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Re: [PATCH, rev2], Fix PR target/pr83862: Fix PowerPC long double signbit with -mabi=ieeelongdouble


On Mon, Jan 22, 2018 at 12:32:13PM -0600, Segher Boessenkool wrote:
> Hi Mike,
> 
> Thanks for doing this!
> 
> On Sun, Jan 21, 2018 at 07:03:58PM -0500, Michael Meissner wrote:
> > With these patches, the generator code issues an UNSPEC (signbit<mode>2_dm)
> > just to get the high double word, and the shift right is then emitted.  The
> > UNSPEC only handles the value being in either vector or GPR register.  There is
> > a second UNSPEC that is created by the combiner if the value is in memory.  On
> > little endian systems, the first split pass (before register allocation) will
> > allocate a pseudo register to hold the initial ADD of the base and index
> > registers for indexed loads, and then forms a LD reg,8(tmp) to load the high
> > word.  Just in case, some code after register allocation reforms the UNSPEC, it
> > uses a base register for the load, and it can use the base register as needed
> > for the temporary.
> 
> But does it have to do an unspec at all?  Can't it just immediately take
> the relevant 64-bit half (during expand), no unspec in sight, and that
> is that?

Yes it needs the UNSPEC.  As I said, you can't use SUBREG due to a combination
of MODES_TIEABLE_P and the way things are represented in vector registers (with
the scalar part being in the upper part of the register).  I tried it, and
could not get it to work.

-- 
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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