[Bug target/70243] New: PowerPC V4DFmode should not use Altivec instructions on VSX systems

[meissner at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70243

            Bug ID: 70243
           Summary: PowerPC V4DFmode should not use Altivec instructions
                    on VSX systems
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: meissner at gcc dot gnu.org
  Target Milestone: ---

The traditional Altivec single precision vector floating point instructions
should not be used if the VSX instructions are available.  This is due to the
fact that the Altivec instruction does not use the current rounding mode, while
the VSX instruction does.

I did a quick glance through vsx.md and altivec.md, and I believe the only
places we use the Altivec instruction by default is for 4 operand fused
multiply-add and 4 operand fused negate multiply-subtract, when the destination
operand does not overlap with the input operands.