[llvm] [DFSan] Change placeholders from `undef` to `poison` (PR #131534)

[Pedro Lobo via llvm-commits]

https://github.com/pedroclobo created https://github.com/llvm/llvm-project/pull/131534

Use `poison` to create dummy phi nodes instead of `undef`.

>From 9b7cca59bb627f49f8cbc7ae0247614c5f224d61 Mon Sep 17 00:00:00 2001
From: Pedro Lobo <pedro.lobo at tecnico.ulisboa.pt>
Date: Tue, 11 Mar 2025 23:13:42 +0000
Subject: [PATCH] [DFSan] Change placeholders from `undef` to `poison`

Use `poison` to create dummy phi nodes instead of `undef`.
---
 llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp b/llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp
index fcabcdfb0ba9b..1ef650723a30b 100644
--- a/llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp
+++ b/llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp
@@ -3447,9 +3447,9 @@ void DFSanVisitor::visitPHINode(PHINode &PN) {
                                       PN.getIterator());
 
   // Give the shadow phi node valid predecessors to fool SplitEdge into working.
-  Value *UndefShadow = UndefValue::get(ShadowTy);
+  Value *PoisonShadow = PoisonValue::get(ShadowTy);
   for (BasicBlock *BB : PN.blocks())
-    ShadowPN->addIncoming(UndefShadow, BB);
+    ShadowPN->addIncoming(PoisonShadow, BB);
 
   DFSF.setShadow(&PN, ShadowPN);
 
@@ -3457,9 +3457,9 @@ void DFSanVisitor::visitPHINode(PHINode &PN) {
   if (DFSF.DFS.shouldTrackOrigins()) {
     OriginPN = PHINode::Create(DFSF.DFS.OriginTy, PN.getNumIncomingValues(), "",
                                PN.getIterator());
-    Value *UndefOrigin = UndefValue::get(DFSF.DFS.OriginTy);
+    Value *PoisonOrigin = PoisonValue::get(DFSF.DFS.OriginTy);
     for (BasicBlock *BB : PN.blocks())
-      OriginPN->addIncoming(UndefOrigin, BB);
+      OriginPN->addIncoming(PoisonOrigin, BB);
     DFSF.setOrigin(&PN, OriginPN);
   }