[DAGCombine] visitEXTRACT_SUBVECTOR - 'little to big' extract_subvector(bitcast()) support

This moves the X86 specific transform from rL364407
into DAGCombiner to generically handle 'little to big' cases
(for example: extract_subvector(v2i64 bitcast(v16i8))). This
allows us to remove both the x86 implementation and the aarch64
bitcast(extract_subvector(bitcast())) combine.

Earlier patches that dealt with regressions initially exposed
by this patch:
rG5e5e99c041e4
rG0b38af89e2c0

Patch by: @RKSimon (Simon Pilgrim)

Differential Revision: https://reviews.llvm.org/D63815

1 files changed, 17 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 1ae2f58415f..02ae11d8a00 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -18515,7 +18515,23 @@ SDValue DAGCombiner::visitEXTRACT_SUBVECTOR(SDNode *N) {
         return DAG.getBitcast(NVT, NewExtract);
       }
     }
-    // TODO - handle (DestNumElts % SrcNumElts) == 0
+    if ((DestNumElts % SrcNumElts) == 0) {
+      unsigned DestSrcRatio = DestNumElts / SrcNumElts;
+      if ((NVT.getVectorNumElements() % DestSrcRatio) == 0) {
+        unsigned NewExtNumElts = NVT.getVectorNumElements() / DestSrcRatio;
+        EVT NewExtVT = EVT::getVectorVT(*DAG.getContext(),
+                                        SrcVT.getScalarType(), NewExtNumElts);
+        if ((N->getConstantOperandVal(1) % DestSrcRatio) == 0 &&
+            TLI.isOperationLegalOrCustom(ISD::EXTRACT_SUBVECTOR, NewExtVT)) {
+          unsigned IndexValScaled = N->getConstantOperandVal(1) / DestSrcRatio;
+          SDLoc DL(N);
+          SDValue NewIndex = DAG.getIntPtrConstant(IndexValScaled, DL);
+          SDValue NewExtract = DAG.getNode(ISD::EXTRACT_SUBVECTOR, DL, NewExtVT,
+                                           V.getOperand(0), NewIndex);
+          return DAG.getBitcast(NVT, NewExtract);
+        }
+      }
+    }
   }
 
   // Combine: