[Instruction] Set metadata uses to undef on deletion

Summary:
Replace any extant metadata uses of a dying instruction with undef to
preserve debug info accuracy. Some alternatives include:

- Treat Instruction like any other Value, and point its extant metadata
  uses to an empty ValueAsMetadata node. This makes extant dbg.value uses
  trivially dead (i.e. fair game for deletion in many passes), leading to
  stale dbg.values being in effect for too long.

- Call salvageDebugInfoOrMarkUndef. Not needed to make instruction removal
  correct. OTOH results in wasted work in some common cases (e.g. when all
  instructions in a BasicBlock are deleted).

This came up while discussing some basic cases in
https://reviews.llvm.org/D80052.

Reviewers: jmorse, TWeaver, aprantl, dexonsmith, jdoerfert

Subscribers: jholewinski, qcolombet, hiraditya, jfb, sstefan1, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80264

llvm/lib/IR/Instruction.cpp

@@ -43,6 +43,19 @@ Instruction::Instruction(Type *ty, unsigned it, Use *Ops, unsigned NumOps,
 
 Instruction::~Instruction() {
   assert(!Parent && "Instruction still linked in the program!");
+
+  // Replace any extant metadata uses of this instruction with undef to
+  // preserve debug info accuracy. Some alternatives include:
+  // - Treat Instruction like any other Value, and point its extant metadata
+  //   uses to an empty ValueAsMetadata node. This makes extant dbg.value uses
+  //   trivially dead (i.e. fair game for deletion in many passes), leading to
+  //   stale dbg.values being in effect for too long.
+  // - Call salvageDebugInfoOrMarkUndef. Not needed to make instruction removal
+  //   correct. OTOH results in wasted work in some common cases (e.g. when all
+  //   instructions in a BasicBlock are deleted).
+  if (isUsedByMetadata())
+    ValueAsMetadata::handleRAUW(this, UndefValue::get(getType()));
+
   if (hasMetadataHashEntry())
     clearMetadataHashEntries();
 }