From: amc96@nospicedham.cam.ac.uk   
      
   On 23/11/2017 00:56, aen@nospicedham.spamtrap.com wrote:   
   > Hi!   
   >   
   > Here is another timing exercise: This one tells me that to create one   
   > permutation from 13! takes an average of 7 cycles.  The algorithm is   
   > again from Don Knuth's TAOCP Vol. 2.   
   >   
   > Any disagreements?   
   >   
   > .intel_syntax noprefix   
   > # as -o posting.o posting.asm   
   > # gcc -static -o posting posting.o   
   > # ./posting gives the output: 7.070950 44030949650 6227020800   
   > # time ./posting gives: real 0m12,992s user 0m12,956s sys 0m,000s   
   > # bc -l   
   > # 12.956*3400000000 gives: 44050400000   
   > .macro TSCStart   
   >   rdtsc   
   >   shl     rdx,32   
   >   or      rax,rdx   
   >   push    rax   
   > .endm # TSCStart   
   >   
   > .macro TSCEnd   
   >   rdtsc   
   >   shl     rdx,32   
   >   or      rax,rdx   
   >   sub     rax,[rsp]   
   >   add     rsp,8   
   > .endm # TSCEnd   
      
   This use of rdtsc isn't accurate.  It will be reordered in the pipeline   
   with the content you are trying to time.   
      
   If you have rdtscp available, use that.  It specifically has fencing   
   properties to prevent reordering.   
      
   If not, use lfence;rdtsc for Intel and mfence;rdtsc for AMD hardware to   
   explicitly serialise the instruction stream before reading the TSC.   
      
   ~Andrew   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)