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Re: [PATCH], RFC, add support for __float128/__ibm128 types on PowerPC


On Tue, Apr 29, 2014 at 06:30:32PM -0400, Michael Meissner wrote:
> This patch adds support for a new type (__float128) on the PowerPC to allow
> people to use the 128-bit IEEE floating point format instead of the traditional
> IBM double-double that has been used in the Linux compilers.  At this time,
> long double still will remain using the IBM double-double format.
> There has been an undocumented option to switch long double to to IEEE 128-bit,
> but right now, there are bugs I haven't ironed out on VSX systems.
> In addition, I added another type (__ibm128) so that when the transition is
> eventually made, people can use this type to get the old long double type.
> I was wondering if people had any comments on the code so far, and things I
> should different.  Note, I will be out on vacation May 6th - 14th, so I don't
> expect to submit the patches until I get back.

For mangling, if you are going to mangle it the same as the -mlong-double-64
long double, is __float128 going to be supported solely for ELFv2 ABI and
are you sure nobody has ever used -mlong-double-64 or
--without-long-double-128 configured compiler for it?

What is the plan for glibc (and for libstdc++)?
Looking at current ppc64le glibc, it seems it mistakenly still supports
the -mlong-double-64 stuff (e.g. printf calls are usually redirected to
__nldbl_printf (and tons of other calls).  So, is the plan to use
yet another set of symbols?  For __nldbl_* it is about 113 entry points
in and 1 in, but if you are going to support all of
-mlong-double-64, -mlong-double-128 as well as __float128, that would be far
more, because the compat -mlong-double-64 support mostly works by
redirecting, either in headers or through a special *.a library, to
corresponding double entry points whenever possible.
So, if you call logl in -mlong-double-64 code, it will be redirected to
log, because it has the same ABI.  But if you call *printf or nexttowardf
etc. where there is no ABI compatible double entrypoint, it needs to be a
new symbol.
But with __float128 vs. __ibm128 and long double being either of those,
you need different logl.

Which is why it is so huge problem that this hasn't been resolved initially
as part of ELFv2 changes.


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