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Posted to commits@nuttx.apache.org by GitBox <gi...@apache.org> on 2022/08/27 13:59:53 UTC
[GitHub] [incubator-nuttx] fjpanag opened a new issue, #6941: Wrong calculations by pow()
fjpanag opened a new issue, #6941:
URL: https://github.com/apache/incubator-nuttx/issues/6941
I just realized that `pow()` produces wrong results.
Specifically:
`pow(-2, 2)` yields `inf` instead of 4.
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[GitHub] [incubator-nuttx] fjpanag commented on issue #6941: Wrong calculations by pow()
Posted by GitBox <gi...@apache.org>.
fjpanag commented on issue #6941:
URL: https://github.com/apache/incubator-nuttx/issues/6941#issuecomment-1235656927
So my fix is indeed correct algorithmically, but the library itself is not very accurate. This is what makes the tests fail.
For example, `2^3` yields `8.0000000000000089` instead of just `8.0`.
My toolchain's library tends to be more accurate, and thus it makes the tests pass.
I will provide a PR to fix the issue, so negative bases will be able to be used with `pow()`, but a new more accurate library is surely needed.
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[GitHub] [incubator-nuttx] fjpanag commented on issue #6941: Wrong calculations by pow()
Posted by GitBox <gi...@apache.org>.
fjpanag commented on issue #6941:
URL: https://github.com/apache/incubator-nuttx/issues/6941#issuecomment-1230942331
Oh nice! It would be great to include a high quality library in NuttX!
For the record, I fixed this specific function. I just haven't pushed it yet.
I haven't done so because the exact same Lua test fails again, although the function seems to behave correctly now.
Thus, either my fix is wrong, or other libm functions are called for the same operation, and there is another bug elsewhere...
Compiling with the toolchain provided library fixes all issues.
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[GitHub] [incubator-nuttx] fjpanag commented on issue #6941: Wrong calculations by pow()
Posted by GitBox <gi...@apache.org>.
fjpanag commented on issue #6941:
URL: https://github.com/apache/incubator-nuttx/issues/6941#issuecomment-1229208352
This is the NuttX implementation of `pow()`:
```c
double pow(double b, double e)
{
return exp(e * log(b));
}
```
A friend of mine advised me that it is incorrect to use `log()` with negative numbers.
*(I am not capable of evaluating such math expressions myself, sorry...)*
So we need special handling for negative bases, I guess?
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[GitHub] [incubator-nuttx] acassis commented on issue #6941: Wrong calculations by pow()
Posted by GitBox <gi...@apache.org>.
acassis commented on issue #6941:
URL: https://github.com/apache/incubator-nuttx/issues/6941#issuecomment-1230912451
@fjpanag I talked with project maintainer of the LibmCS and he trying to get authorization from ESA to let's include it on NuttX to replace our Rhombus' libm implemantation. More info: https://www.youtube.com/watch?v=UcRefVlVL_A
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[GitHub] [incubator-nuttx] fjpanag commented on issue #6941: Wrong calculations by pow()
Posted by GitBox <gi...@apache.org>.
fjpanag commented on issue #6941:
URL: https://github.com/apache/incubator-nuttx/issues/6941#issuecomment-1229975391
*Note to self.*
This is how the C standard defines the `pow()` function:
> The pow function computes x raised to the power y. A domain error occurs if x is negative
and y is not an integral value. A domain error occurs if the result cannot be represented when x
is zero and y is less than or equal to zero. A range error may occur.
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[GitHub] [incubator-nuttx] xiaoxiang781216 closed issue #6941: Wrong calculations by pow()
Posted by GitBox <gi...@apache.org>.
xiaoxiang781216 closed issue #6941: Wrong calculations by pow()
URL: https://github.com/apache/incubator-nuttx/issues/6941
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[GitHub] [incubator-nuttx] nealef commented on issue #6941: Wrong calculations by pow()
Posted by GitBox <gi...@apache.org>.
nealef commented on issue #6941:
URL: https://github.com/apache/incubator-nuttx/issues/6941#issuecomment-1234974722
Just to add to the libm discussion and motivations for libmCS: https://github.com/apache/incubator-nuttx/issues/6767
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