Perform trivial tail call optimization for callees with "C" ABI. These are done

even when -tailcallopt is not specified and it does not require changing ABI.
First case is the most trivial one. Perform tail call optimization when both
the caller and callee do not return values and when the callee does not take
any input arguments.

llvm-svn: 94664

2 files changed, 30 insertions, 5 deletions

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 6e6447574e4..eb7f4815915 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -12,6 +12,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#define DEBUG_TYPE "x86-isel"
 #include "X86.h"
 #include "X86InstrBuilder.h"
 #include "X86ISelLowering.h"
@@ -39,6 +40,7 @@
 #include "llvm/MC/MCSymbol.h"
 #include "llvm/ADT/BitVector.h"
 #include "llvm/ADT/SmallSet.h"
+#include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/VectorExtras.h"
 #include "llvm/Support/CommandLine.h"
@@ -48,6 +50,8 @@
 #include "llvm/Support/raw_ostream.h"
 using namespace llvm;
 
+STATISTIC(NumTailCalls, "Number of tail calls");
+
 static cl::opt<bool>
 DisableMMX("disable-mmx", cl::Hidden, cl::desc("Disable use of MMX"));
 
@@ -1788,7 +1792,7 @@ X86TargetLowering::LowerCall(SDValue Chain, SDValue Callee,
   if (isTailCall)
     // Check if it's really possible to do a tail call.
     isTailCall = IsEligibleForTailCallOptimization(Callee, CallConv, isVarArg,
-                                                   Ins, DAG);
+                                                   Outs, Ins, DAG);
 
   assert(!(isVarArg && CallConv == CallingConv::Fast) &&
          "Var args not supported with calling convention fastcc");
@@ -1806,6 +1810,8 @@ X86TargetLowering::LowerCall(SDValue Chain, SDValue Callee,
 
   int FPDiff = 0;
   if (isTailCall) {
+    ++NumTailCalls;
+
     // Lower arguments at fp - stackoffset + fpdiff.
     unsigned NumBytesCallerPushed =
       MF.getInfo<X86MachineFunctionInfo>()->getBytesToPopOnReturn();
@@ -2237,11 +2243,29 @@ bool
 X86TargetLowering::IsEligibleForTailCallOptimization(SDValue Callee,
                                                      CallingConv::ID CalleeCC,
                                                      bool isVarArg,
-                                      const SmallVectorImpl<ISD::InputArg> &Ins,
+                                    const SmallVectorImpl<ISD::OutputArg> &Outs,
+                                    const SmallVectorImpl<ISD::InputArg> &Ins,
                                                      SelectionDAG& DAG) const {
-  if (CalleeCC == CallingConv::Fast &&
-      DAG.getMachineFunction().getFunction()->getCallingConv() == CalleeCC)
+  // If -tailcallopt is specified, make fastcc functions tail-callable.
+  const Function *F = DAG.getMachineFunction().getFunction();
+  if (PerformTailCallOpt &&
+      CalleeCC == CallingConv::Fast && F->getCallingConv() == CalleeCC)
     return true;
+
+  if (CalleeCC != CallingConv::Fast &&
+      CalleeCC != CallingConv::C)
+    return false;
+
+  // Look for obvious safe cases to perform tail call optimization.
+  // For now, only consider callees which take no arguments and no return
+  // values.
+  if (!Outs.empty())
+    return false;
+
+  if (Ins.empty())
+    // If the caller does not return a value, then this is obviously safe.
+    return F->getReturnType()->isVoidTy();
+
   return false;
 }
 
diff --git a/llvm/lib/Target/X86/X86ISelLowering.h b/llvm/lib/Target/X86/X86ISelLowering.h
index 0642b392dd4..193ef054c99 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.h
+++ b/llvm/lib/Target/X86/X86ISelLowering.h
@@ -630,7 +630,8 @@ namespace llvm {
     bool IsEligibleForTailCallOptimization(SDValue Callee,
                                            CallingConv::ID CalleeCC,
                                            bool isVarArg,
-                                      const SmallVectorImpl<ISD::InputArg> &Ins,
+                                    const SmallVectorImpl<ISD::OutputArg> &Outs,
+                                    const SmallVectorImpl<ISD::InputArg> &Ins,
                                            SelectionDAG& DAG) const;
     bool IsCalleePop(bool isVarArg, CallingConv::ID CallConv);
     SDValue EmitTailCallLoadRetAddr(SelectionDAG &DAG, SDValue &OutRetAddr,