[llvm] [OMPIRBuilder] Avoid crash in BasicBlock::splice. (PR #154987)

[Abid Qadeer via llvm-commits]

https://github.com/abidh updated https://github.com/llvm/llvm-project/pull/154987

>From f8f9835fd09d5e5bff4260474685216c893ab5ee Mon Sep 17 00:00:00 2001
From: Abid Qadeer <haqadeer at amd.com>
Date: Fri, 22 Aug 2025 15:45:02 +0100
Subject: [PATCH 1/2] [OMPIRBuilder] Avoid crash in BasicBlock::splice.

Calling splice when both Old and New are empty is a nop currently but
it can cause a crash once debug records are used instead of debug
intrinsics. This PR makes the call conditional on at least one of
Old or New being non-empty.

Consider the following mlir:
omp.target map_entries() {
  llvm.intr.dbg.declare ...
  llvm.intr.dbg.declare ...
  omp.teams ...
  ...
}

Current code would translate llvm.intr to llvm intrinsics and we will
have 2 of them in the Old BB by the time omp.teams implementation starts.
This implementation creates many BasicBlocks by calling splitBB.

In the new scheme (using debug records), there will be no instruction
in the Old BB after llvm.intr get translated but just 2 trailing debug
records. So effectively both Old and New are empty. When control reaches
BasicBlock::splice, it calls spliceDebugInfoEmptyBlock. This funtion
expects that in this case (Src is empty but has trailing debug records),
the ToIt is valid and it can call adoptDbgRecords on it. This
assumption is not true in this case as New is empty and ToIt is pointing
to end().
---
 llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp b/llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp
index c0e956840f989..f6afa93f21bdc 100644
--- a/llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp
+++ b/llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp
@@ -307,7 +307,8 @@ void llvm::spliceBB(IRBuilderBase::InsertPoint IP, BasicBlock *New,
 
   // Move instructions to new block.
   BasicBlock *Old = IP.getBlock();
-  New->splice(New->begin(), Old, IP.getPoint(), Old->end());
+  if (!Old->empty() || !New->empty())
+    New->splice(New->begin(), Old, IP.getPoint(), Old->end());
 
   if (CreateBranch) {
     auto *NewBr = BranchInst::Create(New, Old);

>From 3654d526ef83fc6b0c30dfe47a955f85f5e8acd8 Mon Sep 17 00:00:00 2001
From: Abid Qadeer <haqadeer at amd.com>
Date: Tue, 26 Aug 2025 15:46:05 +0100
Subject: [PATCH 2/2] Add a unit test.

---
 .../Frontend/OpenMPIRBuilderTest.cpp          | 24 +++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/llvm/unittests/Frontend/OpenMPIRBuilderTest.cpp b/llvm/unittests/Frontend/OpenMPIRBuilderTest.cpp
index b7a060bb3563d..c13570dc803b3 100644
--- a/llvm/unittests/Frontend/OpenMPIRBuilderTest.cpp
+++ b/llvm/unittests/Frontend/OpenMPIRBuilderTest.cpp
@@ -7852,4 +7852,28 @@ TEST_F(OpenMPIRBuilderTest, splitBB) {
     EXPECT_TRUE(DL == AllocaBB->getTerminator()->getStableDebugLoc());
 }
 
+TEST_F(OpenMPIRBuilderTest, spliceBBWithEmptyBB) {
+  OpenMPIRBuilder OMPBuilder(*M);
+  OMPBuilder.Config.IsTargetDevice = false;
+  OMPBuilder.initialize();
+  F->setName("func");
+  IRBuilder<> Builder(BB);
+
+  // Test calling spliceBB with an empty Block (but having trailing debug
+  // records).
+  DIBuilder DIB(*M);
+  DISubprogram *SP = F->getSubprogram();
+  DIType *VoidPtrTy =
+      DIB.createQualifiedType(dwarf::DW_TAG_pointer_type, nullptr);
+  DILocalVariable *Var = DIB.createParameterVariable(
+      SP, "test", /*ArgNo*/ 1, SP->getFile(), /*LineNo=*/0, VoidPtrTy);
+  DIB.insertDeclare(F->getArg(0), Var, DIB.createExpression(), DL,
+                    Builder.GetInsertPoint());
+  BasicBlock *New = BasicBlock::Create(Ctx, "", F);
+  spliceBB(Builder.saveIP(), New, true, DL);
+  Instruction *Terminator = BB->getTerminator();
+  EXPECT_NE(Terminator, nullptr);
+  EXPECT_FALSE(Terminator->getDbgRecordRange().empty());
+}
+
 } // namespace