[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, 20 insertions, 7 deletions

diff --git a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
index 5829af3..24a4c2e 100644
--- a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
+++ b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -4429,12 +4429,17 @@ void ModuleBitcodeWriterBase::writePerModuleGlobalValueSummary() {
     StackIdAbbv->Add(BitCodeAbbrevOp(bitc::FS_STACK_IDS));
     // numids x stackid
     StackIdAbbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Array));
-    // FIXME: The stack ids are hashes that are close to 64 bits in size, so
-    // emitting as a pair of 32-bit fixed-width values, as we do for context
-    // ids, would be more efficient.
-    StackIdAbbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));
+    // 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.
+    StackIdAbbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 32));
     unsigned StackIdAbbvId = Stream.EmitAbbrev(std::move(StackIdAbbv));
-    Stream.EmitRecord(bitc::FS_STACK_IDS, Index->stackIds(), StackIdAbbvId);
+    SmallVector<uint32_t> Vals;
+    Vals.reserve(Index->stackIds().size() * 2);
+    for (auto Id : Index->stackIds()) {
+      Vals.push_back(static_cast<uint32_t>(Id >> 32));
+      Vals.push_back(static_cast<uint32_t>(Id));
+    }
+    Stream.EmitRecord(bitc::FS_STACK_IDS, Vals, StackIdAbbvId);
   }
 
   // n x context id
@@ -4624,9 +4629,17 @@ void IndexBitcodeWriter::writeCombinedGlobalValueSummary() {
     StackIdAbbv->Add(BitCodeAbbrevOp(bitc::FS_STACK_IDS));
     // numids x stackid
     StackIdAbbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Array));
-    StackIdAbbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));
+    // 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.
+    StackIdAbbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 32));
     unsigned StackIdAbbvId = Stream.EmitAbbrev(std::move(StackIdAbbv));
-    Stream.EmitRecord(bitc::FS_STACK_IDS, StackIds, StackIdAbbvId);
+    SmallVector<uint32_t> Vals;
+    Vals.reserve(StackIds.size() * 2);
+    for (auto Id : StackIds) {
+      Vals.push_back(static_cast<uint32_t>(Id >> 32));
+      Vals.push_back(static_cast<uint32_t>(Id));
+    }
+    Stream.EmitRecord(bitc::FS_STACK_IDS, Vals, StackIdAbbvId);
   }
 
   // Abbrev for FS_COMBINED_PROFILE.