[LTO][Legacy] Update TargetOption after parsing debug options

[steven_wu]

Summary

The current libLTO implementation sets up the TargetOption for the
CodeGenerator when the CodeGenerator is created. That is before the
codegen commandline options are parsed because lto_codegen_debug_options
API takes CodeGenerator as a parameter. That causes the codegen options
(e.g. -tailcallopt) passing through lto_codegen_debug_options has no effect.
Fix the issue by updating TargetOptions after parsing commandline
options.

rdar://problem/53595173

[tejohnson]

Can you add a test case?

[steven_wu]

Implementation specific to lto.cpp is really hard to write a test. The TargetOptions are cl::opt bring in through "llvm/CodeGen/CommandFlags.inc" so we need a client for libLTO to test this (which we never had). Let me know if you have any suggestions.

[tejohnson]

I just discovered that there was a nearly identical patch to do this a few years ago, which generated a bunch of discussion and it seems that there was an agreement not to do this (relating to not exposing internal options through the C API, but I haven't parsed it in detail yet). See https://reviews.llvm.org/D19015

However, note that only about the first half of the discussion is on Phabricator, the other half was via email. Search for D19015 here: http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160411/thread.html

Adding @mehdi_amini and @probinson here since they were involved in the earlier patch discussion.

[steven_wu]

Interesting. I did some digging through bug report as well because I am curious why this is only reported now but didn't find anything.

I am fine with not fixing this. If someone really want a certain option, we can add a new LTO interface and linker flag for them. I guess lld or gold doesn't really have this problem since they are probably just set the TargetOption directly.

llvm/tools/lto/lto.cpp

 void lto_codegen_add_must_preserve_symbol(lto_code_gen_t cg,
                                           const char *symbol) {
   unwrap(cg)->addMustPreserveSymbol(symbol);
 }
 
 static void maybeParseOptions(lto_code_gen_t cg) {
   if (!parsedOptions) {
     unwrap(cg)->parseCodeGenDebugOptions();
+    // update the TargetOptions after parsing the debug options.
+    unwrap(cg)->setTargetOptions(InitTargetOptionsFromCodeGenFlags());
     lto_add_attrs(cg);
     parsedOptions = true;
   }
 }
 
 bool lto_codegen_write_merged_modules(lto_code_gen_t cg, const char *path) {
   maybeParseOptions(cg);
   return !unwrap(cg)->writeMergedModules(path);