[SelectionDAG] Handle big endian target BITCAST in computeKnownBits()

The BITCAST handling in computeKnownBits() previously only worked for little
endian.

This patch reverses the iteration over elements for a big endian target which
allows this to work in this case also.

SystemZ test case.

Review: Eli Friedman
https://reviews.llvm.org/D44249

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@327764 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -2363,10 +2363,7 @@ void SelectionDAG::computeKnownBits(SDValue Op, KnownBits &Known,
       break;
     }
 
-    // Support big-endian targets when it becomes useful.
     bool IsLE = getDataLayout().isLittleEndian();
-    if (!IsLE)
-      break;
 
     // Bitcast 'small element' vector to 'large element' scalar/vector.
     if ((BitWidth % SubBitWidth) == 0) {
@@ -2385,8 +2382,9 @@ void SelectionDAG::computeKnownBits(SDValue Op, KnownBits &Known,
       for (unsigned i = 0; i != SubScale; ++i) {
         computeKnownBits(N0, Known2, SubDemandedElts.shl(i),
                          Depth + 1);
-        Known.One |= Known2.One.zext(BitWidth).shl(SubBitWidth * i);
-        Known.Zero |= Known2.Zero.zext(BitWidth).shl(SubBitWidth * i);
+        unsigned Shifts = IsLE ? i : SubScale - 1 - i;
+        Known.One |= Known2.One.zext(BitWidth).shl(SubBitWidth * Shifts);
+        Known.Zero |= Known2.Zero.zext(BitWidth).shl(SubBitWidth * Shifts);
       }
     }
 
@@ -2408,7 +2406,8 @@ void SelectionDAG::computeKnownBits(SDValue Op, KnownBits &Known,
       Known.Zero.setAllBits(); Known.One.setAllBits();
       for (unsigned i = 0; i != NumElts; ++i)
         if (DemandedElts[i]) {
-          unsigned Offset = (i % SubScale) * BitWidth;
+          unsigned Shifts = IsLE ? i : NumElts - 1 - i;
+          unsigned Offset = (Shifts % SubScale) * BitWidth;
           Known.One &= Known2.One.lshr(Offset).trunc(BitWidth);
           Known.Zero &= Known2.Zero.lshr(Offset).trunc(BitWidth);
           // If we don't know any bits, early out.

test/CodeGen/SystemZ/dag-combine-03.ll

@@ -0,0 +1,29 @@
+; Test that DAGCombiner gets helped by getKnownBitsForTargetNode() when
+; BITCAST nodes are involved on a big-endian target.
+;
+; RUN: llc -mtriple=s390x-linux-gnu -mcpu=z13 < %s  | FileCheck %s
+
+define void @fun() {
+entry:
+  br label %lab0
+
+lab0:
+  %phi = phi i64 [ %sel, %lab0 ], [ 0, %entry ]
+  %add = add nuw nsw i64 %phi, 1
+  %cmp = icmp eq i64 %add, undef
+  %ins = insertelement <2 x i1> undef, i1 %cmp, i32 0
+  %xor = xor <2 x i1> %ins, <i1 true, i1 true>
+  %extr = extractelement <2 x i1> %xor, i32 0
+; The EXTRACT_VECTOR_ELT is done first into an i32, and then AND:ed with
+; 1. The AND is not actually necessary since the element contains a CC (i1)
+; value. Test that the BITCAST nodes in the DAG when computing KnownBits is
+; handled so that the AND is removed. If this succeeds, this results in a CHI
+; instead of TMLL.
+
+; CHECK-LABEL: # %bb.0:
+; CHECK:       chi
+; CHECK-NOT:   tmll
+; CHECK:       j
+  %sel = select i1 %extr, i64 %add, i64 0
+  br label %lab0
+}