[LTO] Suppress emission of empty combined module by default

Summary:
That unless the user requested an output object (--lto-obj-path), the an
unused empty combined module is not emitted.

This changed is helpful for some target (ex. RISCV-V) which encoded the
ABI info in IR module flags (target-abi). Empty unused module has no ABI
info so the linker would get the linking error during merging
incompatible ABIs.

Reviewers: tejohnson, espindola, MaskRay

Subscribers: emaste, inglorion, arichardson, hiraditya, simoncook, MaskRay, steven_wu, dexonsmith, PkmX, dang, lenary, s.egerton, luismarques, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78988

1 files changed, 8 insertions, 4 deletions

diff --git a/llvm/lib/LTO/LTO.cpp b/llvm/lib/LTO/LTO.cpp
index e9eb1d1..ab39a08 100644
--- a/llvm/lib/LTO/LTO.cpp
+++ b/llvm/lib/LTO/LTO.cpp
@@ -603,6 +603,7 @@ Error LTO::addModule(InputFile &Input, unsigned ModI,
   if (LTOInfo->IsThinLTO)
     return addThinLTO(BM, ModSyms, ResI, ResE);
 
+  RegularLTO.EmptyCombinedModule = false;
   Expected<RegularLTOState::AddedModule> ModOrErr =
       addRegularLTO(BM, ModSyms, ResI, ResE);
   if (!ModOrErr)
@@ -1026,10 +1027,13 @@ Error LTO::runRegularLTO(AddStreamFn AddStream) {
         !Conf.PostInternalizeModuleHook(0, *RegularLTO.CombinedModule))
       return Error::success();
   }
-  if (Error Err =
-          backend(Conf, AddStream, RegularLTO.ParallelCodeGenParallelismLevel,
-                  std::move(RegularLTO.CombinedModule), ThinLTO.CombinedIndex))
-    return Err;
+
+  if (!RegularLTO.EmptyCombinedModule || Conf.AlwaysEmitRegularLTOObj) {
+    if (Error Err = backend(
+            Conf, AddStream, RegularLTO.ParallelCodeGenParallelismLevel,
+            std::move(RegularLTO.CombinedModule), ThinLTO.CombinedIndex))
+      return Err;
+  }
 
   return finalizeOptimizationRemarks(std::move(*DiagFileOrErr));
 }