Fix a lot of confusion around inserting nops on empty functions.

On MachO, and MachO only, we cannot have a truly empty function since that
breaks the linker logic for atomizing the section.

When we are emitting a frame pointer, the presence of an unreachable will
create a cfi instruction pointing past the last instruction. This is perfectly
fine. The FDE information encodes the pc range it applies to. If some tool
cannot handle this, we should explicitly say which bug we are working around
and only work around it when it is actually relevant (not for ELF for example).

Given the unreachable we could omit the .cfi_def_cfa_register, but then
again, we could also omit the entire function prologue if we wanted to.

llvm-svn: 217801

2 files changed, 7 insertions, 15 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
index b1fc46b4112..b26d8d6f101 100644
--- a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
@@ -731,12 +731,10 @@ void AsmPrinter::EmitFunctionBody() {
 
   // Print out code for the function.
   bool HasAnyRealCode = false;
-  const MachineInstr *LastMI = nullptr;
   for (auto &MBB : *MF) {
     // Print a label for the basic block.
     EmitBasicBlockStart(MBB);
     for (auto &MI : MBB) {
-      LastMI = &MI;
 
       // Print the assembly for the instruction.
       if (!MI.isPosition() && !MI.isImplicitDef() && !MI.isKill() &&
@@ -797,24 +795,14 @@ void AsmPrinter::EmitFunctionBody() {
     EmitBasicBlockEnd(MBB);
   }
 
-  // If the last instruction was a prolog label, then we have a situation where
-  // we emitted a prolog but no function body. This results in the ending prolog
-  // label equaling the end of function label and an invalid "row" in the
-  // FDE. We need to emit a noop in this situation so that the FDE's rows are
-  // valid.
-  bool RequiresNoop = LastMI && LastMI->isCFIInstruction();
-
   // If the function is empty and the object file uses .subsections_via_symbols,
   // then we need to emit *something* to the function body to prevent the
   // labels from collapsing together.  Just emit a noop.
-  if ((MAI->hasSubsectionsViaSymbols() && !HasAnyRealCode) || RequiresNoop) {
+  if ((MAI->hasSubsectionsViaSymbols() && !HasAnyRealCode)) {
     MCInst Noop;
     TM.getSubtargetImpl()->getInstrInfo()->getNoopForMachoTarget(Noop);
-    if (Noop.getOpcode()) {
-      OutStreamer.AddComment("avoids zero-length function");
-      OutStreamer.EmitInstruction(Noop, getSubtargetInfo());
-    } else  // Target not mc-ized yet.
-      OutStreamer.EmitRawText(StringRef("\tnop\n"));
+    OutStreamer.AddComment("avoids zero-length function");
+    OutStreamer.EmitInstruction(Noop, getSubtargetInfo());
   }
 
   const Function *F = MF->getFunction();
diff --git a/llvm/lib/CodeGen/TargetInstrInfo.cpp b/llvm/lib/CodeGen/TargetInstrInfo.cpp
index 277073f0c8a..ab45f89a628 100644
--- a/llvm/lib/CodeGen/TargetInstrInfo.cpp
+++ b/llvm/lib/CodeGen/TargetInstrInfo.cpp
@@ -372,6 +372,10 @@ static const TargetRegisterClass *canFoldCopy(const MachineInstr *MI,
   return nullptr;
 }
 
+void TargetInstrInfo::getNoopForMachoTarget(MCInst &NopInst) const {
+  llvm_unreachable("Not a MachO target");
+}
+
 bool TargetInstrInfo::
 canFoldMemoryOperand(const MachineInstr *MI,
                      const SmallVectorImpl<unsigned> &Ops) const {