--- a/lld/ELF/OutputSections.cpp	Mon May 25 11:55:54 2020 +0900
+++ b/lld/ELF/OutputSections.cpp	Tue Jun 08 06:07:14 2021 +0900
@@ -20,7 +20,9 @@
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Support/Parallel.h"
 #include "llvm/Support/SHA1.h"
+#include "llvm/Support/TimeProfiler.h"
 #include <regex>
+#include <unordered_set>
 
 using namespace llvm;
 using namespace llvm::dwarf;
@@ -77,10 +79,14 @@
 // to be allocated for nobits sections. Other ones don't require
 // any special treatment on top of progbits, so there doesn't
 // seem to be a harm in merging them.
+//
+// NOTE: clang since rL252300 emits SHT_X86_64_UNWIND .eh_frame sections. Allow
+// them to be merged into SHT_PROGBITS .eh_frame (GNU as .cfi_*).
 static bool canMergeToProgbits(unsigned type) {
   return type == SHT_NOBITS || type == SHT_PROGBITS || type == SHT_INIT_ARRAY ||
          type == SHT_PREINIT_ARRAY || type == SHT_FINI_ARRAY ||
-         type == SHT_NOTE;
+         type == SHT_NOTE ||
+         (type == SHT_X86_64_UNWIND && config->emachine == EM_X86_64);
 }
 
 // Record that isec will be placed in the OutputSection. isec does not become
@@ -280,6 +286,8 @@
       !name.startswith(".debug_"))
     return;
 
+  llvm::TimeTraceScope timeScope("Compress debug sections");
+
   // Create a section header.
   zDebugHeader.resize(sizeof(Elf_Chdr));
   auto *hdr = reinterpret_cast<Elf_Chdr *>(zDebugHeader.data());
@@ -376,6 +384,15 @@
   // provides signature of the section group.
   ArrayRef<Symbol *> symbols = section->file->getSymbols();
   os->info = in.symTab->getSymbolIndex(symbols[section->info]);
+
+  // Some group members may be combined or discarded, so we need to compute the
+  // new size. The content will be rewritten in InputSection::copyShtGroup.
+  std::unordered_set<uint32_t> seen;
+  ArrayRef<InputSectionBase *> sections = section->file->getSections();
+  for (const uint32_t &idx : section->getDataAs<uint32_t>().slice(1))
+    if (OutputSection *osec = sections[read32(&idx)]->getOutputSection())
+      seen.insert(osec->sectionIndex);
+  os->size = (1 + seen.size()) * sizeof(uint32_t);
 }
 
 void OutputSection::finalize() {
@@ -401,7 +418,11 @@
   if (!config->copyRelocs || (type != SHT_RELA && type != SHT_REL))
     return;
 
-  if (isa<SyntheticSection>(first))
+  // Skip if 'first' is synthetic, i.e. not a section created by --emit-relocs.
+  // Normally 'type' was changed by 'first' so 'first' should be non-null.
+  // However, if the output section is .rela.dyn, 'type' can be set by the empty
+  // synthetic .rela.plt and first can be null.
+  if (!first || isa<SyntheticSection>(first))
     return;
 
   link = in.symTab->getParent()->sectionIndex;
@@ -430,19 +451,19 @@
   return std::regex_match(s.begin(), s.end(), re);
 }
 
-// .ctors and .dtors are sorted by this priority from highest to lowest.
-//
-//  1. The section was contained in crtbegin (crtbegin contains
-//     some sentinel value in its .ctors and .dtors so that the runtime
-//     can find the beginning of the sections.)
+// .ctors and .dtors are sorted by this order:
 //
-//  2. The section has an optional priority value in the form of ".ctors.N"
-//     or ".dtors.N" where N is a number. Unlike .{init,fini}_array,
-//     they are compared as string rather than number.
+// 1. .ctors/.dtors in crtbegin (which contains a sentinel value -1).
+// 2. The section is named ".ctors" or ".dtors" (priority: 65536).
+// 3. The section has an optional priority value in the form of ".ctors.N" or
+//    ".dtors.N" where N is a number in the form of %05u (priority: 65535-N).
+// 4. .ctors/.dtors in crtend (which contains a sentinel value 0).
 //
-//  3. The section is just ".ctors" or ".dtors".
-//
-//  4. The section was contained in crtend, which contains an end marker.
+// For 2 and 3, the sections are sorted by priority from high to low, e.g.
+// .ctors (65536), .ctors.00100 (65436), .ctors.00200 (65336).  In GNU ld's
+// internal linker scripts, the sorting is by string comparison which can
+// achieve the same goal given the optional priority values are of the same
+// length.
 //
 // In an ideal world, we don't need this function because .init_array and
 // .ctors are duplicate features (and .init_array is newer.) However, there
@@ -457,13 +478,7 @@
   bool endB = isCrtend(b->file->getName());
   if (endA != endB)
     return endB;
-  StringRef x = a->name;
-  StringRef y = b->name;
-  assert(x.startswith(".ctors") || x.startswith(".dtors"));
-  assert(y.startswith(".ctors") || y.startswith(".dtors"));
-  x = x.substr(6);
-  y = y.substr(6);
-  return x < y;
+  return getPriority(a->name) > getPriority(b->name);
 }
 
 // Sorts input sections by the special rules for .ctors and .dtors.
@@ -475,16 +490,17 @@
   llvm::stable_sort(isd->sections, compCtors);
 }
 
-// If an input string is in the form of "foo.N" where N is a number,
-// return N. Otherwise, returns 65536, which is one greater than the
-// lowest priority.
+// If an input string is in the form of "foo.N" where N is a number, return N
+// (65535-N if .ctors.N or .dtors.N). Otherwise, returns 65536, which is one
+// greater than the lowest priority.
 int elf::getPriority(StringRef s) {
   size_t pos = s.rfind('.');
   if (pos == StringRef::npos)
     return 65536;
-  int v;
-  if (!to_integer(s.substr(pos + 1), v, 10))
-    return 65536;
+  int v = 65536;
+  if (to_integer(s.substr(pos + 1), v, 10) &&
+      (pos == 6 && (s.startswith(".ctors") || s.startswith(".dtors"))))
+    v = 65535 - v;
   return v;
 }