Re: Why C is much slower than Fortran

George Neuner <gneuner@dyn.com> wrote:
> The way to get the most out of your compiler is to write simple, clear
> code and not use clever tricks.


Can you say ``five times slower''?


That's gcc's Pentium performance on a simple, clear, straightforward
inner loop---


      long double t0,t1,t2,t3,t4,t7;
      int *buf;
      ...
      for (i = -96;i < 0;++i) {
          t7 = buf[i];


          t0 += doublearray[0][i + 96] * t7;
          t1 += doublearray[1][i + 96] * t7;
          t2 += doublearray[2][i + 96] * t7;
          t3 += doublearray[3][i + 96] * t7;
          t4 += doublearray[4][i + 96] * t7;
      }


---compared to what I get _for exactly the same computation_ by writing
more complicated code.


See http://pobox.com/~djb/hash127/install.html for the complete program.
Try compiling and running ./speed; then try again with x86-idea in
conf-opt and with your favorite Pentium compiler in conf-cc.


> As John said, the language specification is a contract.


Well, yes, we all expect the compiler to behave as documented. But the
documented behavior is inadequate. Saying ``yes, but it's documented''
is missing the point.


---Dan

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