[PATCH], Fix PR target/pr83862: Fix PowerPC long double signbit with -mabi=ieeelongdouble

[Michael Meissner <meissner at linux dot vnet dot ibm dot com>]

PR target/83862 pointed out a problem I put into the 128-bit floating point
type signbit optimization.  The issue is we want to avoid doing a load to a
floating point/vector register and then a direct move to do signbit, so we
change the load to load the upper 64-bits of the floating point value to get
the sign bit.  Unfortunately, if the type is IEEE 128-bit and memory is
addressed with an indexed address on a little endian system, it generates an
illegal address and generates an internal compiler error.

I have tested this on a little endian power8 system, with bootstrap compilers.
There was not regression, and the new test passes.  Can I install this into the
trunk?

The same code is also in GCC 6 and 7.  While, -mabi=ieeelongdouble is not
supported in those releases, you can get a failure if you use an explicit
_Float128 type instead of long double.  Assuming that the bug shows up, can I
apply these patches to those branches as well?

[gcc]
2018-01-16  Michael Meissner  <meissner@linux.vnet.ibm.com>

	PR target/83862
	* config/rs6000/rs6000.c (rs6000_split_signbit): Do not create an
	illegal address on little endian systems if the source value is in
	memory addressed with indexed addressing.
	* config/rs6000/rs6000.md (signbit<mode>2_dm): Likewise.
	(signbit<mode>2_dm_<su>ext): Likewise.

[gcc/testsuite]
2018-01-16  Michael Meissner  <meissner@linux.vnet.ibm.com>

	PR target/83862
	* gcc.target/powerpc/pr83862.c: New test.

-- 
Michael Meissner, IBM
IBM, M/S 2506R, 550 King Street, Littleton, MA 01460-6245, USA
email: meissner@linux.vnet.ibm.com, phone: +1 (978) 899-4797

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