[lld-macho] Fix LOH parsing segfault

[int3]

Summary

advanceSubsection() didn't account for the possibility that a section
could have no subsections.

[int3]

update

[int3]

more detailed comment

[thakis]

thanks!

lld/MachO/InputFiles.cpp

auto updateAddr = [&]() {
     subsectionBase = (*section)->addr + subsection->offset;
     subsectionEnd = subsectionBase + subsection->isec->getSize();
   };
 
   auto advanceSubsection = [&]() {
     if (section == sections.end())
       return;
     ++subsection;
-    if (subsection == (*section)->subsections.end()) {
+    while (subsection == (*section)->subsections.end()) {
       ++section;
       if (section == sections.end())
         return;
       subsection = (*section)->subsections.begin();
     }
   };
 
   updateAddr();
   auto hintStart = optimizationHints.begin();
   for (auto hintEnd = hintStart, end = optimizationHints.end(); hintEnd != end;
        ++hintEnd) {
     if (hintEnd->offset0 >= subsectionEnd) {
       subsection->isec->optimizationHints =
           ArrayRef<OptimizationHint>(&*hintStart, hintEnd - hintStart);
 
       hintStart = hintEnd;
       while (hintStart->offset0 >= subsectionEnd) {
         advanceSubsection();
         if (section == sections.end())
           break;
         updateAddr();
+        assert(hintStart->offset0 >= subsectionBase);
       }
     }
 
     hintEnd->offset0 -= subsectionBase;
     for (int i = 0, count = expectedArgCount(hintEnd->type); i < count - 1;
          ++i) {
       if (LLVM_UNLIKELY(
               hintEnd->delta[i] < -static_cast<int64_t>(hintEnd->offset0) ||

lld/test/MachO/loh-parsing.s

+# REQUIRES: aarch64
+
+# RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %s -o %t.o
+# RUN: %lld -arch arm64 -dylib %t.o -o /dev/null
+
+## Check that we parse the LOH & match it to its referent sections correctly,
+## even when there are other subsections that don't get parsed as regular
+## sections. (We would previously segfault.)
+## __debug_info is one such section that gets special-case handling.
+
+.text
+_foo:
+
+.section __DWARF,__debug_info,regular,debug
+
+## __StaticInit occurs after __debug_info in the input object file, so the
+## LOH-matching code will have to "walk" past __debug_info while searching for
+## __StaticInit. Thus this verifies that we can skip past __debug_info
+## correctly.
+.section __TEXT,__StaticInit
+L1:  adrp  x1, _foo@PAGE
+L2:  ldr   x1, [x1, _foo@PAGEOFF]
+
+.loh AdrpLdr L1, L2