[ThinLTO] Disable "Always import constants" due to compile time issues

Summary:
Disable the always importing of constants introduced in D70404 by
default under a new internal option, since it is causing order of
magnitude compile time regressions during the thin link. Will continue
investigating why the regressions occur.

Reviewers: evgeny777, wmi

Subscribers: mehdi_amini, inglorion, hiraditya, steven_wu, dexonsmith, arphaman, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73724

1 files changed, 8 insertions, 2 deletions

diff --git a/llvm/lib/IR/ModuleSummaryIndex.cpp b/llvm/lib/IR/ModuleSummaryIndex.cpp
index 301df9a..0a43e17 100644
--- a/llvm/lib/IR/ModuleSummaryIndex.cpp
+++ b/llvm/lib/IR/ModuleSummaryIndex.cpp
@@ -31,6 +31,12 @@ static cl::opt<bool> PropagateAttrs("propagate-attrs", cl::init(true),
                                     cl::Hidden,
                                     cl::desc("Propagate attributes in index"));
 
+// FIXME: Enable again when thin link compile time regressions understood and
+// addressed
+static cl::opt<bool> ImportConstantsWithRefs(
+    "import-constants-with-refs", cl::init(false), cl::Hidden,
+    cl::desc("Import constant global variables with references"));
+
 FunctionSummary FunctionSummary::ExternalNode =
     FunctionSummary::makeDummyFunctionSummary({});
 
@@ -221,8 +227,8 @@ bool ModuleSummaryIndex::canImportGlobalVar(GlobalValueSummary *S,
     // c) Link error (external declaration with internal definition).
     // However we do not promote objects referenced by writeonly GV
     // initializer by means of converting it to 'zeroinitializer'
-    return !GVS->isConstant() && !isReadOnly(GVS) && !isWriteOnly(GVS) &&
-           GVS->refs().size();
+    return !(ImportConstantsWithRefs && GVS->isConstant()) &&
+           !isReadOnly(GVS) && !isWriteOnly(GVS) && GVS->refs().size();
   };
   auto *GVS = cast<GlobalVarSummary>(S->getBaseObject());