[PowerPC] Add future work for vector insert/extract to README_ALTIVEC.txt

llvm-svn: 235309

1 files changed, 14 insertions, 0 deletions

diff --git a/llvm/lib/Target/PowerPC/README_ALTIVEC.txt b/llvm/lib/Target/PowerPC/README_ALTIVEC.txt
index 1d5b0922a9f..e40c44b8272 100644
--- a/llvm/lib/Target/PowerPC/README_ALTIVEC.txt
+++ b/llvm/lib/Target/PowerPC/README_ALTIVEC.txt
@@ -313,3 +313,17 @@ Opportunies to use instructions from PPCInstrVSX.td during code gen
 Related to this: we currently do not generate the lxvw4x instruction for either
 v4f32 or v4i32, probably because adding a dag pattern to the recognizer requires
 a single target type.  This should probably be addressed in the PPCISelDAGToDAG logic.
+
+//===----------------------------------------------------------------------===//
+
+Currently EXTRACT_VECTOR_ELT and INSERT_VECTOR_ELT are type-legal only for v2f64 with VSX available.  We should create custom lowering support for the other vector types.  Without this support, we generate sequences with load-hit-store hazards.
+
+v4f32 can be supported with VSX by shifting the correct element into big-endian lane 0, using xscvspdpn to produce a double-precision representation of the single-precision value in big-endian double-precision lane 0, and reinterpreting lane 0 as an FPR or vector-scalar register.
+
+v2i64 can be supported with VSX and P8Vector in the same manner as v2f64, followed by a direct move to a GPR.
+
+v4i32 can be supported with VSX and P8Vector by shifting the correct element into big-endian lane 1, using a direct move to a GPR, and sign-extending the 32-bit result to 64 bits.
+
+v8i16 can be supported with VSX and P8Vector by shifting the correct element into big-endian lane 3, using a direct move to a GPR, and sign-extending the 16-bit result to 64 bits.
+
+v16i8 can be supported with VSX and P8Vector by shifting the correct element into big-endian lane 7, using a direct move to a GPR, and sign-extending the 8-bit result to 64 bits.