Compile this:

short test4(unsigned X) {
  return (X >> 16);
}

to:

_test4:
        movl 4(%esp), %eax
        sarl $16, %eax
        ret

instead of:

_test4:
        movl $-65536, %eax
        andl 4(%esp), %eax
        sarl $16, %eax
        ret


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@28171 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -1575,6 +1575,11 @@ SDOperand DAGCombiner::visitSRA(SDNode *N) {
     }
   }
   
+  // Simplify, based on bits shifted out of the LHS. 
+  if (N1C && SimplifyDemandedBits(SDOperand(N, 0)))
+    return SDOperand(N, 0);
+  
+  
   // If the sign bit is known to be zero, switch this to a SRL.
   if (TLI.MaskedValueIsZero(N0, MVT::getIntVTSignBit(VT)))
     return DAG.getNode(ISD::SRL, VT, N0, N1);