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/* Optimal divide-by-contstant code generator
Advanced RISC Machines
*/
#include <stdio.h>
#include <stdlib.h>
#define BANNER "generated by divc 1.02 (Advanced RISC Machines)"
#define DATE "27 Jan 94"
#define DIVIDE_BY_2_MINUS_1 0
#define DIVIDE_BY_2_PLUS_1 1
typedef unsigned int uint;
uint log2( uint n )
{
uint bit, pow, logn;
for( bit = 0, pow = 1; bit < 31; bit++, pow <<= 1 )
{
if( n == pow ) logn = bit;
}
return( logn );
}
int powerof2( int n )
{
return( n == ( n & (-n) ) );
}
void dodiv2m1( uint logd, uint logmsb )
{
/* Output instructions to do division by 2^n - 1 */
printf( "\tMOV a1, a1, lsr #1\n" );
while( logd < 32 )
{
printf( "\tADD a1, a1, a1, lsr #%d\n", logd );
logd <<= 1;
}
printf( "\tMOV a1, a1, lsr #%d\n", logmsb - 1 );
}
void dodiv2p1( uint logd, uint logmsb )
{
/* Output instructions to do division by 2^n + 1 */
printf( "\tSUB a1, a1, a1, lsr #%d\n", logd );
while( logd < 16 )
{
logd <<= 1;
printf( "\tADD a1, a1, a1, lsr #%d\n", logd );
}
printf( "\tMOV a1, a1, lsr #%d\n", logmsb );
}
void loada4( uint type, uint lsb, uint msb )
{
/* Constant is too big to be used as an immediate constant, */
/* so load it into register a4. */
printf( "\tMOV a4, #0x%x\n", msb );
switch( type )
{
case DIVIDE_BY_2_MINUS_1:
printf( "\tSUB a4, a4, #0x%x\n", lsb );
break;
case DIVIDE_BY_2_PLUS_1:
printf( "\tADD a4, a4, #0x%x\n", lsb );
break;
default:
fputs( "Internal error", stderr );
}
}
void divideby2( uint type, uint n, uint lsb, uint msb )
{
uint loglsb;
uint logmsb;
uint usinga4;
loglsb = log2( lsb );
logmsb = log2( msb );
printf( "; %s [%s]\n\n", BANNER, DATE );
printf( "\tAREA |div%d$code|, CODE, READONLY\n\n", n );
printf( "\tEXPORT udiv%d\n\n", n );
printf( "udiv%d\n", n );
printf( "; takes argument in a1\n" );
printf( "; returns quotient in a1, remainder in a2\n" );
printf( "; cycles could be saved if only divide or remainder is required\n" );
usinga4 = ( n >> loglsb ) > 255;
if( usinga4 )
{
loada4( type, lsb, msb );
printf( "\tSUB a2, a1, a4\n" );
}
else
{
printf( "\tSUB a2, a1, #%d\n", n );
}
/* 1/n as a binary number consists of a simple repeating pattern */
/* The multiply by 1/n is expanded as a sequence of ARM instructions */
/* (there is a rounding error which must be corrected later) */
switch( type )
{
case DIVIDE_BY_2_MINUS_1:
dodiv2m1( logmsb - loglsb, logmsb );
/* Now do multiply-by-n */
printf( "\tRSB a3, a1, a1, asl #%d\n", logmsb - loglsb );
break;
case DIVIDE_BY_2_PLUS_1:
dodiv2p1( logmsb - loglsb, logmsb );
/* Now do multiply-by-n */
printf( "\tADD a3, a1, a1, asl #%d\n", logmsb - loglsb );
break;
default:
fputs( "Internal error", stderr );
}
/* Subtract from adjusted original to obtain remainder */
printf( "\tSUBS a2, a2, a3, asl #%d\n", loglsb );
/* Apply corrections */
printf( "\tADDPL a1, a1, #1\n" );
if( usinga4 )
{
printf( "\tADDMI a2, a2, a4\n" );
}
else
{
printf( "\tADDMI a2, a2, #%d\n", n );
}
/* Additional test required for divide-by-3, as result could be */
/* off by 2 lsb due to accumulated rounding errors. */
if( n == 3 )
{
printf( "\tCMP a2, #3\n" );
printf( "\tADDGE a1, a1, #1\n" );
printf( "\tSUBGE a2, a2, #3\n" );
}
printf( "\tMOV pc, lr\n\n" );
printf( "\tEND\n" );
}
int main( int argc, char *argv[] )
{
if( argc != 2 )
{
printf( "Usage: divc <n>\n" );
printf( "Generates optimal ARM code for divide-by-constant\n" );
printf( "where <n> is one of (2^n-2^m) or (2^n+2^m) eg. 10\n" );
printf( "Advanced RISC Machines [%s]\n", DATE );
}
else
{
int num;
num = atoi( argv[ 1 ] );
if( num <= 1 )
{
fprintf( stderr, "%d is not sensible\n", num );
}
else
{
uint lsb = 1;
/* find least-significant bit */
while( ( num & lsb ) == 0 )
{
lsb <<= 1;
}
if( powerof2( num ) )
{
fprintf( stderr, "%d is an easy case\n", num );
}
else if( powerof2( num + lsb ) )
{
divideby2( DIVIDE_BY_2_MINUS_1, num, lsb, num + lsb );
}
else if( powerof2( num - lsb ) )
{
divideby2( DIVIDE_BY_2_PLUS_1, num, lsb, num - lsb );
}
else
{
fprintf( stderr, "%d is not one of (2^n-2^m) or (2^n+2^m)\n", num );
}
}
}
return( 0 );
}
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