add a note about how we should implement this FIXME from the legalizer:

// FIXME: revisit this when we have some kind of mechanism by which targets
    // can decided legality of vector constants, of which there may be very
    // many.

llvm-svn: 25733

llvm/lib/Target/PowerPC/PPCSubtarget.cpp

@@ -1,4 +1,4 @@
-//===- PowerPCSubtarget.cpp - PPC Subtarget Information ---------*- C++ -*-===//
+//===- PowerPCSubtarget.cpp - PPC Subtarget Information -------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //

llvm/lib/Target/PowerPC/README.txt

@@ -396,3 +396,9 @@ of the GOT on targets with one).
 
 Generate lwbrx and other byteswapping load/store instructions when reasonable.
 
+===-------------------------------------------------------------------------===
+
+Implement TargetConstantVec, and set up PPC to custom lower ConstantVec into
+TargetConstantVec's if it's one of the many forms that are algorithmically
+computable using the spiffy altivec instructions.
+