[3.9.1] Port "[ThinLTO] Disable importing and other cross-module optis at -O0"

[tejohnson]

Summary

There is no point to importing at -O0, since we won't inline. We should
also disable other cross-module optimizations.

This change avoids 3.9 bug PR30774, which is due to the old DIGlobalVariable
references to GlobalVariable, and only at -O0 since the dangling
reference to a non-imported variable is not cleaned up. This is the
simplest workaround for the issue, which was addressed by reversing the
DIGlobalVariable to GlobalVariable edges (r281284).

Fixes PR30774.

[mehdi_amini]

Adding @tstellar since this is targeting 3.9.1.
(This is a bit particular since it is not a cherry-pick)

test/tools/gold/X86/Inputs/thinlto_funcimport.ll

+; ModuleID = 'global_main.o'
+source_filename = "global_main.c"
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; Function Attrs: nounwind uwtable
+define i32 @main() local_unnamed_addr #0 !dbg !6 {
+  %1 = tail call i32 (...) @foo() #2, !dbg !10
+  ret i32 %1, !dbg !11
+}
+
+declare i32 @foo(...) local_unnamed_addr #1
+
+attributes #0 = { nounwind uwtable "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #1 = { "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #2 = { nounwind }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4}
+!llvm.ident = !{!5}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 3.9.1 ", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
+!1 = !DIFile(filename: "global_main.c", directory: ".")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{!"clang version 3.9.1 "}
+!6 = distinct !DISubprogram(name: "main", scope: !1, file: !1, line: 2, type: !7, isLocal: false, isDefinition: true, scopeLine: 2, isOptimized: true, unit: !0, variables: !2)
+!7 = !DISubroutineType(types: !8)
+!8 = !{!9}
+!9 = !DIBasicType(name: "int", size: 32, align: 32, encoding: DW_ATE_signed)
+!10 = !DILocation(line: 3, column: 10, scope: !6)
+!11 = !DILocation(line: 3, column: 3, scope: !6)

test/tools/gold/X86/thinlto_funcimport.ll

+; Do setup work for all below tests: generate bitcode and combined index
+; RUN: opt -module-summary %s -o %t1.bc
+; RUN: opt -module-summary %p/Inputs/thinlto_funcimport.ll -o %t2.bc
+
+; RUN: %gold -plugin %llvmshlibdir/LLVMgold.so \
+; RUN:    --plugin-opt=save-temps \
+; RUN:    --plugin-opt=thinlto \
+; RUN:    -u main %t1.bc %t2.bc -o %t
+; RUN: llvm-nm %t2.bc.thinlto.o | FileCheck %s --check-prefix=NM
+
+; We shouldn't do any importing at -O0
+; rm -f %t2.bc.3.import.bc
+; RUN: %gold -plugin %llvmshlibdir/LLVMgold.so \
+; RUN:    --plugin-opt=save-temps \
+; RUN:    --plugin-opt=thinlto \
+; RUN:    --plugin-opt=O0 \
+; RUN:    -u main %t1.bc %t2.bc -o %t
+; RUN: llvm-nm %t2.bc.thinlto.o | FileCheck %s --check-prefix=NM
+
+; We shouldn't have imported a reference to global variable foobar at -O0.
+; NM-NOT: U foobar
+
+; ModuleID = 'global.o'
+source_filename = "global.c"
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@foobar = local_unnamed_addr constant i32 10, align 4
+
+; Function Attrs: norecurse nounwind readnone uwtable
+define i32 @foo() local_unnamed_addr #0 !dbg !10 {
+  ret i32 1, !dbg !13
+}
+
+attributes #0 = { norecurse nounwind readnone uwtable "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!7, !8}
+!llvm.ident = !{!9}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 3.9.1 ", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, globals: !3)
+!1 = !DIFile(filename: "global.c", directory: ".")
+!2 = !{}
+!3 = !{!4}
+!4 = distinct !DIGlobalVariable(name: "foobar", scope: !0, file: !1, line: 1, type: !5, isLocal: false, isDefinition: true, variable: i32* @foobar)
+!5 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !6)
+!6 = !DIBasicType(name: "int", size: 32, align: 32, encoding: DW_ATE_signed)
+!7 = !{i32 2, !"Dwarf Version", i32 4}
+!8 = !{i32 2, !"Debug Info Version", i32 3}
+!9 = !{!"clang version 3.9.1 "}
+!10 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 3, type: !11, isLocal: false, isDefinition: true, scopeLine: 3, isOptimized: true, unit: !0, variables: !2)
+!11 = !DISubroutineType(types: !12)
+!12 = !{!6}
+!13 = !DILocation(line: 4, column: 3, scope: !10)
+

tools/gold/gold-plugin.cpp

static ld_plugin_status thinLTOLink(raw_fd_ostream *ApiFile) {
 
   // Collect for each module the list of function it defines (GUID ->
   // Summary).
   StringMap<std::map<GlobalValue::GUID, GlobalValueSummary *>>
       ModuleToDefinedGVSummaries(NextModuleId);
   CombinedIndex.collectDefinedGVSummariesPerModule(ModuleToDefinedGVSummaries);
 
   StringMap<FunctionImporter::ImportMapTy> ImportLists(NextModuleId);
+
+  if (options::OptLevel > 0) {
     StringMap<FunctionImporter::ExportSetTy> ExportLists(NextModuleId);
     ComputeCrossModuleImport(CombinedIndex, ModuleToDefinedGVSummaries,
                              ImportLists, ExportLists);
 
-  auto isPrevailing = [&](GlobalValue::GUID GUID, const GlobalValueSummary *S) {
+    auto isPrevailing = [&](GlobalValue::GUID GUID,
+                            const GlobalValueSummary *S) {
       const auto &Prevailing = PrevailingCopy.find(GUID);
       assert(Prevailing != PrevailingCopy.end());
       return Prevailing->second == S;
     };
 
     // Callback for internalization, to prevent internalization of symbols
     // that were not candidates initially, and those that are being imported
     // (which introduces new cross references).
     auto isExported = [&](StringRef ModuleIdentifier, GlobalValue::GUID GUID) {
       const auto &ExportList = ExportLists.find(ModuleIdentifier);
       return (ExportList != ExportLists.end() &&
               ExportList->second.count(GUID)) ||
              Preserve.count(GUID);
     };
 
     thinLTOResolveWeakForLinkerInIndex(
         CombinedIndex, isPrevailing,
         [](StringRef ModuleIdentifier, GlobalValue::GUID GUID,
            GlobalValue::LinkageTypes NewLinkage) {});
 
     // Use global summary-based analysis to identify symbols that can be
     // internalized (because they aren't exported or preserved as per callback).
     // Changes are made in the index, consumed in the ThinLTO backends.
     thinLTOInternalizeAndPromoteInIndex(CombinedIndex, isExported);
+  }
 
   if (options::thinlto_emit_imports_files && !options::thinlto_index_only)
     message(LDPL_WARNING,
             "thinlto-emit-imports-files ignored unless thinlto-index-only");
 
   if (options::thinlto_index_only) {
     // If the thinlto-prefix-replace option was specified, parse it and
     // extract the old and new prefixes.