[MC] [Win64EH] Fill in FuncletOrFuncEnd if missing

This can happen e.g. for code that declare .seh_proc/.seh_endproc
in assembly, or for code that use .seh_handlerdata (which triggers
the unwind info to be emitted before the end of the function).

The TextSection field must be made non-const to be able to use it
with Streamer.SwitchSection().

Differential Revision: https://reviews.llvm.org/D86528

llvm/include/llvm/MC/MCWinEH.h

@@ -36,7 +36,7 @@ struct FrameInfo {
   const MCSymbol *Function = nullptr;
   const MCSymbol *PrologEnd = nullptr;
   const MCSymbol *Symbol = nullptr;
-  const MCSection *TextSection = nullptr;
+  MCSection *TextSection = nullptr;
 
   bool HandlesUnwind = false;
   bool HandlesExceptions = false;

llvm/lib/MC/MCStreamer.cpp

@@ -692,6 +692,8 @@ void MCStreamer::EmitWinCFIEndProc(SMLoc Loc) {
 
   MCSymbol *Label = emitCFILabel();
   CurFrame->End = Label;
+  if (!CurFrame->FuncletOrFuncEnd)
+    CurFrame->FuncletOrFuncEnd = CurFrame->End;
 }
 
 void MCStreamer::EmitWinCFIFuncletOrFuncEnd(SMLoc Loc) {

llvm/lib/MC/MCWin64EH.cpp

@@ -525,9 +525,7 @@ static void ARM64EmitUnwindInfo(MCStreamer &streamer, WinEH::FrameInfo *info) {
 
   int64_t RawFuncLength;
   if (!info->FuncletOrFuncEnd) {
-    // FIXME: This is very wrong; we emit SEH data which covers zero bytes
-    // of code. But otherwise test/MC/AArch64/seh.s crashes.
-    RawFuncLength = 0;
+    report_fatal_error("FuncletOrFuncEnd not set");
   } else {
     // FIXME: GetAbsDifference tries to compute the length of the function
     // immediately, before the whole file is emitted, but in general
@@ -702,6 +700,16 @@ void llvm::Win64EH::ARM64UnwindEmitter::Emit(MCStreamer &Streamer) const {
 
 void llvm::Win64EH::ARM64UnwindEmitter::EmitUnwindInfo(
     MCStreamer &Streamer, WinEH::FrameInfo *info) const {
+  // Called if there's an .seh_handlerdata directive before the end of the
+  // function. This forces writing the xdata record already here - and
+  // in this case, the function isn't actually ended already, but the xdata
+  // record needs to know the function length. In these cases, if the funclet
+  // end hasn't been marked yet, the xdata function length won't cover the
+  // whole function, only up to this point.
+  if (!info->FuncletOrFuncEnd) {
+    Streamer.SwitchSection(info->TextSection);
+    info->FuncletOrFuncEnd = Streamer.emitCFILabel();
+  }
   // Switch sections (the static function above is meant to be called from
   // here and from Emit().
   MCSection *XData = Streamer.getAssociatedXDataSection(info->TextSection);

llvm/test/MC/AArch64/seh.s

@@ -46,6 +46,12 @@
 // CHECK-NEXT:   }
 // CHECK-NEXT: ]
 
+// CHECK-NEXT: UnwindInformation [
+// CHECK-NEXT:   RuntimeFunction {
+// CHECK-NEXT:     Function: func
+// CHECK-NEXT:     ExceptionRecord: .xdata
+// CHECK-NEXT:     ExceptionData {
+// CHECK-NEXT:       FunctionLength: 8
 
     .text
     .globl func