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.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@94664 91177308-0d34-0410-b5e6-96231b3b80d8

5 files changed, 45 insertions, 8 deletions

diff --git a/lib/Target/X86/X86ISelLowering.cpp b/lib/Target/X86/X86ISelLowering.cpp
index 6e6447574e..eb7f481591 100644
--- a/lib/Target/X86/X86ISelLowering.cpp
+++ b/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/lib/Target/X86/X86ISelLowering.h b/lib/Target/X86/X86ISelLowering.h
index 0642b392dd..193ef054c9 100644
--- a/lib/Target/X86/X86ISelLowering.h
+++ b/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,
diff --git a/test/CodeGen/X86/pic.ll b/test/CodeGen/X86/pic.ll
index e886ba06b7..d3c28a055a 100644
--- a/test/CodeGen/X86/pic.ll
+++ b/test/CodeGen/X86/pic.ll
@@ -190,7 +190,7 @@ bb12:
 ; LINUX: .L8$pb:
 ; LINUX:   addl	$_GLOBAL_OFFSET_TABLE_+(.Lpicbaseref8-.L8$pb),
 ; LINUX:   addl	.LJTI8_0@GOTOFF(
-; LINUX:   jmpl	*%ecx
+; LINUX:   jmpl	*
 
 ; LINUX: .LJTI8_0:
 ; LINUX:   .long	 .LBB8_2@GOTOFF
diff --git a/test/CodeGen/X86/tailcall1.ll b/test/CodeGen/X86/tailcall1.ll
index 42f8cdd384..96c4cad979 100644
--- a/test/CodeGen/X86/tailcall1.ll
+++ b/test/CodeGen/X86/tailcall1.ll
@@ -4,8 +4,8 @@ declare fastcc i32 @tailcallee(i32 %a1, i32 %a2, i32 %a3, i32 %a4)
 
 define fastcc i32 @tailcaller(i32 %in1, i32 %in2) nounwind {
 entry:
-	%tmp11 = tail call fastcc i32 @tailcallee(i32 %in1, i32 %in2, i32 %in1, i32 %in2)
-	ret i32 %tmp11
+  %tmp11 = tail call fastcc i32 @tailcallee(i32 %in1, i32 %in2, i32 %in1, i32 %in2)
+  ret i32 %tmp11
 }
 
 declare fastcc i8* @alias_callee()
diff --git a/test/CodeGen/X86/tailcall2.ll b/test/CodeGen/X86/tailcall2.ll
new file mode 100644
index 0000000000..f1eb1529c1
--- /dev/null
+++ b/test/CodeGen/X86/tailcall2.ll
@@ -0,0 +1,12 @@
+; RUN: llc < %s -march=x86    -asm-verbose=false | FileCheck %s
+; RUN: llc < %s -march=x86-64 -asm-verbose=false | FileCheck %s
+
+define void @bar(i32 %x) nounwind ssp {
+entry:
+; CHECK: bar:
+; CHECK: jmp _foo
+  tail call void @foo() nounwind
+  ret void
+}
+
+declare void @foo()