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From: jonathan <johnscpg@googlemail.com>
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 07:04:25 -0800 (PST)
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On Feb 15, 2:54=A0pm, jonathan <johns...@googlemail.com> wrote:
> On Feb 15, 10:58=A0am, Colin Paul Gloster <Colin_Paul_Glos...@ACM.org>
> wrote:
>
> > Hello,
>
> > I have been improving a program by suppressing C++ in it. After
> > speeding it up a lot by making changes, I have found one considerable
> > part which calls a library routine, which is unfortunately very slow
> > as provided as standard with a number of Ada compilers but which is
> > very fast with implementations of other languages....
>
> Here is my own bench ... easier to play with:
>
> with ada.numerics.generic_elementary_functions;
> with text_io; use text_io;
>
> procedure log_bench is
>
> =A0 type Real is digits 15;
> =A0 package Math is new ada.numerics.generic_elementary_functions
> (Real);
> =A0 use Math;
> =A0 x, y : Real :=3D 0.1;
>
> =A0 -- might (or might not!) be faster to use:
> =A0 -- =A0 =A0log_base_10 (x) =3D log_base_10_of_e * log_base_e (x)
>
> =A0 Log_base_10_of_e : constant :=3D 0.434_294_481_903_251_827_651_129;
>
> begin
>
> =A0 for i in 1 .. 1_000_000 loop
> =A0 =A0 x :=3D x + Log_base_10_of_e * Log (y);
> =A0 =A0 y :=3D y + 0.0000000000001;
> =A0 end loop;
> =A0 put (Real'Image(x));
>
>
>
> gnatmake =A0gnatnp -O2 -march=3Dnative log_bench.adb

Opps, accidently hit the send button!  Continuing discussion:

gnatmake  gnatnp -O2 -march=3Dnative log_bench.adb

-9.99999682845800E+05
real    0m0.024s
user    0m0.024s
sys     0m0.000s

NOW, comment out the y :=3D y + ... so that the inner loop
is constant, and use the same compilation switches:

gnatmake  gnatnp -O2 -march=3Dnative log_bench.adb

-9.99999900000000E+05
real    0m0.002s
user    0m0.000s
sys     0m0.000s

You see now what's happening.  With the gnatn switch the
compiler is smart enough to call the Log just once, rather
than 10**6 times.

If you remove the -gnatn or -gnatN switches, then it runs in
 0m0.024s again.

Jonathan