Legal action type for BUILD_VECTOR

Hello,

I'm working on extending the current PowerPC backend to handle a vector
instruction set for floating-point operations (IBM's double-hummer
instruction set used on the BG/P supercomputers). In this instruction
set, each of the existing floating-point registers becomes the first of
two vector elements. I am having trouble optimizing the BUILD_VECTOR
operation for the case where I am building a v2f64 vector out of two
operands. I tried writing this pattern as:

def : Pat<(v2f64 (build_vector F8RC:$A, F8RC:$B)),
          (FSMFP (INSERT_SUBREG (v2f64 (IMPLICIT_DEF)), F8RC:$A,
sub_64),
                 (INSERT_SUBREG (v2f64 (IMPLICIT_DEF)), F8RC:$B,
sub_64))>;

Where the FSMFP instruction copies the first vector element of the
second operand to the second vector element of the first operand. In
other words, if I already have one of the floating point numbers in a
floating point register I want to leave it there, and then copy in the
second floating-point number to the second slot.

The problem that I am having seems to be that I can't match build_vector
this way. When I try, I receive the following assertion:
clang: LegalizeDAG.cpp:994:
llvm::SDValue<unnamed>::SelectionDAGLegalize::LegalizeOp(llvm::SDValue):
Assertion `0 && "This action is not supported yet!"' failed.

So it seems that the corresponding case statement does not even handle
TargetLowering::Legal? Can this be changed to do what I want? Otherwise,
how should I work around this?

Thank you in advance,
Hal

You could try just marking it Expand; I think the default expansion
will do the right thing if you've implemented VECTOR_SHUFFLE and
SCALAR_TO_VECTOR correctly. Otherwise, mark it custom and lower it to
something appropriate. Legalization does weird things to BUILD_VECTOR
at the moment; we do plan on fixing that at some point.

-Eli