Fix failure to invoke exception handler on Win64

When the last instruction prior to a function epilogue is a call, we
need to emit a nop so that the return address is not in the epilogue IP
range.  This is consistent with MSVC's behavior, and may be a workaround
for a bug in the Win64 unwinder.

Differential Revision: http://reviews.llvm.org/D4751

Patch by Vadim Chugunov!

llvm-svn: 214775

3 files changed, 48 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86FrameLowering.cpp b/llvm/lib/Target/X86/X86FrameLowering.cpp
index 8c029a8c22d..b1aa35048c9 100644
--- a/llvm/lib/Target/X86/X86FrameLowering.cpp
+++ b/llvm/lib/Target/X86/X86FrameLowering.cpp
@@ -849,6 +849,11 @@ void X86FrameLowering::emitEpilogue(MachineFunction &MF,
   unsigned FramePtr = RegInfo->getFrameRegister(MF);
   unsigned StackPtr = RegInfo->getStackRegister();
 
+  bool IsWinEH =
+      MF.getTarget().getMCAsmInfo()->getExceptionHandlingType() ==
+      ExceptionHandling::WinEH;
+  bool NeedsWinEH = IsWinEH && MF.getFunction()->needsUnwindTableEntry();
+
   switch (RetOpcode) {
   default:
     llvm_unreachable("Can only insert epilog into returning blocks");
@@ -933,17 +938,29 @@ void X86FrameLowering::emitEpilogue(MachineFunction &MF,
       unsigned Opc = getLEArOpcode(IsLP64);
       addRegOffset(BuildMI(MBB, MBBI, DL, TII.get(Opc), StackPtr),
                    FramePtr, false, -CSSize);
+      --MBBI;
     } else {
       unsigned Opc = (Is64Bit ? X86::MOV64rr : X86::MOV32rr);
       BuildMI(MBB, MBBI, DL, TII.get(Opc), StackPtr)
         .addReg(FramePtr);
+      --MBBI;
     }
   } else if (NumBytes) {
     // Adjust stack pointer back: ESP += numbytes.
     emitSPUpdate(MBB, MBBI, StackPtr, NumBytes, Is64Bit, IsLP64, UseLEA,
                  TII, *RegInfo);
+    --MBBI;
   }
 
+  // Windows unwinder will not invoke function's exception handler if IP is
+  // either in prologue or in epilogue.  This behavior causes a problem when a
+  // call immediately precedes an epilogue, because the return address points
+  // into the epilogue.  To cope with that, we insert an epilogue marker here,
+  // then replace it with a 'nop' if it ends up immediately after a CALL in the
+  // final emitted code.
+  if (NeedsWinEH)
+    BuildMI(MBB, MBBI, DL, TII.get(X86::SEH_Epilogue));
+
   // We're returning from function via eh_return.
   if (RetOpcode == X86::EH_RETURN || RetOpcode == X86::EH_RETURN64) {
     MBBI = MBB.getLastNonDebugInstr();
diff --git a/llvm/lib/Target/X86/X86InstrCompiler.td b/llvm/lib/Target/X86/X86InstrCompiler.td
index fb69255d5ef..4f9bec6b191 100644
--- a/llvm/lib/Target/X86/X86InstrCompiler.td
+++ b/llvm/lib/Target/X86/X86InstrCompiler.td
@@ -214,6 +214,8 @@ let isPseudo = 1 in {
                             "#SEH_PushFrame $mode", []>;
   def SEH_EndPrologue : I<0, Pseudo, (outs), (ins),
                             "#SEH_EndPrologue", []>;
+  def SEH_Epilogue : I<0, Pseudo, (outs), (ins),
+                            "#SEH_Epilogue", []>;
 }
 
 //===----------------------------------------------------------------------===//
diff --git a/llvm/lib/Target/X86/X86MCInstLower.cpp b/llvm/lib/Target/X86/X86MCInstLower.cpp
index 2471667d95b..7f0af3e581a 100644
--- a/llvm/lib/Target/X86/X86MCInstLower.cpp
+++ b/llvm/lib/Target/X86/X86MCInstLower.cpp
@@ -826,6 +826,20 @@ void X86AsmPrinter::LowerPATCHPOINT(const MachineInstr &MI) {
            getSubtargetInfo());
 }
 
+// Returns instruction preceding MBBI in MachineFunction.
+// If MBBI is the first instruction of the first basic block, returns null.
+static MachineBasicBlock::const_iterator
+PrevCrossBBInst(MachineBasicBlock::const_iterator MBBI) {
+  const MachineBasicBlock *MBB = MBBI->getParent();
+  while (MBBI == MBB->begin()) {
+    if (MBB == MBB->getParent()->begin())
+      return nullptr;
+    MBB = MBB->getPrevNode();
+    MBBI = MBB->end();
+  }
+  return --MBBI;
+}
+
 void X86AsmPrinter::EmitInstruction(const MachineInstr *MI) {
   X86MCInstLower MCInstLowering(*MF, *this);
   const X86RegisterInfo *RI =
@@ -967,6 +981,21 @@ void X86AsmPrinter::EmitInstruction(const MachineInstr *MI) {
     OutStreamer.EmitWinCFIEndProlog();
     return;
 
+  case X86::SEH_Epilogue: {
+    MachineBasicBlock::const_iterator MBBI(MI);
+    // Check if preceded by a call and emit nop if so.
+    for (MBBI = PrevCrossBBInst(MBBI); MBBI; MBBI = PrevCrossBBInst(MBBI)) {
+      // Conservatively assume that pseudo instructions don't emit code and keep
+      // looking for a call. We may emit an unnecessary nop in some cases.
+      if (!MBBI->isPseudo()) {
+        if (MBBI->isCall())
+          EmitAndCountInstruction(MCInstBuilder(X86::NOOP));
+        break;
+      }
+    }
+    return;
+  }
+
   case X86::PSHUFBrm:
   case X86::VPSHUFBrm:
     // Lower PSHUFB normally but add a comment if we can find a constant