Re: Optimizing for speed

Eric Young (eay@cryptsoft.com)
Thu, 19 Feb 1998 00:39:59 +1000 (EST)

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On Tue, 17 Feb 1998, Chuck McManis wrote:
> So I've got some algorithms that I'm trying to squeeze more speed
> out of, and while I'm not quite as fanatical as the DES search
> team I do find myself at a loss for tools. My environment is
> VC++5 on a Win95 platform and what I'd like is something like :
>
> long long t1, t2;
> t1 = snap_time(); /* time in nSecs */
> ... do something ...
> t2 = snap_time();
> printf("This run took %ld nS\n", (long)(t2 - t1));
>
> I've yet to find something in my search through various
> manuals...

Timing of this type of thing can be done at a cycle level via the following
function

//
// gettsc.inl
//
// gives access to the Pentium's (secret) cycle counter
//
// This software was written by Leonard Janke (janke@unixg.ubc.ca)
// in 1996-7 and is entered, by him, into the public domain.

#if defined(__WATCOMC__)
void GetTSC(unsigned long&);
#pragma aux GetTSC = 0x0f 0x31 "mov [edi], eax" parm [edi] modify [edx eax];
#elif defined(__GNUC__)
inline
void GetTSC(unsigned long& tsc)
{
  asm volatile(".byte 15, 49\n\t"
               : "=eax" (tsc)
               :
               : "%edx", "%eax");
}
#elif defined(_MSC_VER)
inline
void GetTSC(unsigned long& tsc)
{
  unsigned long a;
  __asm _emit 0fh
  __asm _emit 31h
  __asm mov a, eax;
  tsc=a;
}
#endif

Now while this works for x86 c++ compilers (and gcc), it is not what you
really want. You probably want to run you code for a few seconds and then
do the divide to work out the speed. For Win32 (96/NT), look at
GetThreadTimes().

eric

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