[libc-commits] [libc] [libc][math][c23] Fix X86_Binary80 special cases for canonicalize functions. (PR #86924)

[via libc-commits]

lntue wrote:

> Fixed tests. Ran into a rather interesting observation. Why is that
> 
> ```
> A = (UInt128(0x0000) << 64) + UInt128(0x8000000000000000)
> B = (UInt128(0x0001) << 64) + UInt128(0x8000000000000000)
> ```
> 
> Both have a `get_biased_exponent()` value of 1 ?

`get_biased_exponent` follows the IEEE 754 convention, and is used to compute the real values of the bit patterns.  So once you added a hidden bit to your mantissa, the real value is simply
```
  2^(get_biased_exponent() - EXPONENT_BIAS) * m.
```
Another way to think about it is that:
```
  2^(get_biased_exponent() - EXPONENT_BIAS - FRACTION_LEN) is the value of the least significant bit.
```

https://github.com/llvm/llvm-project/pull/86924