[MemProf] Change the STACK_ID record to fixed width values (#116448)

The stack ids are hashes that are close to 64 bits in size, so emitting
as a pair of 32-bit fixed-width values is more efficient than a VBR.
This reduced the summary bitcode size for a large target by about 1%.

Bump the index version and ensure we can read the old format.

1 files changed, 10 insertions, 1 deletions

diff --git a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
index 9ca76b5..3e6abac 100644
--- a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -7997,7 +7997,16 @@ Error ModuleSummaryIndexBitcodeReader::parseEntireSummary(unsigned ID) {
     case bitc::FS_STACK_IDS: { // [n x stackid]
       // Save stack ids in the reader to consult when adding stack ids from the
       // lists in the stack node and alloc node entries.
-      StackIds = ArrayRef<uint64_t>(Record);
+      if (Version <= 11) {
+        StackIds = ArrayRef<uint64_t>(Record);
+        break;
+      }
+      // This is an array of 32-bit fixed-width values, holding each 64-bit
+      // context id as a pair of adjacent (most significant first) 32-bit words.
+      assert(Record.size() % 2 == 0);
+      StackIds.reserve(Record.size() / 2);
+      for (auto R = Record.begin(); R != Record.end(); R += 2)
+        StackIds.push_back(*R << 32 | *(R + 1));
       break;
     }