Re: [musl] Unexpected exceptions in math functions

[Rich Felker <dalias@libc.org>]

On Wed, Apr 22, 2020 at 01:56:46PM +0000, leijitang wrote:
> Hi all,
> 
> Recently we update musl in our project from 1.1.2 to version 1.2.0. But there are some errors about math functions when running
> the opensource test cases for musl, which are passed in musl 1.1.2. These errors are about exp2(), pow() and powf().
> Logs are listed below.
> According to the commit message, the underflow exception is signaled if the result is in the subnormal range even if
> the result is exact (e.g. exp2(-1023.0)). But we got INEXACT exception at the same time, which is not as expected.
> Is this a problem?

For practical purposes, probably not, because basically nothing uses
fenv, and clang doesn't even support it (its optimizer makes invalid
transformations that will break use of fenv).

Strictly speaking, yes, and this should be fixed. exp2 with integer
argument should behave like scalbn/ldexp.

> Otherwise, there is a overflow exception in powf(0x1.fffffep+127,0x1p+0) with FE_UPWARD rounding mode(log is also behind. As
> 0x1.fffffep+127 is the largest number that float can represent, there should not be INEXACT|OVERFLOW exceptions.

I think that's correct. pow has various special cases that should be
exact, but catching them all to make sure they're exact is a bit of
work, and possibly hard to do without impacting the performance of
common cases. Still, use of pow in ways that should be exact is common
enough that I think this should be fixed.

> If these are problems affirmed, is there any patch to fix this?

Not yet.