Re: [Tinycc-devel] Major bug in the pow function resulting in: 0.993013 = 0.860892

[Vincent Lefevre]

Hi,

On 2024-01-05 13:47:25 +0100, Aurélie Tisson (BastaPrint) wrote:
> The same calculation gives me 2 different values : 0.993013 0.860892
> wheras it should give 0.993013 only (tells GCC).

Well, your code is not portable.

> #include <stdio.h> 
> #include <math.h> 
> int main(){ 
> double s = .7; 
> double a = (1. - .1 * s) / (1. + .1 * s); 
              ^^^^^^^^^^^     ^^^^^^^^^^^
> double b = pow(a, .05); 
> double c = pow( ((1. - .1 * s) / (1. + .1 * s)), .05); 
                    ^^^^^^^^^^^     ^^^^^^^^^^^

> printf("%f %f \n", b, c); 
> } 

You should not depend on contraction of FP expressions.
Here, GCC may generate a FMA, which generally increases
the accuracy of the results.

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
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