Thanks Robert,

I've seen the disassembled output and thought (in my case) 
it could be tuned to be faster but I just hadn't found a way.  
My hashing algorithm was too slow.

I think your method will be faster and hopefully the empirical 
data will match my belief.


W.

-----Original Message-----
From: Robert Purves [mailto:robert.purves@...]
Sent: Wednesday, April 24, 2002 1:12 AM
To: futurebasic@...
Subject: Re: [FB] Fast lookups and jumps


>I have the need for speed.  Does anyone have a
>better idea for a jump table to replace a SELECT/CASE
>structure?
>
>What I would like to do is to jump to a certain section
>based on the contents of a string.  The string will be one
>to four characters in length, all capitals.
>
>  Ultimately, I want to see if I can gain
>enough speed to warrant some ugly inflexible code.
>
>SELECT myString$
>CASE "BD"
>// do this ...
>CASE "LI"
>// do this ...
>CASE "SIMM"
< snip >
>END SELECT


Case with a string is slow because there are two calls to runtime routines
(GETSTRCONST and FBCompareStrings), as shown by this disassembly:

CASE "BD"
'     +00000014          li          r3,0x0B12
'     +00000018          bl          GETSTRCONST+0x0004
'     +0000001C          li          r31,0x0021
'     +00000020          mr          r3,r31
'     +00000024          bl          FBCompareStrings+0x0004
'     +00000028          mr.         r3,r3
'     +0000002C          bne         _FCOD_1001+0x0048
'     +00000030          b           _FCOD_1001+0x004C


Convert your string's 1-4 characters to a long, and use underscore
constants in the case statements:

dim as ptr   myStringPtr
dim as long  myLong, stringLen
myStringPtr = @myString$
myLong = 0
stringLen = myStringPtr.nil``
while stringLen
stringLen--
myStringPtr++
myLong = (myLong << 8) + myStringPtr.nil``
wend
select myLong
case _"BD"
case _"LI"
case _"SIMM"
end select


Case is now completely inline, and much faster:
case _"BD"
'     +000000F4          li          r3,0x4244
'     +000000F8          lwz         r4,0x0020(SP)
'     +000000FC          cmpw        r3,r4
'     +00000100          beq         _FCOD_1001+0x011C
'     +00000104          b           _FCOD_1001+0x0120


Robert P.



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