Consider this function:

void foo() { __builtin_unreachable(); }

It will output the following on Darwin X86:

_func1:
Leh_func_begin0:
        pushq %rbp
Ltmp0:
        movq %rsp, %rbp
Ltmp1:
Leh_func_end0:

This prolog adds a new Call Frame Information (CFI) row to the FDE with an
address that is not within the address range of the code it describes -- part is
equal to the end of the function -- and therefore results in an invalid EH
frame. If we emit a nop in this situation, then the CFI row is now within the
address range.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@108568 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -599,12 +599,15 @@ void AsmPrinter::EmitFunctionBody() {
   
   // Print out code for the function.
   bool HasAnyRealCode = false;
+  const MachineInstr *LastMI = 0;
   for (MachineFunction::const_iterator I = MF->begin(), E = MF->end();
        I != E; ++I) {
     // Print a label for the basic block.
     EmitBasicBlockStart(I);
     for (MachineBasicBlock::const_iterator II = I->begin(), IE = I->end();
          II != IE; ++II) {
+      LastMI = II;
+
       // Print the assembly for the instruction.
       if (!II->isLabel() && !II->isImplicitDef() && !II->isKill() &&
           !II->isDebugValue()) {
@@ -653,10 +656,17 @@ void AsmPrinter::EmitFunctionBody() {
     }
   }
 
+  // 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->getOpcode()==TargetOpcode::PROLOG_LABEL;
+
   // 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) {
+  if ((MAI->hasSubsectionsViaSymbols() && !HasAnyRealCode) || RequiresNoop) {
     MCInst Noop;
     TM.getInstrInfo()->getNoopForMachoTarget(Noop);
     if (Noop.getOpcode()) {

test/CodeGen/PowerPC/2008-01-25-EmptyFunction.ll

@@ -1,8 +0,0 @@
-; RUN: llc < %s -march=ppc32 | grep nop
-target triple = "powerpc-apple-darwin8"
-
-
-define void @bork() noreturn nounwind  {
-entry:
-        unreachable
-}

test/CodeGen/PowerPC/empty-functions.ll

@@ -0,0 +1,12 @@
+; RUN: llc < %s -mtriple=powerpc-apple-darwin | FileCheck -check-prefix=CHECK-NO-FP %s
+; RUN: llc < %s -mtriple=powerpc-apple-darwin -disable-fp-elim | FileCheck -check-prefix=CHECK-FP %s
+
+define void @func() {
+entry:
+  unreachable
+}
+; CHECK-NO-FP:     _func:
+; CHECK-NO-FP:     nop
+
+; CHECK-FP:      _func:
+; CHECK-FP:      nop

test/CodeGen/X86/2008-01-25-EmptyFunction.ll

@@ -1,8 +0,0 @@
-; RUN: llc < %s -march=x86 | grep nop
-target triple = "i686-apple-darwin8"
-
-
-define void @bork() noreturn nounwind  {
-entry:
-        unreachable
-}

test/CodeGen/X86/empty-functions.ll

@@ -0,0 +1,15 @@
+; RUN: llc < %s -mtriple=x86_64-apple-darwin | FileCheck -check-prefix=CHECK-NO-FP %s
+; RUN: llc < %s -mtriple=x86_64-apple-darwin -disable-fp-elim | FileCheck -check-prefix=CHECK-FP %s
+
+define void @func() {
+entry:
+  unreachable
+}
+; CHECK-NO-FP:     _func:
+; CHECK-NO-FP-NOT: movq %rsp, %rbp
+; CHECK-NO-FP:     nop
+
+; CHECK-FP:      _func:
+; CHECK-FP:      movq %rsp, %rbp
+; CHECK-FP-NEXT: Ltmp1:
+; CHECK-FP:      nop