[TargetLowering][AMDGPU][X86] Improve SimplifyDemandedBits bitcast handling

This patch adds support for BigBitWidth -> SmallBitWidth bitcasts, splitting the DemandedBits/Elts accordingly.

The AMDGPU backend needed an extra  (srl (and x, c1 << c2), c2) -> (and (srl(x, c2), c1) combine to encourage BFE creation, I investigated putting this in DAGCombine but it caused a lot of noise on other targets - some improvements, some regressions.

The X86 changes are all definite wins.

Differential Revision: https://reviews.llvm.org/D60462

llvm-svn: 358887

1 files changed, 25 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index 636fa6c1dab..eb089e5201f 100644
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -1471,12 +1471,36 @@ bool TargetLowering::SimplifyDemandedBits(
       if (SimplifyDemandedBits(Src, DemandedSrcBits, DemandedSrcElts,
                                KnownSrcBits, TLO, Depth + 1))
         return true;
+    } else if ((NumSrcEltBits % BitWidth) == 0 &&
+               TLO.DAG.getDataLayout().isLittleEndian()) {
+      unsigned Scale = NumSrcEltBits / BitWidth;
+      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) * BitWidth;
+          DemandedSrcBits.insertBits(DemandedBits, Offset);
+          DemandedSrcElts.setBit(i / Scale);
+        }
+
+      if (SrcVT.isVector()) {
+        APInt KnownSrcUndef, KnownSrcZero;
+        if (SimplifyDemandedVectorElts(Src, DemandedSrcElts, KnownSrcUndef,
+                                       KnownSrcZero, TLO, Depth + 1))
+          return true;
+      }
+
+      KnownBits KnownSrcBits;
+      if (SimplifyDemandedBits(Src, DemandedSrcBits, DemandedSrcElts,
+                               KnownSrcBits, TLO, Depth + 1))
+        return true;
     }
 
     // If this is a bitcast, let computeKnownBits handle it.  Only do this on a
     // recursive call where Known may be useful to the caller.
     if (Depth > 0) {
-      Known = TLO.DAG.computeKnownBits(Op, Depth);
+      Known = TLO.DAG.computeKnownBits(Op, DemandedElts, Depth);
       return false;
     }
     break;