Thread overview  


January 24, 2012 floatingWTF  

 
alias double Real; //alias float Real; // simple linear interpolation; I partitioned the internals to help me figure out what was happening. Real lerp(Real t, Real a, Real b){ Real s1 = (1.0  t) * a; Real s2 = t * b; Real rt1 = s1 + s2; Real rt2 = ((1.0  t) * a) + (t * b); writefln("t=%.2f, a=%.2f, b=%.2f, s1=%.2f, s2=%.2f, rt1=%.2f, rt2=%.2f :::", t, a, b, s1, s2, rt1, rt2); return rt2; } unittest{ writeln(lerp(0.75, 2.0, 2.0)); // the correct result is 1.0 } compile and run with 'dmd inline unittest' and the output should be: t=0.75, a=2.00, b=2.00, s1=0.50, s2=1.50, rt1=1.00, rt2=1.00 ::: 1 Now, change 'Real' to float by uncommenting the second line and compile and run with 'dmd unittest'. This is what I get for the output: t=0.75, a=0.00, b=2.00, s1=0.50, s2=0.00, rt1=0.50, rt2=1.50 ::: 1.5 I have no idea why 'a' is zero. 'rt1' and 'rt2' do not have the same value, and 'lerp' does not return 1.0. Compiling with 'dmd inline unittest' does produce the correct result as before, though. You can play with different compiler options, such as O, to get the same weird behavior. I presume this is another DMD bug? 
January 24, 2012 Re: floatingWTF  

 
Posted in reply to Caligo  On 01/24/2012 04:13 AM, Caligo wrote: > alias double Real; > //alias float Real; > > // simple linear interpolation; I partitioned the internals to help me > figure out what was happening. > Real lerp(Real t, Real a, Real b){ > Real s1 = (1.0  t) * a; > Real s2 = t * b; > Real rt1 = s1 + s2; > Real rt2 = ((1.0  t) * a) + (t * b); > writefln("t=%.2f, a=%.2f, b=%.2f, s1=%.2f, s2=%.2f, rt1=%.2f, > rt2=%.2f :::", t, a, b, s1, s2, rt1, rt2); > return rt2; > } > > unittest{ > > writeln(lerp(0.75, 2.0, 2.0)); // the correct result is 1.0 > } > > compile and run with 'dmd inline unittest' and the output should be: > > t=0.75, a=2.00, b=2.00, s1=0.50, s2=1.50, rt1=1.00, rt2=1.00 ::: > 1 > > Now, change 'Real' to float by uncommenting the second line and > compile and run with 'dmd unittest'. This is what I get for the > output: > > t=0.75, a=0.00, b=2.00, s1=0.50, s2=0.00, rt1=0.50, rt2=1.50 ::: > 1.5 This: (with float) writefln("t=%.2f, a=%.2f, b=%.2f, s1=%.2f, s2=%.2f, rt1=%.2f, rt2=%.2f :::", t, a, b, s1, s2, rt1, rt2); writefln("t=%.2f, a=%f, b=%.2f, s1=%.2f, s2=%.2f, rt1=%.2f, rt2=%.2f :::", t, a, b, s1, s2, rt1, rt2); Outputs: (with float) t=0.75, a=0.00, b=2.00, s1=0.50, s2=0.00, rt1=0.50, rt2=1.50 ::: t=0.75, a=2.000000, b=2.00, s1=0.50, s2=0.00, rt1=0.50, rt2=1.50 ::: difference: %.2f > %f > > I have no idea why 'a' is zero. 'rt1' and 'rt2' do not have the same > value, and 'lerp' does not return 1.0. Compiling with 'dmd inline > unittest' does produce the correct result as before, though. > > You can play with different compiler options, such as O, to get the > same weird behavior. > > I presume this is another DMD bug? 
January 24, 2012 Re: floatingWTF  

