| Issue |
89668
|
| Summary |
Fixed point integral sqrt (uksqrtui) crashes with LIBC_FAST_MATH and produces an incorrect value without LIBC_FAST_MATH
|
| Labels |
bug,
libc
|
| Assignees |
lntue
|
| Reporter |
PiJoules
|
```
#include <stdio.h>
#include <stdint.h>
extern "C" unsigned _Accum uksqrtui(uint32_t);
int main() {
unsigned _Accum x = uksqrtui(65529);
printf("%f\n", (float)x);
}
```
This will crash with `integer divide by zero` on an x86-64 linux. It looks like the main reason for this is the `FastFractType` for `SqrtConfig<unsigned int>` is a 16-bit `unsigned fract`, so when we get to `FracType r = a * x_frac + b;`, we end up with `(0x820c * 0xfff9) // 2**16 + 0x7df8 == 0x10000` but we only retain the lower 16 bits for an unsigned fract. This is with `LIBC_FAST_MATH`.
Without `LIBC_FAST_MATH`, the error seems less clear and this will produce a value of `3.953979`. The final decimal result for either of these modes should be around `255.98632775990205`.
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