On Mon, 18 Feb 2019 16:34:57 +0100, Terje Mathisen   
    wrote:   
   >> ...   
   >> The test was:   
   >>      mov rcx,4294967295   
   >> L0: bt  [rbx],rcx   
   >>      dec rcx   
   >>      jnz L0   
   >>   
   The cycle count for that was:   
   33393694911 / 4294967296 = 7,78 cycles   
      
   >>...   
   >...   
   >Please try two more tests:   
   >   
   >a) Starting at 2 and incrementing until wraparound, i.e. the same JNZ L0   
   >as the last instruction. Count the number of primes found like this:   
   >   
   >      xor eax,eax   
   >      mov ecx,2   
   >L0:  bt  [rbx],rcx   
   >      adc eax,0   
   >      inc rcx   
   >      jnz L0   
   >;; EAX has prime count for verification   
   >   
   I did of course inc ecx, but the above gives me:   
   33699932204 cycles / 4294967296 = 7,84 cycles, and eax = 203280221   
      
   >b) Do the same with the compressed 2:1 (odd) table   
   >   
   >      mov eax,1 ; Counting 2 as a prime   
   >      mov ecx,1 ; 3 SHR 1   
   >L0:  bt  [rbx],rcx   
   >      adc eax,0   
   >      inc rcx   
   >      jns L0	; Stops at 2^31   
   >   
   Here I get:   
   16860994234 cycles / 2147483648 = 7,85 cycles and eax = 105097566   
      
   And I don't quite get what's wrong with 8 cycles compared to how long   
   a single divide takes!   
   --   
   aen   
      
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