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November 03, 2011
Double implicitly converted to real
Hi. I noticed that one of the guarantees in TDPL is that any code that is valid in both C
and D should compile with the same result. But I'm seeing a different behavior here.
I'm trying to find the smallest double for which the comparison x+1/x = x is true.
I take a number way too small, and a number way too large, and then binary search (for 100
iterations) to get the desired number:

//add appropriate import std.stdio or #include <stdio.h>
int main(){
   double min = 1;
   double max = 10000000000;
   int iters = 0;
   double average;
   for(;iters<100; iters++){
       average = (min+max)/2;
       if( average + 1/average == average)
           max = average;
       else
           min = average;
   }
   printf("%f",average);
   return 0;
}

Here's the problem:
D (under DMD v2.051) gives this answer: 4294967296.000000
C (gcc version 3.4.6 20060404): 134217728.000000

It seems D is implicitly converting double to real. Is this the usual behavior?

Cheers,
Charles.
November 03, 2011
Re: Double implicitly converted to real
Le 03/11/2011 15:39, Charles McAnany a écrit :
> Hi. I noticed that one of the guarantees in TDPL is that any code that is valid in both C
> and D should compile with the same result. But I'm seeing a different behavior here.
> I'm trying to find the smallest double for which the comparison x+1/x = x is true.
> I take a number way too small, and a number way too large, and then binary search (for 100
> iterations) to get the desired number:
>
> //add appropriate import std.stdio or #include<stdio.h>
> int main(){
>      double min = 1;
>      double max = 10000000000;
>      int iters = 0;
>      double average;
>      for(;iters<100; iters++){
>          average = (min+max)/2;
>          if( average + 1/average == average)
>              max = average;
>          else
>              min = average;
>      }
>      printf("%f",average);
>      return 0;
> }
>
> Here's the problem:
> D (under DMD v2.051) gives this answer: 4294967296.000000
> C (gcc version 3.4.6 20060404): 134217728.000000
>
> It seems D is implicitly converting double to real. Is this the usual behavior?
>
> Cheers,
> Charles.

As long as you don't loose information, you can cast implicitely in D. 
If you loose information (or the compiler cannot prove that your are not 
loosing information) then an explicit cast is required.

So this implicit cast is expected.

Now, you are not using real in your code, so you shouldn't use real 
anywhere. Are you sure this is the actual issue ?

Finally, both compiler you are using are rather old ones. dmd is in 
version 2.056 now and gdc has 4.6.2 version (and using 2.055 frontend).
November 03, 2011
Re: Double implicitly converted to real
Charles McAnany Wrote:

> I noticed that one of the guarantees in TDPL is that any code that is valid in both C
> and D should compile with the same result.

This is almost true (there are few differences, in D fixed-size arrays are managed by value instead of by pointer, and global floating point variables are initialized to NaN instead of 0), but only where all C compilers themselves give the same results.

Floating point operations are based on a standard implementation, but I think in practice different optimizations cause different C compilers to not give exactly the same result when floating point values are used (even the same CPU gives different results if you use the FP stack with 80 bit reals or if you use 64 bit doubles in SSE registers). So small differences are expected. Your program is designed to blow up those small differences.


> It seems D is implicitly converting double to real. Is this the usual behavior?

DMD compiler sometimes uses real values for intermediate values, so such differences are not so unexpected.

Bye,
bearophile
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