LegalizeIntegerTypes: Reenable the large shift with small amount optimization.

To avoid problems with zero shifts when getting the bits that move between words
we use a trick: first shift the by amount-1, then do another shift by one. When
amount is 0 (and size 32) we first shift by 31, then by one, instead of by 32.

Also fix a latent bug that emitted the low and high words in the wrong order
when shifting right.

Fixes PR12113.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151637 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 21 insertions, 11 deletions

diff --git a/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp b/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp
index d18766a..38467c5 100644
--- a/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp
+++ b/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp
@@ -1397,15 +1397,15 @@ ExpandShiftWithKnownAmountBit(SDNode *N, SDValue &Lo, SDValue &Hi) {
     }
   }
 
-#if 0
-  // FIXME: This code is broken for shifts with a zero amount!
   // If we know that all of the high bits of the shift amount are zero, then we
   // can do this as a couple of simple shifts.
   if ((KnownZero & HighBitMask) == HighBitMask) {
-    // Compute 32-amt.
-    SDValue Amt2 = DAG.getNode(ISD::SUB, ShTy,
-                                 DAG.getConstant(NVTBits, ShTy),
-                                 Amt);
+    // Calculate 31-x. 31 is used instead of 32 to avoid creating an undefined
+    // shift if x is zero.  We can use XOR here because x is known to be smaller
+    // than 31.
+    SDValue Amt2 = DAG.getNode(ISD::XOR, dl, ShTy, Amt,
+                               DAG.getConstant(NVTBits-1, ShTy));
+
     unsigned Op1, Op2;
     switch (N->getOpcode()) {
     default: llvm_unreachable("Unknown shift");
@@ -1414,13 +1414,23 @@ ExpandShiftWithKnownAmountBit(SDNode *N, SDValue &Lo, SDValue &Hi) {
     case ISD::SRA:  Op1 = ISD::SRL; Op2 = ISD::SHL; break;
     }
 
-    Lo = DAG.getNode(N->getOpcode(), NVT, InL, Amt);
-    Hi = DAG.getNode(ISD::OR, NVT,
-                     DAG.getNode(Op1, NVT, InH, Amt),
-                     DAG.getNode(Op2, NVT, InL, Amt2));
+    // When shifting right the arithmetic for Lo and Hi is swapped.
+    if (N->getOpcode() != ISD::SHL)
+      std::swap(InL, InH);
+
+    // Use a little trick to get the bits that move from Lo to Hi. First
+    // calculate the shift with amount-1.
+    SDValue Sh1 = DAG.getNode(Op2, dl, NVT, InL, Amt2);
+    // Then shift one bit further to get the right result.
+    SDValue Sh2 = DAG.getNode(Op2, dl, NVT, Sh1, DAG.getConstant(1, ShTy));
+
+    Lo = DAG.getNode(N->getOpcode(), dl, NVT, InL, Amt);
+    Hi = DAG.getNode(ISD::OR, dl, NVT, DAG.getNode(Op1, dl, NVT, InH, Amt),Sh2);
+
+    if (N->getOpcode() != ISD::SHL)
+      std::swap(Hi, Lo);
     return true;
   }
-#endif
 
   return false;
 }