[TargetLowering] SimplifyMultipleUseDemandedBits - add BITCAST pass through support (Reapplied)

This allows us to peek through BITCASTs, attempt to simplify the source operand, and then bitcast back.

This reapplies rL367091 which was reverted at rL367118 - we were inconsistently peeking through the bitcasts to the source value.

Fixes PR42777

llvm-svn: 367174

1 files changed, 59 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index 92706e1f116..9633a8c873a 100644
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -565,15 +565,72 @@ bool TargetLowering::SimplifyDemandedBits(SDValue Op, const APInt &DemandedBits,
 }
 
 // TODO: Can we merge SelectionDAG::GetDemandedBits into this?
-// TODO: Under what circumstances can we create nodes? BITCAST? Constant?
+// TODO: Under what circumstances can we create nodes? Constant folding?
 SDValue TargetLowering::SimplifyMultipleUseDemandedBits(
     SDValue Op, const APInt &DemandedBits, const APInt &DemandedElts,
     SelectionDAG &DAG, unsigned Depth) const {
   if (Depth >= 6) // Limit search depth.
     return SDValue();
 
+  unsigned NumElts = DemandedElts.getBitWidth();
   KnownBits LHSKnown, RHSKnown;
   switch (Op.getOpcode()) {
+  case ISD::BITCAST: {
+    SDValue Src = peekThroughBitcasts(Op.getOperand(0));
+    EVT SrcVT = Src.getValueType();
+    EVT DstVT = Op.getValueType();
+    unsigned NumSrcEltBits = SrcVT.getScalarSizeInBits();
+    unsigned NumDstEltBits = DstVT.getScalarSizeInBits();
+
+    if (NumSrcEltBits == NumDstEltBits)
+      if (SDValue V = SimplifyMultipleUseDemandedBits(
+              Src, DemandedBits, DemandedElts, DAG, Depth + 1))
+        return DAG.getBitcast(DstVT, V);
+
+    // TODO - bigendian once we have test coverage.
+    if (SrcVT.isVector() && (NumDstEltBits % NumSrcEltBits) == 0 &&
+        DAG.getDataLayout().isLittleEndian()) {
+      unsigned Scale = NumDstEltBits / NumSrcEltBits;
+      unsigned NumSrcElts = SrcVT.getVectorNumElements();
+      APInt DemandedSrcBits = APInt::getNullValue(NumSrcEltBits);
+      APInt DemandedSrcElts = APInt::getNullValue(NumSrcElts);
+      for (unsigned i = 0; i != Scale; ++i) {
+        unsigned Offset = i * NumSrcEltBits;
+        APInt Sub = DemandedBits.extractBits(NumSrcEltBits, Offset);
+        if (!Sub.isNullValue()) {
+          DemandedSrcBits |= Sub;
+          for (unsigned j = 0; j != NumElts; ++j)
+            if (DemandedElts[j])
+              DemandedSrcElts.setBit((j * Scale) + i);
+        }
+      }
+
+      if (SDValue V = SimplifyMultipleUseDemandedBits(
+              Src, DemandedSrcBits, DemandedSrcElts, DAG, Depth + 1))
+        return DAG.getBitcast(DstVT, V);
+    }
+
+    // TODO - bigendian once we have test coverage.
+    if ((NumSrcEltBits % NumDstEltBits) == 0 &&
+        DAG.getDataLayout().isLittleEndian()) {
+      unsigned Scale = NumSrcEltBits / NumDstEltBits;
+      unsigned NumSrcElts = SrcVT.isVector() ? SrcVT.getVectorNumElements() : 1;
+      APInt DemandedSrcBits = APInt::getNullValue(NumSrcEltBits);
+      APInt DemandedSrcElts = APInt::getNullValue(NumSrcElts);
+      for (unsigned i = 0; i != NumElts; ++i)
+        if (DemandedElts[i]) {
+          unsigned Offset = (i % Scale) * NumDstEltBits;
+          DemandedSrcBits.insertBits(DemandedBits, Offset);
+          DemandedSrcElts.setBit(i / Scale);
+        }
+
+      if (SDValue V = SimplifyMultipleUseDemandedBits(
+              Src, DemandedSrcBits, DemandedSrcElts, DAG, Depth + 1))
+        return DAG.getBitcast(DstVT, V);
+    }
+
+    break;
+  }
   case ISD::AND: {
     LHSKnown = DAG.computeKnownBits(Op.getOperand(0), DemandedElts, Depth + 1);
     RHSKnown = DAG.computeKnownBits(Op.getOperand(1), DemandedElts, Depth + 1);
@@ -625,7 +682,7 @@ SDValue TargetLowering::SimplifyMultipleUseDemandedBits(
     // If all the demanded elts are from one operand and are inline,
     // then we can use the operand directly.
     bool AllUndef = true, IdentityLHS = true, IdentityRHS = true;
-    for (unsigned i = 0, NumElts = ShuffleMask.size(); i != NumElts; ++i) {
+    for (unsigned i = 0; i != NumElts; ++i) {
       int M = ShuffleMask[i];
       if (M < 0 || !DemandedElts[i])
         continue;