[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

llvm-svn: 327764

1 files changed, 5 insertions, 6 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index aac79075e6e..db355cf68f1 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/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.