[Bitcode] Avoid duplicating linker option when upgrading

[steven_wu]

Summary

The upgrading path from old ModuleFlag based linker options to the new
NamedMetadata based linker option in in materializeMetadata() which gets
called once for the module and once for every GV. The linker options are
getting dup'ed every time and it can create massive amount of the linker
options in the object file that gets created from old bitcode. Fix the
problem by checking if the new option exists or not before upgrade
again.

rdar://64543389

[steven_wu]

Add comments in the tests.

[steven_wu]

Ping

[steven_wu]

rebase the patch and ping.

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

for (uint64_t BitPos : DeferredMetadataInfo) {
     // Move the bit stream to the saved position.
     if (Error JumpFailed = Stream.JumpToBit(BitPos))
       return JumpFailed;
     if (Error Err = MDLoader->parseModuleMetadata())
       return Err;
   }
 
   // Upgrade "Linker Options" module flag to "llvm.linker.options" module-level
-  // metadata.
+  // metadata. Only upgrade if the new option doesn't exist to avoid upgrade
+  // multiple times.
+  if (!TheModule->getNamedMetadata("llvm.linker.options")) {
     if (Metadata *Val = TheModule->getModuleFlag("Linker Options")) {
       NamedMDNode *LinkerOpts =
           TheModule->getOrInsertNamedMetadata("llvm.linker.options");
       for (const MDOperand &MDOptions : cast<MDNode>(Val)->operands())
         LinkerOpts->addOperand(cast<MDNode>(MDOptions));
     }
+  }
 
   DeferredMetadataInfo.clear();
   return Error::success();
 }
 
 void BitcodeReader::setStripDebugInfo() { StripDebugInfo = true; }
 
 /// When we see the block for a function body, remember where it is and then

llvm/test/Bitcode/upgrade-linker-options-2.ll

+;; Test upgrade linker option doesn't create duplicated linker options.
+;; Inputs is generated from IR and checked in as bitcode as it will get rejected by verifier.
+;; define void @test() {
+;;   ret void
+;; }
+;; !llvm.module.flags = !{!0}
+;; !0 = !{i32 6, !"Linker Options", !1}
+;; !1 = !{!2}
+;; !2 = !{!"-framework", !"Foundation"}
+
+; RUN: llvm-dis %S/Inputs/linker-options.bc -o - | FileCheck %s
+; CHECK: !llvm.linker.options = !{!2}