[ThinLTO] Remove imported available externally defs from comdats.

[tejohnson]

Summary

Available externally definitions are considered declarations for the
linker and eventually dropped. As such they are not allowed to be
in comdats. Remove any such imported functions from comdats.

[tejohnson]

Ping

[tejohnson]

Ping.

Thanks,
Teresa

[mehdi_amini]

lib/Linker/LinkModules.cpp

733 ↗

(On Diff #44655)

I don't understand why we need to this?

737 ↗

(On Diff #44655)

This comment looks like it is a bug in the IRMover that you're working around here?

[tejohnson]

lib/Linker/LinkModules.cpp

733 ↗

(On Diff #44655)

Declarations (more generally declarations for the linker, so including available_externally) may not be in a comdat. It is illegal (and checked by Verifier::visitGlobalValue).

737 ↗

(On Diff #44655)

No, it is describing the below assert. I.e. the only declarations for the linker that are in a comdat at this point would be imported definitions, which are available externally. I will try to clarify the comment a bit.

[tejohnson]

• Expand comments to better explain what and what this code is doing.

[mehdi_amini]

LGTM

lib/Linker/LinkModules.cpp

733 ↗

(On Diff #47210)

Ok I didn't know it was illegal :)

llvm/trunk/lib/Linker/LinkModules.cpp

void ThinLTOGlobalProcessing::processGlobalForThinLTO(GlobalValue &GV) {
   if (GV.hasLocalLinkage() &&
       (doPromoteLocalToGlobal(&GV) || isPerformingImport())) {
     GV.setName(getName(&GV));
     GV.setLinkage(getLinkage(&GV));
     if (!GV.hasLocalLinkage())
       GV.setVisibility(GlobalValue::HiddenVisibility);
     if (isModuleExporting())
       NewExportedValues.insert(&GV);
-    return;
-  }
+  } else
     GV.setLinkage(getLinkage(&GV));
+
+  // Remove functions imported as available externally defs from comdats,
+  // as this is a declaration for the linker, and will be dropped eventually.
+  // It is illegal for comdats to contain declarations.
+  auto *GO = dyn_cast_or_null<GlobalObject>(&GV);
+  if (GO && GO->isDeclarationForLinker() && GO->hasComdat()) {
+    // The IRMover should not have placed any imported declarations in
+    // a comdat, so the only declaration that should be in a comdat
+    // at this point would be a definition imported as available_externally.
+    assert(GO->hasAvailableExternallyLinkage() &&
+           "Expected comdat on definition (possibly available external)");
+    GO->setComdat(nullptr);
+  }
 }
 
 void ThinLTOGlobalProcessing::processGlobalsForThinLTO() {
   for (GlobalVariable &GV : M.globals())
     processGlobalForThinLTO(GV);
   for (Function &SF : M)
     processGlobalForThinLTO(SF);
   for (GlobalAlias &GA : M.aliases())

llvm/trunk/test/Linker/Inputs/funcimport_comdat.ll

+define i32 @main() #0 {
+entry:
+  ret i32 0
+}

llvm/trunk/test/Linker/funcimport_comdat.ll

+; Do setup work for all below tests: generate bitcode and combined index
+; RUN: llvm-as -function-summary %s -o %t.bc
+; RUN: llvm-as -function-summary %p/Inputs/funcimport_comdat.ll -o %t2.bc
+; RUN: llvm-lto -thinlto -o %t3 %t.bc %t2.bc
+
+; Ensure linking of comdat containing external linkage global and function
+; removes the imported available_externally defs from comdat.
+; RUN: llvm-link %t2.bc -functionindex=%t3.thinlto.bc -import=comdat1_func1:%t.bc -S | FileCheck %s --check-prefix=IMPORTCOMDAT
+; IMPORTCOMDAT-NOT: $comdat1 = comdat any
+; IMPORTCOMDAT-NOT: comdat($comdat1)
+
+; Ensure linking of comdat containing internal linkage function with alias
+; removes the imported and promoted available_externally defs from comdat.
+; RUN: llvm-link %t2.bc -functionindex=%t3.thinlto.bc -import=comdat2_func1:%t.bc -S | FileCheck %s --check-prefix=IMPORTCOMDAT2
+; IMPORTCOMDAT2-NOT: $comdat2 = comdat any
+; IMPORTCOMDAT2-NOT: comdat($comdat2)
+
+$comdat1 = comdat any
+@comdat1_glob = global i32 0, comdat($comdat1)
+define void @comdat1_func1() comdat($comdat1) {
+  ret void
+}
+
+$comdat2 = comdat any
+@comdat2_alias = alias void (), void ()* @comdat2_func1
+define internal void @comdat2_func1() comdat($comdat2) {
+  ret void
+}