[ThinLTO] - Fix for "ThinLTO inlines variables that should be discarded".

[grimar]

Summary

This fixes PR36187.

Patch teaches ThinLTO to drop non-prevailing variables,
just like we recently did for functions (in r323633).

[tejohnson]

LGTM with some comment requests below.

lib/LTO/LTOBackend.cpp

410

add a FIXME comment somewhere here that we eventually want to drop dead aliases, but that this needs support in convertToDeclaration. Can you also add a FIXME into convertToDeclaration that alias support is needed for dropDeadSymbols to behave correctly for aliases? It would also be great to file a bug to track the alias handling, similar to the bug you filed for global variables for this patch.

411

add comment that this is walking functions (when investigating this I had to go look at Module.h to confirm what is returned by the default iterator).

[grimar]

lib/LTO/LTOBackend.cpp

410

I added comment to dropDeadSymbols before commit.
Will try to prepare testcase, fill the bug and add the second comment in a follow up.

lib/LTO/LTOBackend.cpp

splitCodeGen(C, TM.get(), AddStream, ParallelCodeGenParallelismLevel,
                  std::move(Mod));
   }
   finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
   return Error::success();
 }
 
 static void dropDeadSymbols(Module &Mod, const GVSummaryMapTy &DefinedGlobals,
                             const ModuleSummaryIndex &Index) {
-  for (auto &GV : Mod) {
+  auto MaybeDrop = [&](GlobalValue &GV) {
     auto It = DefinedGlobals.find(GV.getGUID());
-    if (It == DefinedGlobals.end())
-      continue;
+    if (It != DefinedGlobals.end())
       if (!Index.isGlobalValueLive(It->second))
         convertToDeclaration(GV);
-  }
+  };
+
+  for (auto &GV : Mod)
+    MaybeDrop(GV);
+  for (auto &GV : Mod.globals())
+    MaybeDrop(GV);
 }
 
 Error lto::thinBackend(Config &Conf, unsigned Task, AddStreamFn AddStream,
                        Module &Mod, const ModuleSummaryIndex &CombinedIndex,
                        const FunctionImporter::ImportMapTy &ImportList,
                        const GVSummaryMapTy &DefinedGlobals,
                        MapVector<StringRef, BitcodeModule> &ModuleMap) {
   Expected<const Target *> TOrErr = initAndLookupTarget(Conf, Mod);

test/LTO/Resolution/X86/not-prevailing-variables.ll

+; RUN: opt -module-summary %s -o %t1.o
+; RUN: llvm-lto2 run -save-temps -o %t2.o %t1.o      \
+; RUN:   -r %t1.o,testVar1,plx -r %t1.o,testVar2,plx \
+; RUN:   -r %t1.o,var1,pl -r %t1.o,var2,lx
+
+; Test contains two variables: var1 and var2.
+; var2 is not prevailing and here we check it is not inlined.
+
+; Check 'var2' was not inlined.
+; RUN: llvm-objdump -d %t2.o.1 | FileCheck %s
+; CHECK:      testVar1:
+; CHECK-NEXT:   movl $10, %eax
+; CHECK-NEXT:   retq
+; CHECK:      testVar2:
+; CHECK-NEXT:   movl  (%rip), %eax
+; CHECK-NEXT:   retq
+
+; Check 'var2' is undefined.
+; RUN: llvm-readelf --symbols %t2.o.1 | FileCheck %s --check-prefix=UND
+; UND: NOTYPE  GLOBAL DEFAULT UND var2
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@var1 = global i32 10, align 4
+define i32 @testVar1() {
+  %1 = load i32, i32* @var1, align 4
+  ret i32 %1
+}
+
+@var2 = global i32 11, align 4
+define i32 @testVar2() {
+  %1 = load i32, i32* @var2, align 4
+  ret i32 %1
+}

test/tools/gold/X86/global_with_section.ll

 target triple = "x86_64-unknown-linux-gnu"
 
 ; We should not internalize @var_with_section due to section
 ; CHECK-DAG: @var_with_section = dso_local global i32 0, section "some_section"
 @var_with_section = global i32 0, section "some_section"
 
 ; Confirm via a variable with a non-C identifier section that we are getting
 ; the expected internalization.
-; CHECK-DAG: @var_with_nonC_section = internal global i32 0, section ".nonCsection"
+; CHECK-REGULARLTO-DAG: @var_with_nonC_section = internal global i32 0, section ".nonCsection"
+; Check we dropped definition of dead variable.
+; CHECK-THINLTO-DAG: @var_with_nonC_section = external dso_local global i32, section ".nonCsection"
 @var_with_nonC_section = global i32 0, section ".nonCsection"
 
 ; We should not internalize @deadfunc_with_section due to section
 ; CHECK-DAG: define dso_local void @deadfunc_with_section() section "some_other_section"
 define void @deadfunc_with_section() section "some_other_section" {
   call void @deadfunc2_called_from_section()
   ret void
 }