Fix a long standing tail call optimization bug. When a libcall is emitted

legalizer always use the DAG entry node. This is wrong when the libcall is
emitted as a tail call since it effectively folds the return node. If
the return node's input chain is not the entry (i.e. call, load, or store)
use that as the tail call input chain.

PR12419
rdar://9770785
rdar://11195178

llvm-svn: 154370

2 files changed, 9 insertions, 3 deletions

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 2bef7581d4b..4f14a0e20b4 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -1578,18 +1578,20 @@ X86TargetLowering::LowerReturn(SDValue Chain,
                      MVT::Other, &RetOps[0], RetOps.size());
 }
 
-bool X86TargetLowering::isUsedByReturnOnly(SDNode *N) const {
+bool X86TargetLowering::isUsedByReturnOnly(SDNode *N, SDValue &Chain) const {
   if (N->getNumValues() != 1)
     return false;
   if (!N->hasNUsesOfValue(1, 0))
     return false;
 
+  SDValue TCChain = Chain;
   SDNode *Copy = *N->use_begin();
   if (Copy->getOpcode() == ISD::CopyToReg) {
     // If the copy has a glue operand, we conservatively assume it isn't safe to
     // perform a tail call.
     if (Copy->getOperand(Copy->getNumOperands()-1).getValueType() == MVT::Glue)
       return false;
+    TCChain = Copy->getOperand(0);
   } else if (Copy->getOpcode() != ISD::FP_EXTEND)
     return false;
 
@@ -1601,7 +1603,11 @@ bool X86TargetLowering::isUsedByReturnOnly(SDNode *N) const {
     HasRet = true;
   }
 
-  return HasRet;
+  if (!HasRet)
+    return false;
+
+  Chain = TCChain;
+  return true;
 }
 
 EVT
diff --git a/llvm/lib/Target/X86/X86ISelLowering.h b/llvm/lib/Target/X86/X86ISelLowering.h
index 6e5eda6b7f0..ca8efe64dd4 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.h
+++ b/llvm/lib/Target/X86/X86ISelLowering.h
@@ -805,7 +805,7 @@ namespace llvm {
                   const SmallVectorImpl<SDValue> &OutVals,
                   DebugLoc dl, SelectionDAG &DAG) const;
 
-    virtual bool isUsedByReturnOnly(SDNode *N) const;
+    virtual bool isUsedByReturnOnly(SDNode *N, SDValue &Chain) const;
 
     virtual bool mayBeEmittedAsTailCall(CallInst *CI) const;