Recommit r358887 "[TargetLowering][AMDGPU][X86] Improve SimplifyDemandedBits bitcast handling"

I've included a new fix in X86RegisterInfo to prevent PR41619 without
reintroducing r359392. We might be able to improve that in the base class
implementation of shouldRewriteCopySrc somehow. But this hopefully enables
forward progress on SimplifyDemandedBits improvements for now.

Original commit message:

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 DAGComb
but it caused a lot of noise on other targets - some improvements, some regressions.

The X86 changes are all definite wins.

llvm-svn: 360552

1 files changed, 25 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index 502913886e3..fc7e7d9c3ce 100644
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -1581,12 +1581,36 @@ 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;