Fix .seh_stackalloc 0

[vadimcn]

Summary

seh_stackalloc 0 is not representable in Win64 SEH info, so emitting it is a bug.

[rnk]

lgtm, I can commit this.

lib/MC/MCStreamer.cpp

518–519

MCStreamer needs a way to report errors.... :(

[rnk]

I'll add a test for this.

[vadimcn]

This was committed by @rnk in r212081.

lib/MC/MCStreamer.cpp

void MCStreamer::EmitWin64EHSetFrame(unsigned Register, unsigned Offset) {
   MCWin64EHInstruction Inst(Win64EH::UOP_SetFPReg, Label, Register, Offset);
   EmitLabel(Label);
   CurFrame->LastFrameInst = CurFrame->Instructions.size();
   CurFrame->Instructions.push_back(Inst);
 }
 
 void MCStreamer::EmitWin64EHAllocStack(unsigned Size) {
   EnsureValidW64UnwindInfo();
+  if (Size == 0)
+    report_fatal_error("Allocation size must be non-zero!");
   if (Size & 7)
     report_fatal_error("Misaligned stack allocation!");
   MCWin64EHUnwindInfo *CurFrame = CurrentW64UnwindInfo;
   MCSymbol *Label = getContext().CreateTempSymbol();
   MCWin64EHInstruction Inst(Label, Size);
   EmitLabel(Label);
   CurFrame->Instructions.push_back(Inst);
 }

lib/Target/X86/X86FrameLowering.cpp

if (HasFP) {
       // will restore SP to (BP - SEHFrameOffset)
       for (const CalleeSavedInfo &Info : MFI->getCalleeSavedInfo()) {
         int offset = MFI->getObjectOffset(Info.getFrameIdx());
         SEHFrameOffset = std::max(SEHFrameOffset, abs(offset));
       }
       SEHFrameOffset += SEHFrameOffset % 16; // ensure alignmant
 
       // This only needs to account for XMM spill slots, GPR slots
-      // are covered by .seh_pushreg's emitted above.
+      // are covered by the .seh_pushreg's emitted above.
+      unsigned Size = SEHFrameOffset - X86FI->getCalleeSavedFrameSize();
+      if (Size) {
         BuildMI(MBB, MBBI, DL, TII.get(X86::SEH_StackAlloc))
-          .addImm(SEHFrameOffset - X86FI->getCalleeSavedFrameSize())
+            .addImm(Size)
             .setMIFlag(MachineInstr::FrameSetup);
+      }
 
       BuildMI(MBB, MBBI, DL, TII.get(X86::SEH_SetFrame))
           .addImm(FramePtr)
           .addImm(SEHFrameOffset)
           .setMIFlag(MachineInstr::FrameSetup);
     } else {
       // SP will be the base register for restoring XMMs
       if (NumBytes) {