[PATCH] Correct type used for VADD_SPLAT optimization on PowerPC

In PPCISelLowering.cpp: PPCTargetLowering::LowerBUILD_VECTOR(), there
is an optimization for certain patterns to generate one or two vector
splats followed by a vector add or subtract.  This operation is
represented by a VADD_SPLAT in the selection DAG.  Prior to this
patch, it was possible for the VADD_SPLAT to be assigned the wrong
data type, causing incorrect code generation.  This patch corrects the
problem.

Specifically, the code previously assigned the value type of the
BUILD_VECTOR node to the newly generated VADD_SPLAT node.  This is
correct much of the time, but not always.  The problem is that the
call to isConstantSplat() may return a SplatBitSize that is not the
same as the number of bits in the original element vector type.  The
correct type to assign is a vector type with the same element bit size
as SplatBitSize.

The included test case shows an example of this, where the
BUILD_VECTOR node has a type of v16i8.  The vector to be built is {0,
16, 0, 16, 0, 16, 0, 16, 0, 16, 0, 16, 0, 16, 0, 16}.  isConstantSplat
detects that we can generate a splat of 16 for type v8i16, which is
the type we must assign to the VADD_SPLAT node.  If we do not, we
generate a vspltisb of 8 and a vaddubm, which generates the incorrect
result {16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16,
16}.  The correct code generation is a vspltish of 8 and a vadduhm.

This patch also corrected code generation for
CodeGen/PowerPC/2008-07-10-SplatMiscompile.ll, which had been marked
as an XFAIL, so we can remove the XFAIL from the test case.

llvm-svn: 209662

1 files changed, 8 insertions, 4 deletions

diff --git a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
index 214c8692060..cf4c9e61a58 100644
--- a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
+++ b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
@@ -5531,10 +5531,14 @@ SDValue PPCTargetLowering::LowerBUILD_VECTOR(SDValue Op,
     // we convert to a pseudo that will be expanded later into one of
     // the above forms.
     SDValue Elt = DAG.getConstant(SextVal, MVT::i32);
-    EVT VT = Op.getValueType();
-    int Size = VT == MVT::v16i8 ? 1 : (VT == MVT::v8i16 ? 2 : 4);
-    SDValue EltSize = DAG.getConstant(Size, MVT::i32);
-    return DAG.getNode(PPCISD::VADD_SPLAT, dl, VT, Elt, EltSize);
+    EVT VT = (SplatSize == 1 ? MVT::v16i8 :
+              (SplatSize == 2 ? MVT::v8i16 : MVT::v4i32));
+    SDValue EltSize = DAG.getConstant(SplatSize, MVT::i32);
+    SDValue RetVal = DAG.getNode(PPCISD::VADD_SPLAT, dl, VT, Elt, EltSize);
+    if (VT == Op.getValueType())
+      return RetVal;
+    else
+      return DAG.getNode(ISD::BITCAST, dl, Op.getValueType(), RetVal);
   }
 
   // If this is 0x8000_0000 x 4, turn into vspltisw + vslw.  If it is