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Message-ID: <Z8IZNno-AL0sqYjn@voyager>
Date: Fri, 28 Feb 2025 21:14:46 +0100
From: Markus Wichmann <nullplan@....net>
To: musl@...ts.openwall.com
Cc: Trevor Gross <tmgross@...ch.edu>
Subject: Re: f128 aliases for long double math symbols
Am Fri, Feb 28, 2025 at 06:36:09AM -0500 schrieb Trevor Gross:
> Hello all,
>
> On platforms where `long double` is `_Float128`, glibc provides
> aliases of math symbols. For example, on aarch64 `sinl` and `sinf128`
> refer to the same function.
>
> Would the same be welcome in musl? The advantage this provides is
> certainty that the correct symbol is linked for `_Float128` operations
> without knowledge of what the platform `long double` is configured to
> (e.g. the ongoing PowerPC double-double to IEEE128 migration).
>
> Best regards,
> Trevor
Is there any standard reserving these names and specifying their
meaning? As far as I can tell, this is purely a GNU extension. It
doesn't appear to be present on any other implementation whatsoever.
Therefore, musl will not be able to adopt these interfaces, as it would
run the risk of future incompatible standardization. Strictly speaking,
these names are in the application namespace, too.
Applications can verify their implementation's long double
representation with the values of the constants from <float.h>. Linking
a libc with an application when they have different ideas about what
long double means will break badly and must be prevented. But sinf128()
doesn't help with that.
In the past, similar ABI changes have been enabled by additional tooling
in the linker to recognize the disparity, e.g. by adding notes into the
object file, or writing special tags into the dynamic section (even
though I recall the error messages for these cases being subpar).
Mayhaps this is a better way forward.
Ciao,
Markus
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