[PATCH] Provide _Float128 for Clang

[Joseph Myers]

On Wed, 18 Dec 2024, H.J. Lu wrote:

> Since Clang doesn't provide _Float128:
> 
> test-fesetexcept-traps.c:67:3: error: unknown type name '_Float128'
>    67 |   math_force_eval (b);
>       |   ^
> ../sysdeps/x86/fpu/math-barriers.h:34:49: note: expanded from macro 'math_force_eval'
>    34 |         || __builtin_types_compatible_p (__typeof (x), _Float128))      \
>       |                                                        ^
> 
> provide it for Clang.

The correct place for compatibility definitions for _Float128 is 
bits/floatn.h (an installed header) - for x86, that's 
sysdeps/x86/bits/floatn.h.

That header already knows about older GCC supporting __float128 but not 
_Float128 and has appropriate conditionals to use that support (including 
corresponding definitions of __CFLOAT128, __builtin_huge_valf128, etc. as 
needed).  So what should be done is to add appropriate Clang conditionals 
to that header, then ensure it's included anywhere it's needed internally 
in glibc.

As a starting point, see the "x86: Define __HAVE_FLOAT128 for Clang and 
use __builtin_*f128 code path" commit in the azanella/clang branch 
(actually, that branch should be a starting point for all Clang-related 
fixes - sometimes the patches in it may not follow a good approach for 
inclusion in glibc, but it's still worth checking what that branch does 
for an issue before developing your own patch if you disagree with the one 
in that branch).

The patch in that branch is only a starting point, probably not ready for 
inclusion as is, because it doesn't have any Clang version conditionals.  
Since bits/floatn.h is an installed header, Clang support changes there 
should handle *all* released Clang versions that supported relevant glibc 
targets.  So unless every Clang release supporting glibc's x86 targets 
also supported __float128 on those targets (and likewise for built-in 
function etc. conditionals in the header as well), some Clang version 
conditionals are needed as well.  But once you've checked what Clang 
versions supported __float128 and related features, something based on 
that branch can be used to add that information to bits/floatn.h, which 
should solve your problem.  (Of course other architectures might need 
their own changes to their own bits/floatn.h files, but those could be 
dealt with separately.)

-- 
Joseph S. Myers
josmyers@redhat.com