[Issue 8476] New: float comparison operand not truncated from real

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=8476

           Summary: float comparison operand not truncated from real
           Product: D
           Version: D2
          Platform: x86
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: ellery-newcomer at utulsa.edu


--- Comment #0 from Ellery Newcomer <ellery-newcomer at utulsa.edu> 2012-07-30 14:28:26 PDT ---
The following code produces different results on a comparison.
The problem is in fitnessCompare; it looks like one of the two operands has
float precision, while the other has real precision.

assembly dump:

08065710
<_D19travelling_salesman4mainFZv14fitnessCompareMFS19travelling_salesman10chromosomeS19travelling_salesman10chromosomeZb>:
 8065710:       55                      push   ebp
 8065711:       8b ec                   mov    ebp,esp
 8065713:       83 ec 10                sub    esp,0x10
 8065716:       ff 75 14                push   DWORD PTR [ebp+0x14]
 8065719:       ff 75 10                push   DWORD PTR [ebp+0x10]
 806571c:       e8 27 00 00 00          call   8065748
<_D19travelling_salesman7fitnessFNaxS19travelling_salesman10chromosomeZf>
 8065721:       ff 75 0c                push   DWORD PTR [ebp+0xc]
 8065724:       ff 75 08                push   DWORD PTR [ebp+0x8]
 8065727:       d9 5d f0                fstp   DWORD PTR [ebp-0x10]
 806572a:       e8 19 00 00 00          call   8065748
<_D19travelling_salesman7fitnessFNaxS19travelling_salesman10chromosomeZf>
 806572f:       d9 45 f0                fld    DWORD PTR [ebp-0x10]
 8065732:       d9 c9                   fxch   st(1)
 8065734:       de d9                   fcompp
 8065736:       df e0                   fnstsw ax
 8065738:       9e                      sahf
 8065739:       b8 01 00 00 00          mov    eax,0x1
 806573e:       7a 02                   jp     8065742
<_D19travelling_salesman4mainFZv14fitnessCompareMFS19travelling_salesman10chromosomeS19travelling_salesman10chromosomeZb+0x32>
 8065740:       72 02                   jb     8065744
<_D19travelling_salesman4mainFZv14fitnessCompareMFS19travelling_salesman10chromosomeS19travelling_salesman10chromosomeZb+0x34>
 8065742:       31 c0                   xor    eax,eax
 8065744:       c9                      leave
 8065745:       c2 10 00                ret    0x10



actual code:



import std.stdio;  
import std.random;
import std.array;
import std.math;

struct city{
    int x;
    int y;
}

struct chromosome{
    city[] dna;
}    

void main(){
    bool fitnessCompare(chromosome first,chromosome second){ 
        return fitness(first)>fitness(second);
    }
    auto less = &fitnessCompare;
    auto z = chromosome([city(0, 10), city(25, 25), city(10, 65), city(50, 50),
city(75, 30), city(20, 0)]);
    auto f = fitness(z); 
    writeln("fitness(z) > fitness(z) ?",f > f);
    writeln("fitness(z) > fitness(z) ?",less(z,z));
}

float fitness(const chromosome victim) pure{
    const city[] cities=city(0,0) ~ victim.dna ~ city(0,0);

    //we need to start from home and return to home

    float travelled=0f;
    for(int x=0;x<cities.length-1;x++)
        travelled+=distance(cities[x],cities[x+1]);
    //writeln(100/travelled);
    return 100/travelled;
}

float distance(city from,city to) pure{
    return sqrt(cast(float)(pow(to.x-from.x,2) + pow(to.y-from.y,2)));
}

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