[NFCI]Remove EntryCount from FunctionSummary and clean up surrounding synthetic count passes. (#107471)

The primary motivation is to remove `EntryCount` from `FunctionSummary`.
This frees 8 bytes out of `sizeof(FunctionSummary)` (136 bytes as of
https://github.com/llvm/llvm-project/commit/64498c54831bed9cf069e0923b9b73678c6451d8).

While I'm at it, this PR clean up {SummaryBasedOptimizations,
SyntheticCountsPropagation} since they were not used and there are no
plans to further invest on them.

With this patch, bitcode writer writes a placeholder 0 at the byte
offset of `EntryCount` and bitcode reader can parse the function entry
count at the correct byte offset. Added a TODO to stop writing
`EntryCount` and bump bitcode version

1 files changed, 2 insertions, 6 deletions

diff --git a/llvm/lib/IR/ModuleSummaryIndex.cpp b/llvm/lib/IR/ModuleSummaryIndex.cpp
index 6713d32..a788e63 100644
--- a/llvm/lib/IR/ModuleSummaryIndex.cpp
+++ b/llvm/lib/IR/ModuleSummaryIndex.cpp
@@ -91,12 +91,11 @@ constexpr uint64_t ModuleSummaryIndex::BitcodeSummaryVersion;
 
 uint64_t ModuleSummaryIndex::getFlags() const {
   uint64_t Flags = 0;
+  // Flags & 0x4 is reserved. DO NOT REUSE.
   if (withGlobalValueDeadStripping())
     Flags |= 0x1;
   if (skipModuleByDistributedBackend())
     Flags |= 0x2;
-  if (hasSyntheticEntryCounts())
-    Flags |= 0x4;
   if (enableSplitLTOUnit())
     Flags |= 0x8;
   if (partiallySplitLTOUnits())
@@ -124,10 +123,7 @@ void ModuleSummaryIndex::setFlags(uint64_t Flags) {
   // Set on combined index only.
   if (Flags & 0x2)
     setSkipModuleByDistributedBackend();
-  // 1 bit: HasSyntheticEntryCounts flag.
-  // Set on combined index only.
-  if (Flags & 0x4)
-    setHasSyntheticEntryCounts();
+  // Flags & 0x4 is reserved. DO NOT REUSE.
   // 1 bit: DisableSplitLTOUnit flag.
   // Set on per module indexes. It is up to the client to validate
   // the consistency of this flag across modules being linked.