[mlir][LLVM] Use `undef` operand instead of removing `llvm.intr.dbg.value`

[zero9178]

Summary

Jeremy Morse noted in https://reviews.llvm.org/D154451 that LLVM doesn't drop a dbg.value when its value is being removed, but rather sets the operand to undef. This preserves the debug info and gives allows the debugger to instead inform the user that the variable has been optimized out, rather than not displaying the variable at all.

This patch fixes that mistake done in the previous revision by mirroring that behaviour in MLIR as well.

[gysit]

Thanks for the improvement. LGTM!

mlir/lib/Dialect/LLVMIR/IR/LLVMMemorySlot.cpp

 DeletionKind LLVM::DbgDeclareOp::removeBlockingUses(
     const SmallPtrSetImpl<OpOperand *> &blockingUses, RewriterBase &rewriter) {
   return DeletionKind::Delete;
 }
 
 bool LLVM::DbgValueOp::canUsesBeRemoved(
     const SmallPtrSetImpl<OpOperand *> &blockingUses,
     SmallVectorImpl<OpOperand *> &newBlockingUses) {
-  return true;
+  // There is only one operand that we can remove the use of.
+  if (blockingUses.size() != 1)
+    return false;
+
+  return (*blockingUses.begin())->get() == getValue();
 }
 
 DeletionKind LLVM::DbgValueOp::removeBlockingUses(
     const SmallPtrSetImpl<OpOperand *> &blockingUses, RewriterBase &rewriter) {
-  return DeletionKind::Delete;
+  // Rewriter by default is after '*this', but we need it before '*this'.
+  rewriter.setInsertionPoint(*this);
+
+  // Rather than dropping the debug value, replace it with undef to preserve the
+  // debug local variable info. This allows the debugger to inform the user that
+  // the variable has been optimized out.
+  auto undef =
+      rewriter.create<UndefOp>(getValue().getLoc(), getValue().getType());
+  rewriter.updateRootInPlace(*this, [&] { getValueMutable().assign(undef); });
+  return DeletionKind::Keep;
 }
 
 //===----------------------------------------------------------------------===//
 // Interfaces for GEPOp
 //===----------------------------------------------------------------------===//
 
 static bool hasAllZeroIndices(LLVM::GEPOp gepOp) {
   return llvm::all_of(gepOp.getIndices(), [](auto index) {

mlir/test/Dialect/LLVMIR/mem2reg-dbginfo.mlir

^bb1:
   // CHECK-NOT: llvm.intr.dbg.value
 // CHECK: ^{{.*}}(%[[BLOCKARG2:.*]]: i64):
 ^bb2:
   // CHECK: llvm.intr.dbg.value #[[$VAR]] = %[[BLOCKARG2]]
   llvm.br ^bb1
 }
 
 // CHECK-LABEL: llvm.func @always_drop_promoted_declare
+// CHECK: %[[UNDEF:.*]] = llvm.mlir.undef
 // CHECK-NOT: = llvm.alloca
 // CHECK-NOT: llvm.intr.dbg.declare
-// CHECK-NOT: llvm.intr.dbg.value
+// CHECK: llvm.intr.dbg.value #{{.*}} = %[[UNDEF]]
 llvm.func @always_drop_promoted_declare() {
   %0 = llvm.mlir.constant(1 : i32) : i32
   %1 = llvm.alloca %0 x i64 {alignment = 8 : i64} : (i32) -> !llvm.ptr
   llvm.intr.dbg.declare #di_local_variable = %1 : !llvm.ptr
   llvm.intr.dbg.value #di_local_variable = %1 : !llvm.ptr
   llvm.return
 }