 
Posted in reply to sclytrack  t=0.750000, a=2.000000, b=2.000000, s1=0.500000, s2=0.000000, rt1=0.500000, rt2=1.500000 :::
1.5
t = 0.75 ok
a = 2.0 ok
b = 2.0 ok
s1 = 0.5 ok
s2 = 1.5 //<different above says s2 = 0.0
rt1= 1.0 = s1 + s2 semiok depends on s2.
rt2 = 1.0 = ((1.0  t) * a) + (t * b); goes nuts.
On 01/24/2012 10:03 AM, sclytrack wrote:
> On 01/24/2012 04:13 AM, Caligo wrote:
>> alias double Real;
>> //alias float Real;
>>
>> // simple linear interpolation; I partitioned the internals to help me
>> figure out what was happening.
>> Real lerp(Real t, Real a, Real b){
>> Real s1 = (1.0  t) * a;
>> Real s2 = t * b;
>> Real rt1 = s1 + s2;
>> Real rt2 = ((1.0  t) * a) + (t * b);
>> writefln("t=%.2f, a=%.2f, b=%.2f, s1=%.2f, s2=%.2f, rt1=%.2f,
>> rt2=%.2f :::", t, a, b, s1, s2, rt1, rt2);
>> return rt2;
>> }
>>
>> unittest{
>>
>> writeln(lerp(0.75, 2.0, 2.0)); // the correct result is 1.0
>> }
>>
>> compile and run with 'dmd inline unittest' and the output should be:
>>
>> t=0.75, a=2.00, b=2.00, s1=0.50, s2=1.50, rt1=1.00, rt2=1.00 :::
>> 1
>>
>> Now, change 'Real' to float by uncommenting the second line and
>> compile and run with 'dmd unittest'. This is what I get for the
>> output:
>>
>> t=0.75, a=0.00, b=2.00, s1=0.50, s2=0.00, rt1=0.50, rt2=1.50 :::
>> 1.5
>
> This: (with float)
>
> writefln("t=%.2f, a=%.2f, b=%.2f, s1=%.2f, s2=%.2f, rt1=%.2f, rt2=%.2f
> :::", t, a, b, s1, s2, rt1, rt2);
> writefln("t=%.2f, a=%f, b=%.2f, s1=%.2f, s2=%.2f, rt1=%.2f, rt2=%.2f
> :::", t, a, b, s1, s2, rt1, rt2);
>
> Outputs: (with float)
>
> t=0.75, a=0.00, b=2.00, s1=0.50, s2=0.00, rt1=0.50, rt2=1.50 :::
> t=0.75, a=2.000000, b=2.00, s1=0.50, s2=0.00, rt1=0.50, rt2=1.50 :::
>
> difference:
>
> %.2f > %f
>
>
>
>>
>> I have no idea why 'a' is zero. 'rt1' and 'rt2' do not have the same
>> value, and 'lerp' does not return 1.0. Compiling with 'dmd inline
>> unittest' does produce the correct result as before, though.
>>
>> You can play with different compiler options, such as O, to get the
>> same weird behavior.
>>
>> I presume this is another DMD bug?
>

January 24, 2012 Re: floatingWTF  

 
Posted in reply to sclytrack   alias float Real; Real lerp(Real t, Real a, Real b) { Real s2 = t * b; writeln("t*b= ", s2); writeln("t*b= ", t * b); return s2; } Output: t*b= 1.5 t*b= 1.5  alias float Real; Real lerp(Real t, Real a, Real b) { Real s1 = (1.0  t) * a; Real s2 = t * b; writeln("t*b= ", s2); writeln("t*b= ", t * b); return s2; } Output: t*b= 0 t*b= 1.5  >> >>> I presume this is another DMD bug? >> > yes. 
January 24, 2012 Re: floatingWTF  

 
Posted in reply to sclytrack  No, it is not a bug. Here is a hint: import std.stdio; int main() { float f; writeln(f); return 0; } /+ output + nan + end of output +/ 
January 24, 2012 Re: floatingWTF  

