[LTO] Don't apply LTOPostLink module flag during writeMergedModule

For `ld64` which uses legacy LTOCodeGenerator, it relies on
writeMergedModule to perform `ld -r` (generates a linked object file).
If all the inputs to `ld -r` is fullLTO bitcode, `ld64` will linked the
bitcode module, internalize all the symbols and write out another
fullLTO bitcode object file. This bitcode file doesn't have all the
bitcode inputs and it should not have LTOPostLink module flag. It will
also cause error when this bitcode object file is linked with other LTO
object file.
Fix the issue by not applying LTOPostLink flag during writeMergedModule
function. The flag should only be added when all the bitcode are linked
and ready to be optimized.

rdar://problem/58462798

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D84789

Author: cachemeifyoucan

llvm/lib/LTO/LTOCodeGenerator.cpp

@@ -466,8 +466,6 @@ void LTOCodeGenerator::applyScopeRestrictions() {
 
   internalizeModule(*MergedModule, mustPreserveGV);
 
-  MergedModule->addModuleFlag(Module::Error, "LTOPostLink", 1);
-
   ScopeRestrictionsDone = true;
 }
 
@@ -559,6 +557,9 @@ bool LTOCodeGenerator::optimize(bool DisableVerify, bool DisableInline,
   // Mark which symbols can not be internalized
   this->applyScopeRestrictions();
 
+  // Write LTOPostLink flag for passes that require all the modules.
+  MergedModule->addModuleFlag(Module::Error, "LTOPostLink", 1);
+
   // Instantiate the pass manager to organize the passes.
   legacy::PassManager passes;
 

llvm/test/LTO/ARM/lto-linking-metadata.ll

@@ -1,7 +1,8 @@
 ; RUN: opt %s -o %t1.bc
 
-; RUN: llvm-lto %t1.bc -o %t1.save.opt -save-merged-module -O1 --exported-symbol=foo
+; RUN: llvm-lto %t1.bc -o %t1.save.opt -save-linked-module -save-merged-module -O1 --exported-symbol=foo
 ; RUN: llvm-dis < %t1.save.opt.merged.bc | FileCheck %s
+; RUN: llvm-dis < %t1.save.opt.linked.bc | FileCheck %s --check-prefix=CHECK-LINKED
 
 ; RUN: llvm-lto2 run %t1.bc -o %t.out.o -save-temps \
 ; RUN:     -r=%t1.bc,foo,pxl
@@ -17,3 +18,6 @@ entry:
 
 ; CHECK: !llvm.module.flags = !{[[MD_NUM:![0-9]+]]}
 ; CHECK: [[MD_NUM]] = !{i32 1, !"LTOPostLink", i32 1}
+
+; CHECK-LINKED: @foo
+; CHECK-LINKED-NOT: LTOPostLink

llvm/tools/llvm-lto/llvm-lto.cpp

@@ -181,6 +181,10 @@ static cl::opt<std::string> ThinLTOGeneratedObjectsDir(
     cl::desc("Save ThinLTO generated object files using filenames created in "
              "the given directory."));
 
+static cl::opt<bool> SaveLinkedModuleFile(
+    "save-linked-module", cl::init(false),
+    cl::desc("Write linked LTO module to file before optimize"));
+
 static cl::opt<bool>
     SaveModuleFile("save-merged-module", cl::init(false),
                    cl::desc("Write merged LTO module to file before CodeGen"));
@@ -1029,6 +1033,15 @@ int main(int argc, char **argv) {
     CodeGen.setFileType(FT.getValue());
 
   if (!OutputFilename.empty()) {
+    if (SaveLinkedModuleFile) {
+      std::string ModuleFilename = OutputFilename;
+      ModuleFilename += ".linked.bc";
+      std::string ErrMsg;
+
+      if (!CodeGen.writeMergedModules(ModuleFilename))
+        error("writing linked module failed.");
+    }
+
     if (!CodeGen.optimize(DisableVerify, DisableInline, DisableGVNLoadPRE,
                           DisableLTOVectorization)) {
       // Diagnostic messages should have been printed by the handler.