Use an abbrev to reduce size of VALUE_GUID records in ThinLTO summaries (#90497)

GUID often have content in the higher bits of a 64-bit entry so using
the unabbrev encoding is inefficient (lots of VBR control bits).
Instead, use an abbrev with two 32-bit fixed width chunks.
The abbrev also helps encode the "count" in one place instead of
in every record.

Reduces size of distributed backend summary files by 8.7% in one
example app.

Co-authored-by: Jan Voung <jvoung@google.com>

1 files changed, 7 insertions, 2 deletions

diff --git a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
index 0b7fcd8..a0779f9 100644
--- a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -7513,9 +7513,14 @@ Error ModuleSummaryIndexBitcodeReader::parseEntireSummary(unsigned ID) {
       TheIndex.setFlags(Record[0]);
       break;
     }
-    case bitc::FS_VALUE_GUID: { // [valueid, refguid]
+    case bitc::FS_VALUE_GUID: { // [valueid, refguid_upper32, refguid_lower32]
       uint64_t ValueID = Record[0];
-      GlobalValue::GUID RefGUID = Record[1];
+      GlobalValue::GUID RefGUID;
+      if (Version >= 10) {
+        RefGUID = Record[1] << 32 | Record[2];
+      } else {
+        RefGUID = Record[1];
+      }
       ValueIdToValueInfoMap[ValueID] = std::make_tuple(
           TheIndex.getOrInsertValueInfo(RefGUID), RefGUID, RefGUID);
       break;