arm program to generate optimal devision code for constants
This commit is contained in:
Torsten Ruger
parent
054f94d68b
commit
918ede1c02
213
test/bench/divc.c
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213
test/bench/divc.c
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/* Optimal divide-by-contstant code generator
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* Advanced RISC Machines
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* */
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#include <stdio.h>
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#include <stdlib.h>
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#define BANNER "generated by divc 1.02 (Advanced RISC Machines)"
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#define DATE "27 Jan 94"
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#define DIVIDE_BY_2_MINUS_1 0
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#define DIVIDE_BY_2_PLUS_1 1
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typedef unsigned int uint;
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uint log2( uint n )
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{
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uint bit, pow, logn;
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for( bit = 0, pow = 1; bit < 31; bit++, pow <<= 1 )
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{
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if( n == pow ) logn = bit;
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}
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return( logn );
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}
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int powerof2( int n )
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{
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return( n == ( n & (-n) ) );
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}
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void dodiv2m1( uint logd, uint logmsb )
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{
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/* Output instructions to do division by 2^n - 1 */
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printf( "\tMOV a1, a1, lsr #1\n" );
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while( logd < 32 )
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{
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printf( "\tADD a1, a1, a1, lsr #%d\n", logd );
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logd <<= 1;
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}
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printf( "\tMOV a1, a1, lsr #%d\n", logmsb - 1 );
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}
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void dodiv2p1( uint logd, uint logmsb )
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{
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/* Output instructions to do division by 2^n + 1 */
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printf( "\tSUB a1, a1, a1, lsr #%d\n", logd );
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while( logd < 16 )
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{
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logd <<= 1;
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printf( "\tADD a1, a1, a1, lsr #%d\n", logd );
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}
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printf( "\tMOV a1, a1, lsr #%d\n", logmsb );
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}
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void loada4( uint type, uint lsb, uint msb )
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{
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/* Constant is too big to be used as an immediate constant, */
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/* so load it into register a4. */
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printf( "\tMOV a4, #0x%x\n", msb );
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switch( type )
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{
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case DIVIDE_BY_2_MINUS_1:
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printf( "\tSUB a4, a4, #0x%x\n", lsb );
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break;
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case DIVIDE_BY_2_PLUS_1:
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printf( "\tADD a4, a4, #0x%x\n", lsb );
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break;
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default:
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fputs( "Internal error", stderr );
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}
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}
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void divideby2( uint type, uint n, uint lsb, uint msb )
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{
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uint loglsb;
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uint logmsb;
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uint usinga4;
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loglsb = log2( lsb );
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logmsb = log2( msb );
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printf( "; %s [%s]\n\n", BANNER, DATE );
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printf( "\tAREA |div%d$code|, CODE, READONLY\n\n", n );
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printf( "\tEXPORT udiv%d\n\n", n );
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printf( "udiv%d\n", n );
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printf( "; takes argument in a1\n" );
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printf( "; returns quotient in a1, remainder in a2\n" );
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printf( "; cycles could be saved if only divide or remainder is required\n" );
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usinga4 = ( n >> loglsb ) > 255;
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if( usinga4 )
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{
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loada4( type, lsb, msb );
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printf( "\tSUB a2, a1, a4\n" );
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}
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else
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{
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printf( "\tSUB a2, a1, #%d\n", n );
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}
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/* 1/n as a binary number consists of a simple repeating pattern */
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/* The multiply by 1/n is expanded as a sequence of ARM instructions */
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/* (there is a rounding error which must be corrected later) */
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switch( type )
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{
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case DIVIDE_BY_2_MINUS_1:
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dodiv2m1( logmsb - loglsb, logmsb );
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/* Now do multiply-by-n */
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printf( "\tRSB a3, a1, a1, asl #%d\n", logmsb - loglsb );
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break;
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case DIVIDE_BY_2_PLUS_1:
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dodiv2p1( logmsb - loglsb, logmsb );
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/* Now do multiply-by-n */
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printf( "\tADD a3, a1, a1, asl #%d\n", logmsb - loglsb );
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break;
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default:
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fputs( "Internal error", stderr );
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}
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/* Subtract from adjusted original to obtain remainder */
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printf( "\tSUBS a2, a2, a3, asl #%d\n", loglsb );
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/* Apply corrections */
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printf( "\tADDPL a1, a1, #1\n" );
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if( usinga4 )
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{
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printf( "\tADDMI a2, a2, a4\n" );
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}
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else
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{
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printf( "\tADDMI a2, a2, #%d\n", n );
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}
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/* Additional test required for divide-by-3, as result could be */
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/* off by 2 lsb due to accumulated rounding errors. */
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if( n == 3 )
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{
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printf( "\tCMP a2, #3\n" );
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printf( "\tADDGE a1, a1, #1\n" );
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printf( "\tSUBGE a2, a2, #3\n" );
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}
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printf( "\tMOV pc, lr\n\n" );
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printf( "\tEND\n" );
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}
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int main( int argc, char *argv[] )
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{
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if( argc != 2 )
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{
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printf( "Usage: divc <n>\n" );
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printf( "Generates optimal ARM code for divide-by-constant\n" );
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printf( "where <n> is one of (2^n-2^m) or (2^n+2^m) eg. 10\n" );
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printf( "Advanced RISC Machines [%s]\n", DATE );
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}
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else
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{
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int num;
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num = atoi( argv[ 1 ] );
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if( num <= 1 )
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{
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fprintf( stderr, "%d is not sensible\n", num );
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}
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else
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{
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uint lsb = 1;
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/* find least-significant bit */
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while( ( num & lsb ) == 0 )
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{
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lsb <<= 1;
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}
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if( powerof2( num ) )
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{
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fprintf( stderr, "%d is an easy case\n", num );
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}
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else if( powerof2( num + lsb ) )
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{
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divideby2( DIVIDE_BY_2_MINUS_1, num, lsb, num + lsb );
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}
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else if( powerof2( num - lsb ) )
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{
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divideby2( DIVIDE_BY_2_PLUS_1, num, lsb, num - lsb );
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}
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else
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{
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fprintf( stderr, "%d is not one of (2^n-2^m) or (2^n+2^m)\n", num );
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}
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}
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}
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return( 0 );
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}
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