[ThinLTO] Always emit a summary when compiling in ThinLTO mode

Summary:
Emit an empty summary section, instead of no summary section, when
there are no global variables in the index. This ensures that LTO
will treat these files as ThinLTO inputs, instead of as regular
LTO inputs.

In addition to not being what the user likely intended when
compiling with -flto=thin, the current behavior is problematic for
distributed build systems that expect to get ThinLTO index and imports
files back for each input compiled with -flto=thin. Combining into
a single regular LTO module also reduces the backend parallelism.
And in the case where the index was suppressed due to uses in
inline assembly, combining into a single LTO module could provoke
renaming of duplicates that we were trying to prevent by suppressing
the index.

This change required a couple of fixes to handle the empty summary
section.

Reviewers: mehdi_amini

Subscribers: mehdi_amini, llvm-commits, pcc

Differential Revision: https://reviews.llvm.org/D24779

llvm-svn: 282037

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -6097,13 +6097,18 @@ std::error_code ModuleSummaryIndexBitcodeReader::parseModule() {
           return error("Invalid record");
         break;
       case bitc::GLOBALVAL_SUMMARY_BLOCK_ID:
-        assert(VSTOffset > 0 && "Expected non-zero VST offset");
         assert(!SeenValueSymbolTable &&
                "Already read VST when parsing summary block?");
-        if (std::error_code EC =
-                parseValueSymbolTable(VSTOffset, ValueIdToLinkageMap))
-          return EC;
-        SeenValueSymbolTable = true;
+        // We might not have a VST if there were no values in the
+        // summary. An empty summary block generated when we are
+        // performing ThinLTO compiles so we don't later invoke
+        // the regular LTO process on them.
+        if (VSTOffset > 0) {
+          if (std::error_code EC =
+                  parseValueSymbolTable(VSTOffset, ValueIdToLinkageMap))
+            return EC;
+          SeenValueSymbolTable = true;
+        }
         SeenGlobalValSummary = true;
         if (std::error_code EC = parseEntireSummary())
           return EC;