[llvm] [AssignmentTracking] Skip large types in redundant debug info pruning (PR #74329)

[Nikita Popov via llvm-commits]

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@@ -2295,33 +2295,41 @@ removeRedundantDbgLocsUsingBackwardScan(const BasicBlock *BB,
       DebugAggregate Aggr =
           getAggregate(FnVarLocs.getVariable(RIt->VariableID));
       uint64_t SizeInBits = Aggr.first->getSizeInBits().value_or(0);
+      uint64_t SizeInBytes = SizeInBits + 7 / 8;
 
-      if (SizeInBits == 0) {
+      // Cutoff for large variables to prevent expensive bitvector operations.
+      const uint64_t MaxSizeBytes = 2048;
+
+      if (SizeInBytes == 0 || SizeInBytes > MaxSizeBytes) {
         // If the size is unknown (0) then keep this location def to be safe.
+        // Do the same for defs of large variables, which would be expensive
+        // to represent with a BitVector.
+        // FIXME: Don't use a BitVector.
         NewDefsReversed.push_back(*RIt);
         continue;
       }
 
       // Only keep this location definition if it is not fully eclipsed by
       // other definitions in this wedge that come after it
 
-      // Inert the bits the location definition defines.
+      // Inert the bytes the location definition defines.
       auto InsertResult =
-          VariableDefinedBits.try_emplace(Aggr, BitVector(SizeInBits));
+          VariableDefinedBytes.try_emplace(Aggr, BitVector(SizeInBytes));
       bool FirstDefinition = InsertResult.second;
-      BitVector &DefinedBits = InsertResult.first->second;
+      BitVector &DefinedBytes = InsertResult.first->second;
 
       DIExpression::FragmentInfo Fragment =
           RIt->Expr->getFragmentInfo().value_or(
               DIExpression::FragmentInfo(SizeInBits, 0));
       bool InvalidFragment = Fragment.endInBits() > SizeInBits;
+      uint64_t StartInBytes = Fragment.startInBits() + 7 / 8;
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nikic wrote:

Not familiar with this code, but shouldn't the start be rounded down?

https://github.com/llvm/llvm-project/pull/74329