[WebAssembly] Change placeholder from undef to poison

[pedroclobo]

Use poison instead of undef as a placeholder for phi entries of unreachable predecessors.

[llvmbot]

@llvm/pr-subscribers-backend-webassembly

Author: Pedro Lobo (pedroclobo)

Changes

Use poison instead of undef as a placeholder for phi entries of unreachable predecessors.

---

Full diff: https://github.com/llvm/llvm-project/pull/131536.diff

1 Files Affected:

• (modified) llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp (+2-2)

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp
index 2d11019291a7a..0e79a13d4ccaa 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp
@@ -1767,7 +1767,7 @@ void WebAssemblyLowerEmscriptenEHSjLj::handleLongjmpableCallsForWasmSjLj(
     I->eraseFromParent();
 
   // Add entries for new predecessors to phis in unwind destinations. We use
-  // 'undef' as a placeholder value. We should make sure the phis have a valid
+  // 'poison' as a placeholder value. We should make sure the phis have a valid
   // set of predecessors before running SSAUpdater, because SSAUpdater
   // internally can use existing phis to gather predecessor info rather than
   // scanning the actual CFG (See FindPredecessorBlocks in SSAUpdater.cpp for
@@ -1776,7 +1776,7 @@ void WebAssemblyLowerEmscriptenEHSjLj::handleLongjmpableCallsForWasmSjLj(
     for (PHINode &PN : UnwindDest->phis()) {
       for (auto *NewPred : NewPreds) {
         assert(PN.getBasicBlockIndex(NewPred) == -1);
-        PN.addIncoming(UndefValue::get(PN.getType()), NewPred);
+        PN.addIncoming(PoisonValue::get(PN.getType()), NewPred);
       }
     }
   }

[aheejin]

Are there any differences in these two options in this case? I think the value is used only as a placeholder that is to be replaced later.

[pedroclobo]

Are there any differences in these two options in this case? I think the value is used only as a placeholder that is to be replaced later.

Conceptually, as the value is later replaced, there is no difference between using poison or undef here.
Nonetheless, we should prefer poison, helping the ongoing effort to remove undef and replace it with poison.