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From: "(see below)" <yaldnif.w@blueyonder.co.uk>
Newsgroups: comp.lang.ada
Subject: Re: Is there an Ada compiler whose
 Ada.Numerics.Generic_Elementary_Functions.Log(Base=>10, X=>variable) is
 efficient?
Date: Mon, 15 Feb 2010 18:26:31 +0000
Message-ID: <C79F4257.1365BE%yaldnif.w@blueyonder.co.uk>
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Thread-Topic: Is there an Ada compiler whose
 Ada.Numerics.Generic_Elementary_Functions.Log(Base=>10, X=>variable) is
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Date: 2010-02-15T18:26:31+00:00
List-Id: <comp.lang.ada>

On 15/02/2010 10:58, in article
alpine.LNX.2.00.1002151055530.17315@Bluewhite64.example.net, "Colin Paul
Gloster" <Colin_Paul_Gloster@ACM.org> wrote:

> Of the two programs shown, the fastest C++ implementation on one test
> platform took less than one millisecond and the fastest Ada
> implementation took one minute and 31 seconds and 874 milliseconds on
> the same platform. Both g++ and gnatmake were from the same
> installation of GCC 4.1.2 20080704 (Red Hat 4.1.2-44).

Is that 1 millisecond for 1e6 calls? This implies 1ns per call in C++.
I find it incredible that a log function could be so fast.
I think the loop body must be evaluated at compile-time in C++.

On my system your Ada code gives:

6.34086408536266E+08

real    0m33.918s
user    0m33.864s
sys     0m0.025s

And your original C++ code gives:

6.34086e+08
real    0m0.110s
user    0m0.003s
sys     0m0.003s

But if I replace the C++ loop body by:

      for(int j=1; j<=500; ++j)
         answer += std::log10(j*0.100000000000000000000);
It now gives:

6.34086e+08
real    0m18.112s
user    0m18.082s
sys    0m0.015s

This less than twice as fast as the more generalized Ada code.

The simpler inner loop:
      for(int j=1; j<=500; ++j)
         answer += j;
gives:

1.2525e+11
real    0m0.677s
user    0m0.614s
sys    0m0.003s

So the difference cannot be due to loop overhead.
-- 
Bill Findlay
<surname><forename> chez blueyonder.co.uk