gold-plugin: Do not set codegen opt level based on LTO opt level.

[pcc]

Summary

The LTO opt level should not affect the codegen opt level, and indeed
it does not affect it in lld. Ideally the codegen opt level should
be controlled by an IR-level attribute based on the compile-time opt
level, but that hasn't been implemented yet.

[mehdi_amini]

Ideally the codegen opt level should be controlled by an IR-level attribute based on the compile-time opt level, but that hasn't been implemented yet.

Why isn't it a good default in the meantime?
What will the default be?

[mehdi_amini]

(never mind this is an old revision, it just happen to popup in my inbox because of an unrelated update to it)

llvm/tools/gold/gold-plugin.cpp

std::error_code EC =
         sys::fs::openFileForWrite(NewFilename, FD, sys::fs::F_None);
     if (EC)
       message(LDPL_FATAL, "Could not open file %s: %s", NewFilename.c_str(),
               EC.message().c_str());
   }
   return FD;
 }
 
-static CodeGenOpt::Level getCGOptLevel() {
-  switch (options::OptLevel) {
-  case 0:
-    return CodeGenOpt::None;
-  case 1:
-    return CodeGenOpt::Less;
-  case 2:
-    return CodeGenOpt::Default;
-  case 3:
-    return CodeGenOpt::Aggressive;
-  }
-  llvm_unreachable("Invalid optimization level");
-}
-
 /// Parse the thinlto_prefix_replace option into the \p OldPrefix and
 /// \p NewPrefix strings, if it was specified.
 static void getThinLTOOldAndNewPrefix(std::string &OldPrefix,
                                       std::string &NewPrefix) {
   StringRef PrefixReplace = options::thinlto_prefix_replace;
   assert(PrefixReplace.empty() || PrefixReplace.find(";") != StringRef::npos);
   std::tie(OldPrefix, NewPrefix) = PrefixReplace.split(';');
 }
static std::unique_ptr<LTO> createLTO(IndexWriteCallback OnIndexWrite) {
   Conf.Options.RelaxELFRelocations = false;
 
   // Enable function/data sections by default.
   Conf.Options.FunctionSections = true;
   Conf.Options.DataSections = true;
 
   Conf.MAttrs = MAttrs;
   Conf.RelocModel = RelocationModel;
-  Conf.CGOptLevel = getCGOptLevel();
   Conf.DisableVerify = options::DisableVerify;
   Conf.OptLevel = options::OptLevel;
   if (options::Parallelism)
     Backend = createInProcessThinBackend(options::Parallelism);
   if (options::thinlto_index_only) {
     std::string OldPrefix, NewPrefix;
     getThinLTOOldAndNewPrefix(OldPrefix, NewPrefix);
     Backend = createWriteIndexesThinBackend(