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Torello
Joined: 29 Sep 2006
Posts: 118
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| Int32 to string conversion; fastest itoa |
 Posted: Fri Jan 02, 2009 2:57 am |
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Hi,
First, best wishes to everybody on this forum. A C-full 2009!
Tweaking my application to the minimum energy consumption (thus also execution speed of a task), I found out that sprintf 4 int32 values take up about 92 ms (18LF4620 @4Mhz)! Quite shocked!
Replaced the sprintf with the use of ccs-itoa, but that did not make any difference; probably sprintf uses ccs-itoa. Then took a look at the standard-c itoa. It is using the % (mod) operator heavily. The code looks very resources efficient. I've come up with an replacement below. Uses probably more recourses, but It does the same 4 int32 in 10ms!. So nine times faster. Or 90% discount on energy consumption. Heh, thats worth while spending on a little more bytes.
Can we make this function still faster? Or has somebody an other, faster routine to share?
Regards,
Edwin
| Code: | //==============================================================================
// Function converts int32 to signed decimal string
// returns pointer to the '/0' ended string. (handy for formatting more values)
char *i32toa(int32 n, char *s) {
int8 i,j; //decade counter
int8 idx=0; //string index
int32 const Div[10] = { //decades table
1000000000L,100000000L,10000000L,1000000L
100000L,10000L,1000L,100L,10L,1};
int32 b; //i32 to hold table read
int fdd; //first digit detect (suppress leading zero function)
fdd=0; //clear detection
if (bit_test(n,31)) { //T: n=negative
n=(~n)+1; //convert to positive
s[0]='-'; //mark the negative number
idx++;
}
for (i=0; i<10;i++) { //do all the decades, start with biggest
j=0; //clear the decimal digit counter
b=Div[i]; //read the table once;
while (n>=b) { //T: "left-over" still bigger then decade; substr. and count
j++;
n-=b;
}
if ((j)||(fdd)) { //T: decade count!=0 or first digit has been detected
fdd=1;
s[idx++]='0'+j; //..then add the decade count
}
}
if (!fdd) s[idx++]='0'; //dont suppress zero when n==0!
s[idx]=0; //end the string
return(s+idx); //return last written pointer
}
//=== code snippet to format 4 int32 into string with comma as separator
i320=-2139999999L;
i321=-2139999999L;
i322=-2139999999L;
i323=-2139999999L;
#ifdef _DM output_high(pin_b3); #endif
ptS = i32toa(i320,tS);
*(ptS++)=',';
ptS = i32toa(i321,ptS);
*(ptS++)=',';
ptS = i32toa(i322,ptS);
*(ptS++)=',';
ptS = i32toa(i323,ptS); //total 10ms
#ifdef _DM output_low(pin_b3); #endif
fprintf(debug,"%s\n",tS);
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Ttelmah
Guest
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| Posted: Fri Jan 02, 2009 3:14 am |
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You might find a more efficient '%', with a search here. Though things may have changed.
CCS, 'waste' a lot of time in several places using %. Their implementation, performs a division, then takes the integer part of the result, and multiplies this, subtracting this result from the original to get the %. In many places, they perform this, and a division again, as well, to get the part needed for the next operation!.....
A long time ago, I found that in fact their division code, had the remainder already present in a temporary store, and the two divisions, and one multiplication used to get the values needed, could be replaced with just one division, if you accessed this. I posted a routine to do this. It was though a long time ago (probably on the old board), and I have no idea what the thread was called, but a search might find it. Made a massive difference to the performance...
Best Wishes |
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