unrolled eight times, eliminating a good deal of branching, and <B>SCASW</B> is used instead of <B>CMP [DI],AX.</B> (Note, however, that <B>SCASW</B> is in fact slower than <B>CMP [DI],AX</B> on the 386 and 486, and is sometimes faster on the 286 and 8088 only because it’s shorter and therefore may prefetch faster.) This advanced tweaking produces a 39 percent improvement over the original C code—substantial, but not a tremendous return for the optimization effort invested.</P>
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<P><B>LISTING 8.4 L8-4.ASM</B></P>
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<!-- CODE //-->
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<PRE>
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; Heavily optimized assembly language version of FindIDAverage.
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; Features an unrolled loop and more efficient pointer use.
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SearchedForID equ 4 ;Passed parameter offsets in the
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BlockPointer equ 6 ; stack frame (skip over pushed BP
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; and the return address)
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NextBlock equ 0 ;Field offsets in struct BlockHeader
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BlockCount equ 2
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BLOCK_HEADER_SIZE equ 4 ;Number of bytes in struct BlockHeader
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ID equ 0 ;struct DataElement field offsets
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Value equ 2
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DATA_ELEMENT_SIZE equ 4 ;Number of bytes in struct DataElement
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.model small
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.code
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public _FindIDAverage
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_FindIDAverage proc near
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push bp ;Save caller’s stack frame
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mov bp,sp ;Point to our stack frame
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push di ;Preserve C register variables
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push si
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mov di,ds ;Prepare for SCASW
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mov es,di
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cld
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sub dx,dx ;IDMatchSum = 0
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mov bx,dx ;IDMatchCount = 0
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mov si,[bp+BlockPointer] ;Pointer to first block
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mov ax,[bp+SearchedForID] ;ID we’re looking for
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; Search through all of the linked blocks until the last block
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; (marked with a NULL pointer to the next block) has been searched.
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BlockLoop:
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; Point to the first DataElement entry within this block.
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lea di,[si+BLOCK_HEADER_SIZE]
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; Search through all the DataElement entries within this block
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; and accumulate data from all that match the desired ID.
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mov cx,[si+BlockCount] ;Number of elements in this block
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jcxz DoNextBlock ;Skip this block if it’s empty
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mov bp,cx ;***stack frame no longer available***
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add cx,7
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shr cx,1 ;Number of repetitions of the unrolled
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shr cx,1 ; loop = (BlockCount + 7) / 8
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shr cx,1
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and bp,7 ;Generate the entry point for the
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shl bp,1 ; first, possibly partial pass through
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jmp cs:[LoopEntryTable+bp] ; the unrolled loop and
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; vector to that entry point
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align 2
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LoopEntryTable label word
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dw LoopEntry8,LoopEntry1,LoopEntry2,LoopEntry3
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dw LoopEntry4,LoopEntry5,LoopEntry6,LoopEntry7
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M_IBL macro P1
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local NoMatch
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LoopEntry&P1&:
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scasw ;Do we have an ID match?
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jnz NoMatch ;No match
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;We have a match
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inc bx ;IDMatchCount++;
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add dx,[di] ;IDMatchSum += DataPointer->Value;
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NoMatch:
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add di,DATA_ELEMENT_SIZE-2 ;point to the next element
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; (SCASW advanced 2 bytes already)
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endm
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align 2
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IntraBlockLoop:
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M_IBL 8
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M_IBL 7
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M_IBL 6
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M_IBL 5
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M_IBL 4
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M_IBL 3
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M_IBL 2
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M_IBL 1
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loop IntraBlockLoop
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; Point to the next block and continue if that pointer isn’t NULL.
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DoNextBlock:
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mov si,[si+NextBlock] ;Get pointer to the next block
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and si,si ;Is it a NULL pointer?
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jnz BlockLoop ;No, continue
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; Calculate the average of all matches.
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sub ax,ax ;Assume we found no matches
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and bx,bx
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jz Done ;We didn’t find any matches, return 0
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xchg ax,dx ;Prepare for division
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div bx ;Return IDMatchSum / IDMatchCount
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Done: pop si ;Restore C register variables
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pop di
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pop bp ;Restore caller’s stack frame
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ret
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_FindIDAverage ENDP
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end
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</PRE>
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<!-- END CODE //-->
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<P><BR></P>
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<CENTER>
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<TABLE BORDER>
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<TR>
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<TD><A HREF="08-03.html">Previous</A></TD>
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<TD><A HREF="index.html">Table of Contents</A></TD>
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<hr width="90%" size="1" noshade>
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<div align="center">
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<font face="Verdana,sans-serif" size="1">Graphics Programming Black Book © 2001 Michael Abrash</font>
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