[DAGCombiner] Fold truncate(build_vector(x,y)) -> build_vector(truncate(x),truncate(y))

If scalar truncates are free, attempt to pre-truncate build_vectors source operands.

Only attempt to do this before legalization as we often end up with truncations/extensions during build_vector lowering.

Differential Revision: https://reviews.llvm.org/D59654

llvm-svn: 357161

1 files changed, 15 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index ded4f04c57a..1c44a5d5ce3 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -9895,10 +9895,11 @@ SDValue DAGCombiner::visitZERO_EXTEND_VECTOR_INREG(SDNode *N) {
 SDValue DAGCombiner::visitTRUNCATE(SDNode *N) {
   SDValue N0 = N->getOperand(0);
   EVT VT = N->getValueType(0);
+  EVT SrcVT = N0.getValueType();
   bool isLE = DAG.getDataLayout().isLittleEndian();
 
   // noop truncate
-  if (N0.getValueType() == N->getValueType(0))
+  if (SrcVT == VT)
     return N0;
 
   // fold (truncate (truncate x)) -> (truncate x)
@@ -9999,6 +10000,19 @@ SDValue DAGCombiner::visitTRUNCATE(SDNode *N) {
     }
   }
 
+  // Attempt to pre-truncate BUILD_VECTOR sources.
+  if (N0.getOpcode() == ISD::BUILD_VECTOR && !LegalOperations &&
+      TLI.isTruncateFree(SrcVT.getScalarType(), VT.getScalarType())) {
+    SDLoc DL(N);
+    EVT SVT = VT.getScalarType();
+    SmallVector<SDValue, 8> TruncOps;
+    for (const SDValue &Op : N0->op_values()) {
+      SDValue TruncOp = DAG.getNode(ISD::TRUNCATE, DL, SVT, Op);
+      TruncOps.push_back(TruncOp);
+    }
+    return DAG.getBuildVector(VT, DL, TruncOps);
+  }
+
   // Fold a series of buildvector, bitcast, and truncate if possible.
   // For example fold
   //   (2xi32 trunc (bitcast ((4xi32)buildvector x, x, y, y) 2xi64)) to