Pull in r321986 from upstream lld trunk (by James Henderson):

[ELF] Compress debug sections after assignAddresses and support
  custom layout

  Previously, in r320472, I moved the calculation of section offsets
  and sizes for compressed debug sections into maybeCompress, which
  happens before assignAddresses, so that the compression had the
  required information. However, I failed to take account of
  relocations that patch such sections. This had two effects:

  1. A race condition existed when a debug section referred to a
     different debug section (see PR35788).
  2. References to symbols in non-debug sections would be patched
     incorrectly.  This is because the addresses of such symbols are not
     calculated until after assignAddresses (this was a partial
     regression caused by r320472, but they could still have been
     broken before, in the event that a custom layout was used in a
     linker script).

  assignAddresses does not need to know about the output section size
  of non-allocatable sections, because they do not affect the value of
  Dot. This means that there is no longer a reason not to support
  custom layout of compressed debug sections, as far as I'm aware.
  These two points allow for delaying when maybeCompress can be called,
  removing the need for the loop I previously added to calculate the
  section size, and therefore the race condition. Furthermore, by
  delaying, we fix the issues of relocations getting incorrect symbol
  values, because they have now all been finalized.

This should fix thread race conditions when linking parts of ruby24.

contrib/llvm/tools/lld/ELF/LinkerScript.cpp

@@ -669,11 +669,6 @@ void LinkerScript::assignOffsets(OutputSection *Sec) {
 
   switchTo(Sec);
 
-  // We do not support custom layout for compressed debug sectons.
-  // At this point we already know their size and have compressed content.
-  if (Ctx->OutSec->Flags & SHF_COMPRESSED)
-    return;
-
   // The Size previously denoted how many InputSections had been added to this
   // section, and was used for sorting SHF_LINK_ORDER sections. Reset it to
   // compute the actual size value.

contrib/llvm/tools/lld/ELF/OutputSections.cpp

@@ -183,15 +183,6 @@ template <class ELFT> void OutputSection::maybeCompress() {
       !Name.startswith(".debug_"))
     return;
 
-  // Calculate the section offsets and size pre-compression.
-  Size = 0;
-  for (BaseCommand *Cmd : SectionCommands)
-    if (auto *ISD = dyn_cast<InputSectionDescription>(Cmd))
-      for (InputSection *IS : ISD->Sections) {
-        IS->OutSecOff = alignTo(Size, IS->Alignment);
-        this->Size = IS->OutSecOff + IS->getSize();
-      }
-
   // Create a section header.
   ZDebugHeader.resize(sizeof(Elf_Chdr));
   auto *Hdr = reinterpret_cast<Elf_Chdr *>(ZDebugHeader.data());

contrib/llvm/tools/lld/ELF/Writer.cpp

@@ -427,13 +427,14 @@ template <class ELFT> void Writer<ELFT>::run() {
   if (errorCount())
     return;
 
+  Script->assignAddresses();
+
   // If -compressed-debug-sections is specified, we need to compress
   // .debug_* sections. Do it right now because it changes the size of
   // output sections.
   parallelForEach(OutputSections,
                   [](OutputSection *Sec) { Sec->maybeCompress<ELFT>(); });
 
-  Script->assignAddresses();
   Script->allocateHeaders(Phdrs);
 
   // Remove empty PT_LOAD to avoid causing the dynamic linker to try to mmap a