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  1. #include <stdint.h>
  2. #include <stdio.h>
  3.  
  4. uint8_t my_log2 (uint32_t v)
  5. {
  6.     uint8_t *bytes = (void *)&v;
  7.  
  8.     uint8_t i = 24;
  9.     while (1) {
  10.         uint8_t byte = bytes[i / 8];
  11.         if (byte) {
  12.             do {
  13.                 i++;
  14.                 byte >>= 1;
  15.             } while (byte);
  16.             return i;
  17.         }
  18.         if (i == 0) {
  19.             return 0;
  20.         }
  21.         i -= 8;
  22.     }
  23. }
  24.  
  25. int main ()
  26. {
  27.     uint32_t x = UINT32_C(0x1FFF0FFF);
  28.     printf("%d\n", my_log2(x));
  29. }
  30.  
  31. #if 0
  32. my_log2:
  33.         push r28
  34.         push r29
  35.         rcall .
  36.         rcall .
  37.         in r28,__SP_L__
  38.         in r29,__SP_H__
  39. /* prologue: function */
  40. /* frame size = 4 */
  41. /* stack size = 6 */
  42. .L__stack_usage = 6
  43.         std Y+1,r22
  44.         std Y+2,r23
  45.         std Y+3,r24
  46.         std Y+4,r25
  47.         ldd r25,Y+4
  48.         cpse r25,__zero_reg__
  49.         rjmp .L6
  50.         ldd r25,Y+3
  51.         cpse r25,__zero_reg__
  52.         rjmp .L7
  53.         ldd r25,Y+2
  54.         cpse r25,__zero_reg__
  55.         rjmp .L8
  56.         ldd r25,Y+1
  57.         ldi r24,0
  58.         cpse r25,__zero_reg__
  59.         rjmp .L5
  60. .L4:
  61. /* epilogue start */
  62.         pop __tmp_reg__
  63.         pop __tmp_reg__
  64.         pop __tmp_reg__
  65.         pop __tmp_reg__
  66.         pop r29
  67.         pop r28
  68.         ret
  69. .L8:
  70.         ldi r24,lo8(8)
  71. .L5:
  72.         subi r24,lo8(-(1))
  73.         lsr r25
  74.         breq .L4
  75.         subi r24,lo8(-(1))
  76.         lsr r25
  77.         brne .L5
  78.         rjmp .L4
  79. .L6:
  80.         ldi r24,lo8(24)
  81.         rjmp .L5
  82. .L7:
  83.         ldi r24,lo8(16)
  84.         rjmp .L5
  85.         .size   my_log2, .-my_log2
  86.         .section        .rodata.str1.1,"aMS",@progbits,1
  87. #endif
Success #stdin #stdout 0s 1832KB
comments (?)
stdin 
Standard input is empty
stdout 
29
https://ideone.com/QkQssj
language:
C99 (gcc 8.3)
created:
10 years ago
visibility:
public

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