[PPC] Fix atomics lowering in DAG lowering.

[timshen]

Summary

I forgot to forward the chain, causing some missing instruction
dependencies. The test crashes the compiler without this patch.

Inspired by the test case, D33519 also tries to remove the extra sync.

[timshen]

I'll have this one checked in first - it's a crasher fix.

llvm/trunk/lib/Target/PowerPC/PPCISelLowering.cpp

 SDValue PPCTargetLowering::LowerINTRINSIC_VOID(SDValue Op,
                                                SelectionDAG &DAG) const {
   // SelectionDAGBuilder::visitTargetIntrinsic may insert one extra chain to
   // the beginning of the argument list.
   int ArgStart = isa<ConstantSDNode>(Op.getOperand(0)) ? 0 : 1;
   SDLoc DL(Op);
   switch (cast<ConstantSDNode>(Op.getOperand(ArgStart))->getZExtValue()) {
   case Intrinsic::ppc_cfence: {
+    assert(ArgStart == 1);
     assert(Subtarget.isPPC64() && "Only 64-bit is supported for now.");
     return SDValue(DAG.getMachineNode(PPC::CFENCE8, DL, MVT::Other,
                                       DAG.getNode(ISD::ANY_EXTEND, DL, MVT::i64,
-                                                  Op.getOperand(ArgStart + 1))),
+                                                  Op.getOperand(ArgStart + 1)),
+                                      Op.getOperand(0)),
                    0);
   }
   default:
     break;
   }
   return SDValue();
 }
 

llvm/trunk/test/CodeGen/PowerPC/atomics-constant.ll

+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s | FileCheck %s
+
+target triple = "powerpc64le-unknown-linux-gnu"
+
+@a = constant i64 zeroinitializer
+
+define i64 @foo() {
+; CHECK-LABEL: foo:
+; CHECK:       # BB#0: # %entry
+; CHECK-NEXT:    addis 3, 2, .LC0@toc@ha
+; CHECK-NEXT:    li 4, 0
+; CHECK-NEXT:    ld 3, .LC0@toc@l(3)
+; CHECK-NEXT:    cmpw 7, 4, 4
+; CHECK-NEXT:    ld 3, 0(3)
+; CHECK-NEXT:    bne- 7, .+4
+; CHECK-NEXT:    isync
+; CHECK-NEXT:    li 3, 0
+; CHECK-NEXT:    blr
+entry:
+  %value = load atomic i64, i64* @a acquire, align 8
+  ret i64 %value
+}