코드 최적화 팁

Before: if( Condition ) { Case A; } else { Case B; }

After: Case B; if( Condition ) { Undo Case B; Case A; }

Before: for(i=0;i<10;i++) { printf("%d\n",i*10); }

After: for(i=0;i<100;i+=10) { printf("%d\n",i); }

Before: char playfield[80][25];

After: char playfield[80][32];

Before: for(i=0;i<100;i++) { map[i].visited = 0; }

After: i=99; do { map[i].visited = 0; i--; } while(i>=0);

Before: char x; int y; y = x;

After: int x, y; y = x;

Before: void swap(int *x, int *y) { int t; t = *y; *y = *x; *x = t; }

After: static void swap(int *x, int *y) { int t; t = *y; *y = *x; *x = t; }

Before: float f[100],sum; ... /* Back to back dependent adds force the thoughput to be equal to the total FADD latency. */ sum = 0; for(i=0;i<100;i++) { sum += f[i]; }

After: float f[100],sum0,sum1; float sum2,sum3,sum; ... /* Optimized for a 4-cycle latency fully pipelined FADD unit. The throughput is one add per clock. */ sum0 = sum1 = sum2 = sum3 = 0; for(i=0;i<100;i+=4) { sum0 += f[i]; sum1 += f[i+1]; sum2 += f[i+2]; sum3 += f[i+3]; } sum = (sum0+sum1) + (sum2+sum3);

Before:     x = y % 32;     x = y * 8;     x = y / w + z / w;     if( a==b &&c==d &&e==f ) {...}     if( (x &1) || (x &4) ) {...}     if( x>=0 &&x<8 &&         y>=0 &&y<8 ) {...}     if( (x==1) || (x==2) ||         (x==4) || (x==8) || ... )     if( (x==2) || (x==3) || (x==5) ||         (x==7) || (x==11) || (x==13) ||         (x==17) || (x==19) ) {...}     #define abs(x) (((x)>0)?(x):-(x))     int a[3][3][3];     int b[3][3][3];     ...     for(i=0;i<3;i++)       for(j=0;j<3;j++)          for(k=0;k<3;k++)             b[i][j][k] = a[i][j][k];     for(i=0;i<3;i++)       for(j=0;j<3;j++)          for(k=0;k<3;k++)             a[i][j][k] = 0;     for(x=0;x<100;x++) {       printf("%d\n",(int)(sqrt(x)));     }             unsigned int x, y, a;     ...     a = (x + y) >>1;     /* overflow fixup */     if( a <x &&a <y ) a += 0x8000000;     c:\>tc myprog.c     user% cc myprog.c     Use the quicksort algorithm.     Use Bresenham's line algorithm.     Look through school notes for ideas.     Ignore suggestions from others.     Code, code, code, code ...

After:     x = y &31;     x = y <<3;     x = (y + z) / w;     if( ((a-b)|(c-d)|(e-f))==0 ) {...}     if( x & 5 ) {...}     if( ((unsigned)(x|y))<8 ) {...}     if( x&(x-1)==0 &&x!=0 )     if( (1<<x) & ((1<<2)|(1<<3)|(1<<5)|(1<<7) \         |(1<<11)|(1<<13)|(1<<17)|(1<<19)) ) {...}     static long abs(long x) {     long y;         y = x>>31;    /* Not portable */         return (x^y)-y;     }     typedef struct {             int element[3][3][3];     } Three3DType;     Three3DType a,b;     ...     b = a;     memset(a,0,sizeof(a));     for(tx=1,sx=0,x=0;x<100;x++) {       if( tx<=x ) {         tx+=2*sx+3;         sx++;       }       printf("%d\n",sx);     }     unsigned int x, y, a;     ...     a = (x & y) + ((x ^ y)>>1);     c:\>wpp386 /6r/otexan myprog.c     user% gcc -O3 myprog.c     Use radix, intro or heap(!) sort.     Use fixed point DDA line algorithm.     Get examples from USENET/WEB/FTP     Get suggestions but be skeptical.     Think, code, think, code ...

http://www.azillionmonkeys.com/qed/optimize.html





http://itguru.tistory.com/129