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Posted to commits@nuttx.apache.org by GitBox <gi...@apache.org> on 2021/07/28 01:38:32 UTC
[GitHub] [incubator-nuttx] nealef opened a new issue #4241: Floating point operations
nealef opened a new issue #4241:
URL: https://github.com/apache/incubator-nuttx/issues/4241
### Summary ###
I have encountered a couple of floating point operation problems.
* `fmod()` returns incorrect value
* `log(x)` never returns for specific values of x (see comment in https://github.com/apache/incubator-nuttx/pull/4115)
### Configuration ###
**Hardware:** stm32f777zit6
**NuttX:** Based on commit 2e80d5e38ed92f67767d87b544894cdc254f9459
### Test case ###
```
#include <stdio.h>
#include <math.h>
int
main(int argc, char **argv)
{
register double z = fmod(8.0, 3.0);
printf("mod: %g (0x%016llx)\n",z,z);
double x = log(318.0);
printf("log: %g\n",x);
}
```
#### Expected Results ####
```
mod: 2 (0x4000000000000000)
log: 5.76205
```
#### Actual Results ####
```
mod: 2 (0x3ffffffffffffffe)
<never returns>
```
#### Analysis ####
The `fmod()` function returns an incorrect value (1.9999999999999996) that is corrected with the `printf()` processing but if that value is used in other calculations or comparisons then the wrong result may be produced. Using `gdb` to trace the result returned confirms the 1.999... value is being returned.
For the second case,`gdb` shows the `log()` function is never returning as the condition:
```
while (y > y_old + epsilon || y < y_old - epsilon)
```
Is always true. The value `y` keeps swapping between 5.7620513827801778 and 5.7620513827801769.
### Circumvention ###
I added some oscillation detection code which checks if the y equals the y before last:
```
double log(double x)
{
double y;
double y_old;
double y_very_old; <---
double ey;
double epsilon;
int relax_factor;
int iter;
y = 0.0;
y_old = 1.0;
y_very_old = 2.0;
epsilon = DBL_EPSILON;
iter = 0;
relax_factor = 1;
while (y > y_old + epsilon || y < y_old - epsilon && y != y_very_old) <---
{
y_very_old = y_old; <---
y_old = y;
```
With this, perhaps naive, approach we exit with the value expected.
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[GitHub] [incubator-nuttx] nealef commented on issue #4241: Floating point operations
Posted by GitBox <gi...@apache.org>.
nealef commented on issue #4241:
URL: https://github.com/apache/incubator-nuttx/issues/4241#issuecomment-1003197834
> @nealef please check #5126 for `fmod` issue.
Tks!
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[GitHub] [incubator-nuttx] pkarashchenko commented on issue #4241: Floating point operations
Posted by GitBox <gi...@apache.org>.
pkarashchenko commented on issue #4241:
URL: https://github.com/apache/incubator-nuttx/issues/4241#issuecomment-1003116192
@nealef please try https://github.com/apache/incubator-nuttx/pull/5125 for `log` issue.
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[GitHub] [incubator-nuttx] pkarashchenko commented on issue #4241: Floating point operations
Posted by GitBox <gi...@apache.org>.
pkarashchenko commented on issue #4241:
URL: https://github.com/apache/incubator-nuttx/issues/4241#issuecomment-1003154565
@nealef please check https://github.com/apache/incubator-nuttx/pull/5126 for `fmod` issue.
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[GitHub] [incubator-nuttx] xiaoxiang781216 closed issue #4241: Floating point operations
Posted by GitBox <gi...@apache.org>.
xiaoxiang781216 closed issue #4241:
URL: https://github.com/apache/incubator-nuttx/issues/4241
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