[KeyInstr] Don't propagate source atoms to new uncond br in splitBasicBlock (#139070)

splitBasicBlock inserts an unconditional branch in the "before" block to
the "after" block. It copies the DebugLoc from the split point. Prevent
it copying the source location atom.

Add unittest.

RFC:
https://discourse.llvm.org/t/rfc-improving-is-stmt-placement-for-better-interactive-debugging/82668

Author: OCHyams

llvm/lib/IR/BasicBlock.cpp

@@ -612,7 +612,7 @@ BasicBlock *BasicBlock::splitBasicBlock(iterator I, const Twine &BBName,
                                        this->getNextNode());
 
   // Save DebugLoc of split point before invalidating iterator.
-  DebugLoc Loc = I->getStableDebugLoc();
+  DebugLoc Loc = I->getStableDebugLoc()->getWithoutAtom();
   // Move all of the specified instructions from the original basic block into
   // the new basic block.
   New->splice(New->end(), this, I, end());
@@ -641,7 +641,7 @@ BasicBlock *BasicBlock::splitBasicBlockBefore(iterator I, const Twine &BBName) {
 
   BasicBlock *New = BasicBlock::Create(getContext(), BBName, getParent(), this);
   // Save DebugLoc of split point before invalidating iterator.
-  DebugLoc Loc = I->getDebugLoc();
+  DebugLoc Loc = I->getDebugLoc()->getWithoutAtom();
   // Move all of the specified instructions from the original basic block into
   // the new basic block.
   New->splice(New->end(), this, begin(), I);

llvm/unittests/IR/BasicBlockDbgInfoTest.cpp

@@ -149,6 +149,75 @@ TEST(BasicBlockDbgInfoTest, SplitBasicBlockBefore) {
   ASSERT_TRUE(I2->hasDbgRecords());
 }
 
+TEST(BasicBlockDbgInfoTest, DropSourceAtomOnSplit) {
+  LLVMContext C;
+  std::unique_ptr<Module> M = parseIR(C, R"---(
+    define dso_local void @func() !dbg !10 {
+      %1 = alloca i32, align 4
+      ret void, !dbg !DILocation(line: 3, column: 2, scope: !10, atomGroup: 1, atomRank: 1)
+    }
+
+    !llvm.dbg.cu = !{!0}
+    !llvm.module.flags = !{!2, !3}
+
+    !0 = distinct !DICompileUnit(language: DW_LANG_C11, file: !1, producer: "dummy", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, splitDebugInlining: false, nameTableKind: None)
+    !1 = !DIFile(filename: "dummy", directory: "dummy")
+    !2 = !{i32 7, !"Dwarf Version", i32 5}
+    !3 = !{i32 2, !"Debug Info Version", i32 3}
+    !10 = distinct !DISubprogram(name: "func", scope: !1, file: !1, line: 1, type: !11, scopeLine: 1, spFlags: DISPFlagDefinition, unit: !0, retainedNodes: !13)
+    !11 = !DISubroutineType(types: !12)
+    !12 = !{null}
+    !13 = !{}
+    !14 = !DILocalVariable(name: "a", scope: !10, file: !1, line: 2, type: !15)
+    !15 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+  )---");
+  ASSERT_TRUE(M);
+
+  Function *F = M->getFunction("func");
+
+  // Test splitBasicBlockBefore.
+  {
+    BasicBlock &BB = F->back();
+    // Split at `ret void`.
+    BasicBlock *Before =
+        BB.splitBasicBlockBefore(std::prev(BB.end(), 1), "before");
+    const DebugLoc &BrToAfterDL = Before->getTerminator()->getDebugLoc();
+    ASSERT_TRUE(BrToAfterDL);
+    EXPECT_EQ(BrToAfterDL->getAtomGroup(), 0u);
+
+    BasicBlock *After = Before->getSingleSuccessor();
+    ASSERT_TRUE(After);
+    const DebugLoc &OrigTerminatorDL = After->getTerminator()->getDebugLoc();
+    ASSERT_TRUE(OrigTerminatorDL);
+#ifdef EXPERIMENTAL_KEY_INSTRUCTIONS
+    EXPECT_EQ(OrigTerminatorDL->getAtomGroup(), 1u);
+#else
+    EXPECT_EQ(OrigTerminatorDL->getAtomGroup(), 0u);
+#endif
+  }
+
+  // Test splitBasicBlock.
+  {
+    BasicBlock &BB = F->back();
+    // Split at `ret void`.
+    BasicBlock *After = BB.splitBasicBlock(std::prev(BB.end(), 1), "before");
+
+    const DebugLoc &OrigTerminatorDL = After->getTerminator()->getDebugLoc();
+    ASSERT_TRUE(OrigTerminatorDL);
+#ifdef EXPERIMENTAL_KEY_INSTRUCTIONS
+    EXPECT_EQ(OrigTerminatorDL->getAtomGroup(), 1u);
+#else
+    EXPECT_EQ(OrigTerminatorDL->getAtomGroup(), 0u);
+#endif
+
+    BasicBlock *Before = After->getSinglePredecessor();
+    ASSERT_TRUE(Before);
+    const DebugLoc &BrToAfterDL = Before->getTerminator()->getDebugLoc();
+    ASSERT_TRUE(BrToAfterDL);
+    EXPECT_EQ(BrToAfterDL->getAtomGroup(), 0u);
+  }
+}
+
 TEST(BasicBlockDbgInfoTest, MarkerOperations) {
   LLVMContext C;
   std::unique_ptr<Module> M = parseIR(C, R"(