 
Posted in reply to sclytrack  void calculate2(float a, float b) { float s1 = 1.0  a; float s2 = a * b; writeln(s2); writeln(a * b); }  Output: t*b= 0 t*b= 1.5  assume CS:.text._D4main10calculate2FffZv _D4main10calculate2FffZv: push RBP mov RBP,RSP sub RSP,020h movss 010h[RBP],XMM0 movss 8[RBP],XMM1 mov RAX,03FF0000000000000h mov 020h[RBP],RAX movsd XMM2,020h[RBP] cvtss2sd XMM1,XMM1 subsd XMM2,XMM1 conversion but xmm2 is no longer used cvtsd2ss XMM2,XMM2 precision error multiplication mulss XMM1,XMM0 movss 018h[RBP],XMM1 movss XMM0,018h[RBP] call _D3std5stdio14__T7writelnTfZ7writelnFfZv@PC32 movss XMM0,8[RBP] movss XMM1,010h[RBP] mulss XMM0,XMM1 call _D3std5stdio14__T7writelnTfZ7writelnFfZv@PC32 leave ret nop nop nop  DMD64 D Compiler v2.057 Ubuntu 11.10 
January 24, 2012 Re: floatingWTF  

 
Posted in reply to Dejan Lekic  On 01/24/2012 01:51 PM, Dejan Lekic wrote:
> No, it is not a bug.
>
> Here is a hint:
>
> import std.stdio;
>
> int main() {
> float f;
> writeln(f);
> return 0;
> }
>
> /+ output +
> nan
> + end of output +/
>
I don't understand.

January 24, 2012 Re: floatingWTF  

 
Posted in reply to sclytrack  On 01/24/2012 04:41 PM, sclytrack wrote:
> On 01/24/2012 01:51 PM, Dejan Lekic wrote:
>> No, it is not a bug.
>>
>> Here is a hint:
>>
>> import std.stdio;
>>
>> int main() {
>> float f;
>> writeln(f);
>> return 0;
>> }
>>
>> /+ output +
>> nan
>> + end of output +/
>>
>
> I don't understand.
I see now I forgot the input values. Sorry my IQ is 87.
void calculate2(float a, float b)
{
float s1 = 1.0  a;
float s2 = a * b;
writeln(s2);
writeln(a * b);
}
INPUT values
int main()
{
calculate2(0.75f,2.0f);
return 0;
}

Output:
t*b= 0
t*b= 1.5


January 24, 2012 Re: floatingWTF  

 
Posted in reply to sclytrack  can you give us a complete outofthebox compileable example (with imports,main,etc.) with both examples the working/and nonworking
does it work correct with dmd(32bit)?
is this "calculate2" needed to reproduce the behavior?
and what compiler did you test: dmd64, (dmd32?), ...?
Am 24.01.2012 16:20, schrieb sclytrack:
>
>
> void calculate2(float a, float b)
> {
> float s1 = 1.0  a;
> float s2 = a * b;
> writeln(s2);
> writeln(a * b);
> }
>
> 
> Output:
> t*b= 0
> t*b= 1.5
> 
>
>
> assume CS:.text._D4main10calculate2FffZv
> _D4main10calculate2FffZv:
> push RBP
> mov RBP,RSP
> sub RSP,020h
> movss 010h[RBP],XMM0
> movss 8[RBP],XMM1
> mov RAX,03FF0000000000000h
> mov 020h[RBP],RAX
> movsd XMM2,020h[RBP]
> cvtss2sd XMM1,XMM1
> subsd XMM2,XMM1
> conversion but xmm2 is no longer used
> cvtsd2ss XMM2,XMM2
> precision error multiplication
> mulss XMM1,XMM0
> movss 018h[RBP],XMM1
> movss XMM0,018h[RBP]
> call _D3std5stdio14__T7writelnTfZ7writelnFfZv@PC32
> movss XMM0,8[RBP]
> movss XMM1,010h[RBP]
> mulss XMM0,XMM1
> call _D3std5stdio14__T7writelnTfZ7writelnFfZv@PC32
> leave
> ret
> nop
> nop
> nop
>
> 
> DMD64 D Compiler v2.057
> Ubuntu 11.10

January 24, 2012 Re: floatingWTF  

 
Posted in reply to dennis luehring  double works correct under dmd64, but float not? what is the behavior of dmd32 in this szenario? 
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