[NFC][SROA][DebugInfo] Reuse existing dbg_assigns where possible (#163938)

Addresses issue #145937

Without this patch SROA generates new dbg_assign for new stores. We can
simply steal the existing dbg_assigns linked to the old store when the
store is not being split.

Author: OCHyams

llvm/lib/Transforms/Scalar/SROA.cpp

@@ -344,6 +344,12 @@ static void migrateDebugInfo(AllocaInst *OldAlloca, bool IsSplit,
                              uint64_t SliceSizeInBits, Instruction *OldInst,
                              Instruction *Inst, Value *Dest, Value *Value,
                              const DataLayout &DL) {
+  // If we want allocas to be migrated using this helper then we need to ensure
+  // that the BaseFragments map code still works. A simple solution would be
+  // to choose to always clone alloca dbg_assigns (rather than sometimes
+  // "stealing" them).
+  assert(!isa<AllocaInst>(Inst) && "Unexpected alloca");
+
   auto DVRAssignMarkerRange = at::getDVRAssignmentMarkers(OldInst);
   // Nothing to do if OldInst has no linked dbg.assign intrinsics.
   if (DVRAssignMarkerRange.empty())
@@ -429,11 +435,22 @@ static void migrateDebugInfo(AllocaInst *OldAlloca, bool IsSplit,
       Inst->setMetadata(LLVMContext::MD_DIAssignID, NewID);
     }
 
-    ::Value *NewValue = Value ? Value : DbgAssign->getValue();
-    DbgVariableRecord *NewAssign = cast<DbgVariableRecord>(cast<DbgRecord *>(
-        DIB.insertDbgAssign(Inst, NewValue, DbgAssign->getVariable(), Expr,
-                            Dest, DIExpression::get(Expr->getContext(), {}),
-                            DbgAssign->getDebugLoc())));
+    DbgVariableRecord *NewAssign;
+    if (IsSplit) {
+      ::Value *NewValue = Value ? Value : DbgAssign->getValue();
+      NewAssign = cast<DbgVariableRecord>(cast<DbgRecord *>(
+          DIB.insertDbgAssign(Inst, NewValue, DbgAssign->getVariable(), Expr,
+                              Dest, DIExpression::get(Expr->getContext(), {}),
+                              DbgAssign->getDebugLoc())));
+    } else {
+      // The store is not split, simply steal the existing dbg_assign.
+      NewAssign = DbgAssign;
+      NewAssign->setAssignId(NewID); // FIXME: Can we avoid generating new IDs?
+      NewAssign->setAddress(Dest);
+      if (Value)
+        NewAssign->replaceVariableLocationOp(0u, Value);
+      assert(Expr == NewAssign->getExpression());
+    }
 
     // If we've updated the value but the original dbg.assign has an arglist
     // then kill it now - we can't use the requested new value.
@@ -464,9 +481,10 @@ static void migrateDebugInfo(AllocaInst *OldAlloca, bool IsSplit,
     // noted as slightly offset (in code) from the store. In practice this
     // should have little effect on the debugging experience due to the fact
     // that all the split stores should get the same line number.
-    NewAssign->moveBefore(DbgAssign->getIterator());
-
-    NewAssign->setDebugLoc(DbgAssign->getDebugLoc());
+    if (NewAssign != DbgAssign) {
+      NewAssign->moveBefore(DbgAssign->getIterator());
+      NewAssign->setDebugLoc(DbgAssign->getDebugLoc());
+    }
     LLVM_DEBUG(dbgs() << "Created new assign: " << *NewAssign << "\n");
   };