[ThinLTOBitcodeWriter] Emit summaries for regular LTO modules

[vlad.tsyrklevich]

Summary

Emit summaries for bitcode modules that are only destined for the
regular LTO portion of the build so they can participate in
summary-based dead stripping.

This change reduces the size of a nacl_helper build with cfi-icall
enabled by 7%, removing the majority of the overhead due to enabling
cfi-icall. The cfi-icall size increase was caused by compiling in lots
of unused code and cfi-icall generating jumptable references to unused
symbols that could no longer be removed by -Wl,-gc-sections. Increasing
the visibility of summary-based dead stripping prevented jumptable
entries being created for unused symbols from the regular LTO portion
of the build.

[pcc]

LGTM

lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp

 // If it's possible to split M into regular and thin LTO parts, do so and write
 // a multi-module bitcode file with the two parts to OS. Otherwise, write only a
 // regular LTO bitcode file to OS.
 void splitAndWriteThinLTOBitcode(
     raw_ostream &OS, raw_ostream *ThinLinkOS,
     function_ref<AAResults &(Function &)> AARGetter, Module &M) {
   std::string ModuleId = getUniqueModuleId(&M);
   if (ModuleId.empty()) {
-    // We couldn't generate a module ID for this module, just write it out as a
-    // regular LTO module.
-    WriteBitcodeToFile(M, OS);
+    // We couldn't generate a module ID for this module, write it out as a
+    // regular LTO module with an index for summary-based dead stripping.
+    ProfileSummaryInfo PSI(M);
+    M.addModuleFlag(Module::Error, "ThinLTO", uint32_t(0));
+    ModuleSummaryIndex Index = buildModuleSummaryIndex(M, nullptr, &PSI);
+    WriteBitcodeToFile(M, OS, /*ShouldPreserveUseListOrder=*/false, &Index);
+
     if (ThinLinkOS)
       // We don't have a ThinLTO part, but still write the module to the
       // ThinLinkOS if requested so that the expected output file is produced.
-      WriteBitcodeToFile(M, *ThinLinkOS);
+      WriteBitcodeToFile(M, *ThinLinkOS, /*ShouldPreserveUseListOrder=*/false,
+                         &Index);
+
     return;
   }
 
   promoteTypeIds(M, ModuleId);
 
   // Returns whether a global has attached type metadata. Such globals may
   // participate in CFI or whole-program devirtualization, so they need to
   // appear in the merged module instead of the thin LTO module.

test/Transforms/ThinLTOBitcodeWriter/unsplittable.ll

 ; RUN: opt -thinlto-bc -thin-link-bitcode-file=%t2 -o %t %s
 ; RUN: llvm-dis -o - %t | FileCheck %s
 ; RUN: llvm-bcanalyzer -dump %t | FileCheck --check-prefix=BCA %s
 ; When not splitting the module, the thin link bitcode file should simply be a
 ; copy of the regular module.
 ; RUN: diff %t %t2
 
+; BCA: <FULL_LTO_GLOBALVAL_SUMMARY_BLOCK
 ; BCA-NOT: <GLOBALVAL_SUMMARY_BLOCK
 
 ; CHECK: @llvm.global_ctors = appending global
 @llvm.global_ctors = appending global [1 x { i32, void ()* }] [{ i32, void ()* } { i32 65535, void ()* @f }]
 
 ; CHECK: @g = internal global i8 42, !type !0
 @g = internal global i8 42, !type !0
 
 declare void @sink(i8*)
 
 ; CHECK: define internal void @f()
 define internal void @f() {
   call void @sink(i8* @g)
   ret void
 }
 
 $h = comdat any
 ; CHECK: define void @h() comdat
 define void @h() comdat {
   ret void
 }
 
+; CHECK: !llvm.module.flags = !{![[FLAG:[0-9]+]]}
+; CHECK: ![[FLAG]] = !{i32 1, !"ThinLTO", i32 0}
+
 !0 = !{i32 0, !"typeid"}