[Bug 1783705] [NEW] clang errors with __float128 is not supported on this target on ppc64le

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Running Ubuntu cosmic on ppc64le, since the GCC 8 update I am unable to
compile programs with clang that include cmath:

$ cat foo.cpp 
#include <cmath>
int main() { }

$ clang++ foo.cpp 
In file included from foo.cpp:1:
In file included from /usr/bin/../lib/gcc/powerpc64le-linux-gnu/8/../../../../include/c++/8/cmath:47:
/usr/bin/../lib/gcc/powerpc64le-linux-gnu/8/../../../../include/c++/8/bits/std_abs.h:102:7: error: 
      __float128 is not supported on this target
  abs(__float128 __x)
      ^
/usr/bin/../lib/gcc/powerpc64le-linux-gnu/8/../../../../include/c++/8/bits/std_abs.h:101:3: error: 
      __float128 is not supported on this target
  __float128
  ^
2 errors generated.

GCC works fine. If I add -mfloat128, it fails with this error:

$ clang++ -mfloat128 foo.cpp 
In file included from foo.cpp:1:
In file included from /usr/bin/../lib/gcc/powerpc64le-linux-gnu/8/../../../../include/c++/8/cmath:45:
In file included from /usr/include/math.h:43:
/usr/include/powerpc64le-linux-gnu/bits/floatn.h:72:52: error: unknown machine mode '__KC__'
typedef _Complex float __cfloat128 __attribute__ ((__mode__ (__KC__)));
                                                   ^

$ clang++ --version
clang version 6.0.1-2ubuntu1 (tags/RELEASE_601/final)
Target: powerpc64le-unknown-linux-gnu


libc6: 2.27-3ubuntu1
clang: 6.0-41ubuntu1
gcc: 4:8.1.0-2ubuntu1

ProblemType: Bug
DistroRelease: Ubuntu 18.10
Package: clang 1:6.0-41ubuntu1
Uname: Linux 4.18.0-041800rc6-generic ppc64le
ApportVersion: 2.20.10-0ubuntu5
Architecture: ppc64el
Date: Thu Jul 26 16:27:16 2018
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
ProcLoadAvg: 0.15 1.70 2.08 1/1654 18418
ProcLocks:
 1: FLOCK  ADVISORY  WRITE 4151 00:32:46558 0 EOF
 2: FLOCK  ADVISORY  WRITE 3971 00:1a:62741 0 EOF
 3: FLOCK  ADVISORY  WRITE 3418 00:32:85157 0 EOF
 4: FLOCK  ADVISORY  WRITE 3104 00:17:44175 0 EOF
 5: POSIX  ADVISORY  WRITE 3109 00:17:39081 0 EOF
ProcSwaps:
 Filename				Type		Size	Used	Priority
 /swapfile                               file		2097088	0	-2
ProcVersion: Linux version 4.18.0-041800rc6-generic (kernel at tangerine) (gcc version 8.1.0 (Ubuntu 8.1.0-11ubuntu1)) #201807221830 SMP Sun Jul 22 23:58:55 UTC 2018
SourcePackage: llvm-defaults
UpgradeStatus: No upgrade log present (probably fresh install)
VarLogDump_list: total 0
cpu_cores: Number of cores present = 32
cpu_coreson: Number of cores online = 32
cpu_smt: SMT=4

** Affects: gcc-8 (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: apport-bug cosmic ppc64el
-- 
clang errors with __float128 is not supported on this target on ppc64le
https://bugs.launchpad.net/bugs/1783705
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