[llvm] [SandboxIR][Tracker] Track eraseFromParent() (PR #99431)

[Arthur Eubanks via llvm-commits]

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@@ -341,10 +341,26 @@ void Instruction::removeFromParent() {
 void Instruction::eraseFromParent() {
   assert(users().empty() && "Still connected to users, can't erase!");
   std::unique_ptr<Value> Detached = Ctx.detach(this);
-  // We don't have Tracking yet, so just erase the LLVM IR instructions.
+  auto LLVMInstrs = getLLVMInstrs();
+
+  auto &Tracker = Ctx.getTracker();
+  if (Tracker.isTracking()) {
+    Tracker.track(
+        std::make_unique<EraseFromParent>(std::move(Detached), Tracker));
+    // We don't actually delete the IR instruction, because then it would be
+    // impossible to bring it back from the dead at the same memory location.
+    // Instead we remove it from its BB and track its current location.
+    for (llvm::Instruction *I : LLVMInstrs)
+      I->removeFromParent();
+    // TODO: Multi-instructions need special treatment because some of the
+    // references are internal to the instruction.
+    for (llvm::Instruction *I : LLVMInstrs)
+      I->dropAllReferences();
+    return;
+  }
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aeubanks wrote:

imo putting an `else` here rather than the `return` is a bit clearer

https://github.com/llvm/llvm-project/pull/99431