3 files changed, 13 insertions, 4 deletions
diff --git a/llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp b/llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp
index 1c85248eb84..c9da6fd83ee 100644
--- a/llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp
@@ -138,6 +138,14 @@ void FunctionLoweringInfo::set(const Function &fn, MachineFunction &mf,
         }
       }
 
+      // Look for calls to the @llvm.va_start intrinsic. We can omit some
+      // prologue boilerplate for variadic functions that don't examine their
+      // arguments.
+      if (const auto *II = dyn_cast<IntrinsicInst>(I)) {
+        if (II->getIntrinsicID() == Intrinsic::vastart)
+          MF->getFrameInfo()->setHasVAStart(true);
+      }
+
       // Mark values used outside their block as exported, by allocating
       // a virtual register for them.
       if (isUsedOutsideOfDefiningBlock(I))
diff --git a/llvm/lib/Target/ARM/ARMISelLowering.cpp b/llvm/lib/Target/ARM/ARMISelLowering.cpp
index 42c8f202e0f..f5355833a34 100644
--- a/llvm/lib/Target/ARM/ARMISelLowering.cpp
+++ b/llvm/lib/Target/ARM/ARMISelLowering.cpp
@@ -3080,7 +3080,7 @@ ARMTargetLowering::LowerFormalArguments(SDValue Chain,
   }
   CCInfo.rewindByValRegsInfo();
   lastInsIndex = -1;
-  if (isVarArg) {
+  if (isVarArg && MFI->hasVAStart()) {
     unsigned ExtraArgRegsSize;
     unsigned ExtraArgRegsSaveSize;
     computeRegArea(CCInfo, MF, CCInfo.getInRegsParamsCount(), 0,
@@ -3222,7 +3222,7 @@ ARMTargetLowering::LowerFormalArguments(SDValue Chain,
   }
 
   // varargs
-  if (isVarArg)
+  if (isVarArg && MFI->hasVAStart())
     VarArgStyleRegisters(CCInfo, DAG, dl, Chain,
                          CCInfo.getNextStackOffset(),
                          TotalArgRegsSaveSize);
diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index afcabdcda7e..a685fdb6c7e 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -2405,8 +2405,9 @@ X86TargetLowering::LowerFormalArguments(SDValue Chain,
     StackSize = GetAlignedArgumentStackSize(StackSize, DAG);
 
   // If the function takes variable number of arguments, make a frame index for
-  // the start of the first vararg value... for expansion of llvm.va_start.
-  if (isVarArg) {
+  // the start of the first vararg value... for expansion of llvm.va_start. We
+  // can skip this if there are no va_start calls.
+  if (isVarArg && MFI->hasVAStart()) {
     if (Is64Bit || (CallConv != CallingConv::X86_FastCall &&
                     CallConv != CallingConv::X86_ThisCall)) {
       FuncInfo->setVarArgsFrameIndex(MFI->CreateFixedObject(1, StackSize,true));