[BOLT][DWARF][NFC] Initialize CloneUnitCtxMap with current partition size (#75876)

We would always allocate maximum amount for vector containing
DWARFUnitInfo. In real usecases what ends up hapenning is we allocate a
giant vector when processing one CU, or for thin-lto case multiple CUs.
This lead to a lot of memory overhead, and 2x BOLT processing slowdown
for at least one service built with monolithic DWARF.

For binaries built with LTO with clang all of CUs that have cross
references will share an abbrev table and will be processed in one
batch. Rest of CUs are processesd in --cu-processing-batch-size size.
Which defaults to 1.

For theoretical cases where cross-cu references are present, but they do
not share abbrev will increase the size of CloneUnitCtxMap as each CU is
being processsed.

Author: ayermolo

bolt/lib/Core/DIEBuilder.cpp

@@ -266,13 +266,11 @@ void DIEBuilder::buildCompileUnits(const bool Init) {
 }
 void DIEBuilder::buildCompileUnits(const std::vector<DWARFUnit *> &CUs) {
   BuilderState.reset(new State());
-  // Initializing to full size because there could be cross CU references with
-  // different abbrev offsets. LLVM happens to output CUs that have cross CU
-  // references with the same abbrev table. So destinations end up in the first
-  // set, even if they themselves don't have src cross cu ref. We could have
-  // cases where this is not the case. In which case this container needs to be
-  // big enough for all.
-  getState().CloneUnitCtxMap.resize(DwarfContext->getNumCompileUnits());
+  // Allocating enough for current batch being processed.
+  // In real use cases we either processing a batch of CUs with no cross
+  // references, or if they do have them it is due to LTO. With clang they will
+  // share the same abbrev table. In either case this vector will not grow.
+  getState().CloneUnitCtxMap.resize(CUs.size());
   getState().Type = ProcessingType::CUs;
   for (DWARFUnit *CU : CUs)
     registerUnit(*CU, false);
@@ -897,6 +895,10 @@ void DIEBuilder::registerUnit(DWARFUnit &DU, bool NeedSort) {
                 });
   }
   getState().UnitIDMap[getHash(DU)] = getState().DUList.size();
+  // This handles the case where we do have cross cu references, but CUs do not
+  // share the same abbrev table.
+  if (getState().DUList.size() == getState().CloneUnitCtxMap.size())
+    getState().CloneUnitCtxMap.emplace_back();
   getState().DUList.push_back(&DU);
 }