[MC] Allow Stackmap sections after DWARF in MachO.

[lhames]

Summary

Mixing stackmaps and DWARF in a single file triggers an assertion in
MCMachOStreamer as stackmap sections are emitted in AsmPrinter::emitEndOfAsmFile
after the DWARF sections have already been emitted.

Since it should be safe to emit stackmap sections after DWARF sections this
patch relaxes the assertion to allow that.

[aprantl]

Doing it this way should be fine, too.

[echristo]

In D75836#1912574, @aprantl wrote:

Doing it this way should be fine, too.

Adrian, can you comment on why we want the assert there? Preferably in the code as well :)

[aprantl]

The order in which we switch to the sections initially influences the section ordering in the Mach-O file. I'm having trouble recalling the details right now, but there was *something* that was sensitive to having a debug info section between two non-debuginfo sections, which would result in -g changing the non-debug part of the object file. I'll see if I can dig it up and comment in the code.

[lhames]

Ok. I'm going to land this to unblock people who are hitting this assertion.

Adrian: I think your suggestion was to switch to the stackmap section early, since it is only switching late that triggers this assertion? I didn't end up digging too far into this option because the StackMaps instances are hidden members of the target-specific AsmPrinter subclasses: At the moment there's no way to know whether a stackmap section will be needed in the generic AsmPrinter code.

If emitting or switching to the stackmap section is the right thing to do then I could add an AsmPrinter hook for that.

Alternatively, if switching to a section that is ultimately unused is safe, I could just unconditionally switch to the stackmap section early in the AsmPrinter.

What do you think?

llvm/lib/MC/MCMachOStreamer.cpp

void MCMachOStreamer::ChangeSection(MCSection *Section,
                                     const MCExpr *Subsection) {
   // Change the section normally.
   bool Created = changeSectionImpl(Section, Subsection);
   const MCSectionMachO &MSec = *cast<MCSectionMachO>(Section);
   StringRef SegName = MSec.getSegmentName();
   if (SegName == "__DWARF")
     CreatedADWARFSection = true;
   else if (Created && DWARFMustBeAtTheEnd && !canGoAfterDWARF(MSec))
-    assert(!CreatedADWARFSection && "Creating regular section after DWARF");
+    assert((!CreatedADWARFSection ||
+            Section == getContext().getObjectFileInfo()->getStackMapSection())
+           && "Creating regular section after DWARF");
 
   // Output a linker-local symbol so we don't need section-relative local
   // relocations. The linker hates us when we do that.
   if (LabelSections && !HasSectionLabel[Section] &&
       !Section->getBeginSymbol()) {
     MCSymbol *Label = getContext().createLinkerPrivateTempSymbol();
     Section->setBeginSymbol(Label);
     HasSectionLabel[Section] = true;