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[ThinLTOBitcodeWriter] Emit summaries for regular LTO modules

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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

llvm-svn: 333768
This commit is contained in:
Vlad Tsyrklevich 2018-06-01 15:20:47 +00:00
parent 13034d4b5f
commit ec4a2d665c
2 changed files with 14 additions and 4 deletions
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14
lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
View File

@ -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;
}

4
test/Transforms/ThinLTOBitcodeWriter/unsplittable.ll
View File

@ -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"}