[ThinLTO] Allow importing of multiple symbols with same GUID

[tejohnson]

Summary

The is the clang side of the fix in D55047, to handle the case where
two different modules have local variables with the same GUID because
they had the same source file name at compilation time. Allow multiple
symbols with the same GUID to be imported, and test that this case works
with the distributed backend path.

Depends on D55047.

[evgeny777]

LGTM

lib/CodeGen/BackendUtil.cpp

static void runThinLTOBackend(ModuleSummaryIndex *CombinedIndex, Module *M,
   // via a WriteIndexesThinBackend.
   FunctionImporter::ImportMapTy ImportList;
   for (auto &GlobalList : *CombinedIndex) {
     // Ignore entries for undefined references.
     if (GlobalList.second.SummaryList.empty())
       continue;
 
     auto GUID = GlobalList.first;
-    assert(GlobalList.second.SummaryList.size() == 1 &&
-           "Expected individual combined index to have one summary per GUID");
-    auto &Summary = GlobalList.second.SummaryList[0];
+    for (auto &Summary : GlobalList.second.SummaryList) {
       // Skip the summaries for the importing module. These are included to
       // e.g. record required linkage changes.
       if (Summary->modulePath() == M->getModuleIdentifier())
         continue;
       // Add an entry to provoke importing by thinBackend.
       ImportList[Summary->modulePath()].insert(GUID);
     }
+  }
 
   std::vector<std::unique_ptr<llvm::MemoryBuffer>> OwnedImports;
   MapVector<llvm::StringRef, llvm::BitcodeModule> ModuleMap;
 
   for (auto &I : ImportList) {
     ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> MBOrErr =
         llvm::MemoryBuffer::getFile(I.first());
     if (!MBOrErr) {

test/CodeGen/Inputs/thinlto_backend_local_name_conflict1.ll

+; ModuleID = 'local_name_conflict_var.o'
+source_filename = "local_name_conflict_var.c"
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@baz = internal global i32 10, align 4
+
+; Function Attrs: noinline nounwind uwtable
+define i32 @a() {
+entry:
+  %0 = load i32, i32* @baz, align 4
+  ret i32 %0
+}

test/CodeGen/Inputs/thinlto_backend_local_name_conflict2.ll

+; ModuleID = 'local_name_conflict_var.o'
+source_filename = "local_name_conflict_var.c"
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@baz = internal global i32 10, align 4
+
+; Function Attrs: noinline nounwind uwtable
+define i32 @b() {
+entry:
+  %0 = load i32, i32* @baz, align 4
+  ret i32 %0
+}

test/CodeGen/thinlto_backend_local_name_conflict.ll

+; Test handling when two files with the same source file name contain
+; static read only variables with the same name (which will have the same GUID
+; in the combined index).
+
+; Do setup work for all below tests: generate bitcode and combined index
+; RUN: opt -module-summary -module-hash %s -o %t.bc
+; RUN: opt -module-summary -module-hash %p/Inputs/thinlto_backend_local_name_conflict1.ll -o %t2.bc
+; RUN: opt -module-summary -module-hash %p/Inputs/thinlto_backend_local_name_conflict2.ll -o %t3.bc
+; RUN: llvm-lto -thinlto-action=thinlink -o %t4.bc %t.bc %t2.bc %t3.bc
+; RUN: llvm-lto -thinlto-action=distributedindexes -exported-symbol=main -thinlto-index=%t4.bc %t.bc
+
+; This module will import a() and b() which should cause the read only copy
+; of baz from each of those modules to be imported. Check that the both are
+; imported as local copies.
+; RUN: %clang -target x86_64-unknown-linux-gnu -O2 -o %t4.o -x ir %t.bc -c -fthinlto-index=%t.bc.thinlto.bc -save-temps=obj
+; RUN: llvm-dis %t.s.3.import.bc -o - | FileCheck --check-prefix=IMPORT %s
+; IMPORT: @baz.llvm.{{.*}} = internal global i32 10
+; IMPORT: @baz.llvm.{{.*}} = internal global i32 10
+
+; ModuleID = 'local_name_conflict_var_main.o'
+source_filename = "local_name_conflict_var_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: noinline nounwind uwtable
+define i32 @main() {
+entry:
+  %call1 = call i32 (...) @a()
+  %call2 = call i32 (...) @b()
+  ret i32 0
+}
+
+declare i32 @a(...)
+declare i32 @b(...)