在 2018/6/5 4:44, Steve Ellcey 写道:
Is there a bug in __builtin_isnormal or am I just confused as to what it
means? There doesn't seem to be any actual definition/documentation for
the function. __builtin_isnormal(0.0) is returning false. That seems
wrong to me, 0.0 is a normal (as opposed to a denormalized) number isn't
it? Or is zero special?
It is documented here (this block of text is copied from GCC 7 manual):
```
6.59 Other Built-in Functions Provided by GCC
GCC provides built-in versions of the ISO C99 floating-point comparison
macros ... . In the same fashion, GCC provides fpclassify, isfinite,
isinf_sign, **isnormal** and signbit built-ins used with __builtin_
prefxed. The isinf and isnan built-in functions appear both with and
without the __builtin_ prefx.
```
ISO C says:
```
Description
2 The isnormal macro determines whether its argument value is normal
(neither zero, subnormal, infinite, nor NaN). First, an argument
represented in a format wider than its semantic type is converted to its
semantic type. Then determination is based on the type of the argument.
```
`isnormal(x)` is roughly equivalent to `fpclassify(x) == FP_NORMAL`.
When `x` is `0.0`, `fpclassify(x)` yields `FP_ZERO`, so the result is
`false`.
--
Best regards,
LH_Mouse
