[llvm] r333768 - [ThinLTOBitcodeWriter] Emit summaries for regular LTO modules

[Vlad Tsyrklevich via llvm-commits]

Author: vlad.tsyrklevich
Date: Fri Jun  1 08:20:47 2018
New Revision: 333768

URL: http://llvm.org/viewvc/llvm-project?rev=333768&view=rev
Log:
[ThinLTOBitcodeWriter] Emit summaries for regular LTO modules

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.

Reviewers: pcc

Reviewed By: pcc

Subscribers: dschuff, mehdi_amini, inglorion, eraman, llvm-commits, kcc

Differential Revision: https://reviews.llvm.org/D47594

Modified:
    llvm/trunk/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
    llvm/trunk/test/Transforms/ThinLTOBitcodeWriter/unsplittable.ll

Modified: llvm/trunk/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp?rev=333768&r1=333767&r2=333768&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp (original)
+++ llvm/trunk/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp Fri Jun  1 08:20:47 2018
@@ -200,13 +200,19 @@ void splitAndWriteThinLTOBitcode(
     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;
   }
 

Modified: llvm/trunk/test/Transforms/ThinLTOBitcodeWriter/unsplittable.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/ThinLTOBitcodeWriter/unsplittable.ll?rev=333768&r1=333767&r2=333768&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/ThinLTOBitcodeWriter/unsplittable.ll (original)
+++ llvm/trunk/test/Transforms/ThinLTOBitcodeWriter/unsplittable.ll Fri Jun  1 08:20:47 2018
@@ -5,6 +5,7 @@
 ; 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
@@ -27,4 +28,7 @@ define void @h() comdat {
   ret void
 }
 
+; CHECK: !llvm.module.flags = !{![[FLAG:[0-9]+]]}
+; CHECK: ![[FLAG]] = !{i32 1, !"ThinLTO", i32 0}
+
 !0 = !{i32 0, !"typeid"}