From: "Joseph S. Myers" <joseph@codesourcery.com>
To: Roland McGrath <roland@hack.frob.com>
Cc: libc-ports@sourceware.org, "Ryan S. Arnold" <ryan.arnold@gmail.com>
Subject: Re: powerpc-nofpu ABI baselines
Date: Wed, 16 May 2012 21:58:00 -0000 [thread overview]
Message-ID: <Pine.LNX.4.64.1205162147020.32674@digraph.polyomino.org.uk> (raw)
In-Reply-To: <20120516214148.19DA22C0A8@topped-with-meat.com>
On Wed, 16 May 2012, Roland McGrath wrote:
> > @@ -1845,6 +1844,31 @@
> > __xpg_sigpause F
> > __xstat64 F
> > _flushlbf F
> > + _q_add F
> >
> > [...]
> >
> > See <http://sourceware.org/ml/libc-ports/2007-10/msg00004.html>. These
> > functions are in GLIBC_2.2 version but would not have been in glibc 2.2
> > and would never actually have been useful. Do we want to record them as
> > part of the GLIBC_2.2 ABI to preserve, or remove them?
>
> The relevant question is when the bug came in such that _q_add was
> exported. If it has been that way for some widely-used versions, then
> it's likely that there are binaries referring to the symbol. In that
> case, we can't really take it out even though it's an inaccurate
> representation of what 2.2 was.
The symbols came in when glibc was built with a compiler supporting
128-bit long double.
If versions before 2.4 were built like that, the result would be entirely
ABI-incompatible with any current libc; it's 2.4 that got the proper
support for 128-bit long double on powerpc, with all the affected
functions having GLIBC_2.4 symbol versions for the versions using 128-bit
long double so that older binaries built with 64-bit long double continue
to use older versions of functions built for 64-bit long double. So we
can consider the symbols to have come in at version 2.4.
In glibc 2.4, sysdeps/unix/sysv/linux/powerpc/configure.in ensures that
128-bit long double is supported and ensures that glibc is built with
-mabi=ibmlongdouble (and sysdeps/ieee754/ldbl-128ibm/Makefile ensures
-mlong-double-128 is used). The ABI since 2.4 has been the use of 128-bit
IBM long double.
The _q_* functions, however, are helper functions for IEEE quad long
double. GCC will only ever generate calls to them if
-mabi=ieeelongdouble. -mabi=ieeelongdouble is entirely ABI-incompatible
with IBM long double, as used in glibc 2.4 and greater. Since the
functions in question are not built with -mabi=ieeelongdouble, they do not
even provide the correct interface for any object built with
-mabi=ieeelongdouble (IEEE quad long doubles being passed by reference);
calls to them would never have done anything useful.
The function _q_utoq, to which GCC might generate calls with
-mabi=ieeelongdouble, is listed in Versions but not exported because the
source file calls it _q_uitoq.
--
Joseph S. Myers
joseph@codesourcery.com
next prev parent reply other threads:[~2012-05-16 21:58 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-05-16 21:23 Joseph S. Myers
2012-05-16 21:37 ` Joseph S. Myers
2012-05-16 21:42 ` Roland McGrath
2012-05-16 21:58 ` Joseph S. Myers [this message]
2012-05-16 22:03 ` Roland McGrath
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