Fix lld on GCC 5.1 after the C++14 move

[jfb]

Summary

libstdc++ in GCC 5.1 has some bugs. The move to C++14 in D66195 triggered one
such bug caused by the new constexpr support in C++14, and the implementation
doing SFINAE wrong with the comparator to std::stable_sort.

Here's a small repro: https://godbolt.org/z/2QC3-n

The fix is to inline the lambdas directly into the llvm::stable_sort call
instead of erasing them through a std::function. The code is more readable as
well.

[thakis]

Thanks!

lld/ELF/LinkerScript.cpp

337 ↗

(On Diff #215431)

nit: remove, to make Default fall through to None. (No fall through annotation needed for completely empty cases)

[jfb]

• Fall through as suggested.

[ruiu]

LGTM

lld/trunk/ELF/LinkerScript.cpp

for (InputSectionDescription *id : keptSections)
     if (id->filePat.match(filename))
       for (SectionPattern &p : id->sectionPatterns)
         if (p.sectionPat.match(s->name))
           return true;
   return false;
 }
 
 // A helper function for the SORT() command.
-static std::function<bool(InputSectionBase *, InputSectionBase *)>
-getComparator(SortSectionPolicy k) {
-  switch (k) {
-  case SortSectionPolicy::Alignment:
-    return [](InputSectionBase *a, InputSectionBase *b) {
-      // ">" is not a mistake. Sections with larger alignments are placed
-      // before sections with smaller alignments in order to reduce the
-      // amount of padding necessary. This is compatible with GNU.
-      return a->alignment > b->alignment;
-    };
-  case SortSectionPolicy::Name:
-    return [](InputSectionBase *a, InputSectionBase *b) {
-      return a->name < b->name;
-    };
-  case SortSectionPolicy::Priority:
-    return [](InputSectionBase *a, InputSectionBase *b) {
-      return getPriority(a->name) < getPriority(b->name);
-    };
-  default:
-    llvm_unreachable("unknown sort policy");
-  }
-}
-
-// A helper function for the SORT() command.
 static bool matchConstraints(ArrayRef<InputSection *> sections,
                              ConstraintKind kind) {
   if (kind == ConstraintKind::NoConstraint)
     return true;
 
   bool isRW = llvm::any_of(
       sections, [](InputSection *sec) { return sec->flags & SHF_WRITE; });
 
   return (isRW && kind == ConstraintKind::ReadWrite) ||
          (!isRW && kind == ConstraintKind::ReadOnly);
 }
 
 static void sortSections(MutableArrayRef<InputSection *> vec,
                          SortSectionPolicy k) {
-  if (k != SortSectionPolicy::Default && k != SortSectionPolicy::None)
-    llvm::stable_sort(vec, getComparator(k));
+  auto alignmentComparator = [](InputSectionBase *a, InputSectionBase *b) {
+    // ">" is not a mistake. Sections with larger alignments are placed
+    // before sections with smaller alignments in order to reduce the
+    // amount of padding necessary. This is compatible with GNU.
+    return a->alignment > b->alignment;
+  };
+  auto nameComparator = [](InputSectionBase *a, InputSectionBase *b) {
+    return a->name < b->name;
+  };
+  auto priorityComparator = [](InputSectionBase *a, InputSectionBase *b) {
+    return getPriority(a->name) < getPriority(b->name);
+  };
+
+  switch (k) {
+  case SortSectionPolicy::Default:
+  case SortSectionPolicy::None:
+    return;
+  case SortSectionPolicy::Alignment:
+    return llvm::stable_sort(vec, alignmentComparator);
+  case SortSectionPolicy::Name:
+    return llvm::stable_sort(vec, nameComparator);
+  case SortSectionPolicy::Priority:
+    return llvm::stable_sort(vec, priorityComparator);
+  }
 }
 
 // Sort sections as instructed by SORT-family commands and --sort-section
 // option. Because SORT-family commands can be nested at most two depth
 // (e.g. SORT_BY_NAME(SORT_BY_ALIGNMENT(.text.*))) and because the command
 // line option is respected even if a SORT command is given, the exact
 // behavior we have here is a bit complicated. Here are the rules.
 //