[PATCH] D23600: [LTO] Stop always creating and running an LTO compilation if there is not a single LTO object

[Mehdi AMINI via llvm-commits]

mehdi_amini created this revision.
mehdi_amini added a reviewer: tejohnson.
mehdi_amini added subscribers: llvm-commits, pcc.
Herald added a subscriber: mehdi_amini.

I assume there was a use case, so maybe this strawman patch will help
clarifying if it is legit.
In any case the current situation is not legit: a ThinLTO compilation
should not trigger an unexpected full LTO compilation.
Right now, adding a --save-temps option triggers this and makes the
number of output differs.

https://reviews.llvm.org/D23600

Files:
  include/llvm/LTO/LTO.h
  lib/LTO/LTO.cpp

Index: lib/LTO/LTO.cpp
===================================================================
--- lib/LTO/LTO.cpp
+++ lib/LTO/LTO.cpp
@@ -221,8 +221,7 @@
 LTO::RegularLTOState::RegularLTOState(unsigned ParallelCodeGenParallelismLevel,
                                       Config &Conf)
     : ParallelCodeGenParallelismLevel(ParallelCodeGenParallelismLevel),
-      Ctx(Conf), CombinedModule(llvm::make_unique<Module>("ld-temp.o", Ctx)),
-      Mover(*CombinedModule) {}
+      Ctx(Conf) {}
 
 LTO::ThinLTOState::ThinLTOState(ThinBackend Backend) : Backend(Backend) {
   if (!Backend)
@@ -305,8 +304,10 @@
 // Add a regular LTO object to the link.
 Error LTO::addRegularLTO(std::unique_ptr<InputFile> Input,
                          ArrayRef<SymbolResolution> Res) {
-  RegularLTO.HasModule = true;
-
+  if (!CombinedModule) {
+    CombinedModule = llvm::make_unique<Module>("ld-temp.o", Ctx);
+    Mover = llvm::make_unique<IRMover>(*CombinedModule);
+  }
   ErrorOr<std::unique_ptr<object::IRObjectFile>> ObjOrErr =
       IRObjectFile::create(Input->Obj->getMemoryBufferRef(), RegularLTO.Ctx);
   if (!ObjOrErr)
@@ -364,14 +365,6 @@
   SmallPtrSet<GlobalValue *, 8> Used;
   collectUsedGlobalVariables(M, Used, /*CompilerUsed*/ false);
 
-  // We need to initialize the target info for the combined regular LTO module
-  // in case we have no regular LTO objects. In that case we still need to build
-  // it as usual because the client may want to add symbol definitions to it.
-  if (RegularLTO.CombinedModule->getTargetTriple().empty()) {
-    RegularLTO.CombinedModule->setTargetTriple(M.getTargetTriple());
-    RegularLTO.CombinedModule->setDataLayout(M.getDataLayout());
-  }
-
   MemoryBufferRef MBRef = Input->Obj->getMemoryBufferRef();
   ErrorOr<std::unique_ptr<object::ModuleSummaryIndexObjectFile>>
       SummaryObjOrErr =
@@ -405,12 +398,8 @@
 }
 
 Error LTO::run(AddOutputFn AddOutput) {
-  // Invoke regular LTO if there was a regular LTO module to start with,
-  // or if there are any hooks that the linker may have used to add
-  // its own resolved symbols to the combined module.
-  if (RegularLTO.HasModule || Conf.PreOptModuleHook ||
-      Conf.PostInternalizeModuleHook || Conf.PostOptModuleHook ||
-      Conf.PreCodeGenModuleHook)
+  // Invoke regular LTO if there was a regular LTO module to start with.
+  if (RegularLTO.CombinedModule)
     if (auto E = runRegularLTO(AddOutput))
       return E;
   return runThinLTO(AddOutput);
Index: include/llvm/LTO/LTO.h
===================================================================
--- include/llvm/LTO/LTO.h
+++ include/llvm/LTO/LTO.h
@@ -307,7 +307,7 @@
     LTOLLVMContext Ctx;
     bool HasModule = false;
     std::unique_ptr<Module> CombinedModule;
-    IRMover Mover;
+    std::unique_ptr<IRMover> Mover;
   } RegularLTO;
 
   struct ThinLTOState {


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