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  1. #include <stdint.h>
  2. #include <stdio.h>
  3.  
  4. #if 0
  5. uint8_t my_log2 (uint32_t v)
  6. {
  7.     uint8_t *bytes = (void *)&v;
  8.  
  9.     uint8_t i = 24;
  10.     while (1) {
  11.         uint8_t byte = bytes[i / 8];
  12.         if (byte) {
  13.             do {
  14.                 i++;
  15.                 byte >>= 1;
  16.             } while (byte);
  17.             return i;
  18.         }
  19.         if (i == 0) {
  20.             return 0;
  21.         }
  22.         i -= 8;
  23.     }
  24. }
  25. #endif
  26.  
  27. uint8_t my_log2 (uint32_t op)
  28. {
  29.     uint8_t res;
  30.     asm(
  31.         "    clr r16\n"
  32.         "    cpse %D[op],__zero_reg__\n"
  33.         "    rjmp foundD%=\n"
  34.         "    cpse %C[op],__zero_reg__\n"
  35.         "    rjmp foundC%=\n"
  36.         "    cpse %B[op],__zero_reg__\n"
  37.         "    rjmp foundB%=\n"
  38.         "    cpse %A[op],__zero_reg__\n"
  39.         "    rjmp foundA%=\n"
  40.         "    rjmp end%=\n"
  41.         "foundD%=:\n"
  42.         "    ori r16,$18\n"
  43.         "    mov r17,%D[op]\n"
  44.         "    rjmp examine%=\n"
  45.         "foundC%=:\n"
  46.         "    ori r16,$10\n"
  47.         "    mov r17,%C[op]\n"
  48.         "    rjmp examine%=\n"
  49.         "foundB%=:\n"
  50.         "    ori r16,$08\n"
  51.         "    mov r17,%B[op]\n"
  52.         "    rjmp examine%=\n"
  53.         "foundA%=:\n"
  54.         "    mov r17,%A[op]\n"
  55.         "examine%=:\n"
  56.         "    cpse r17,__zero_reg__\n"
  57.         "    inc r16\n"
  58.         "    lsr r17\n"
  59.         "    cpse r17,__zero_reg__\n"
  60.         "    inc r16\n"
  61.         "    lsr r17\n"
  62.         "    cpse r17,__zero_reg__\n"
  63.         "    inc r16\n"
  64.         "    lsr r17\n"
  65.         "    cpse r17,__zero_reg__\n"
  66.         "    inc r16\n"
  67.         "    lsr r17\n"
  68.         "    cpse r17,__zero_reg__\n"
  69.         "    inc r16\n"
  70.         "    lsr r17\n"
  71.         "    cpse r17,__zero_reg__\n"
  72.         "    inc r16\n"
  73.         "    lsr r17\n"
  74.         "    cpse r17,__zero_reg__\n"
  75.         "    inc r16\n"
  76.         "    lsr r17\n"
  77.         "    cpse r17,__zero_reg__\n"
  78.         "    inc r16\n"
  79.         "end%=:\n"
  80.         "    mov %[res],r16\n"
  81.         : [res] "=r" (res)
  82.         : [op] "r" (op)
  83.         : "r16", "r17"
  84.     );
  85.     return res;
  86. }
  87.  
  88. int main ()
  89. {
  90.     uint32_t x = UINT32_C(0x1FFF0FFF);
  91.     printf("%d\n", my_log2(x));
  92. }
  93.  
  94.  
  95. /tmp/ccSWpg3O.s: Assembler messages:
  96. /tmp/ccSWpg3O.s:30: Error: junk at end of line, first unrecognized character is `1'
  97. /tmp/ccSWpg3O.s:34: Error: junk at end of line, first unrecognized character is `1'
  98. /tmp/ccSWpg3O.s:38: Error: junk at end of line, first unrecognized character is `0
Not running #stdin #stdout 0s 0KB
comments (?)
stdin 
Standard input is empty
stdout 
Standard output is empty
https://ideone.com/5xjpH2
language:
C++ (gcc 8.3)
created:
10 years ago
visibility:
public

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