[NFC] Fix invalid iterator dereferencing in MachineLoop::getBottomBlock

[kachkov98]

Summary

If bottom block of the loop is the last basic block in function,
dereferencing of end() iterator can occur (and for other basic
blocks the condition is always false); fix typo in this check.

[fhahn]

Could you add a test case for the issue?

[kachkov98]

NFC because for now this method is not used by any LLVM pass; this looks like a typo of some old code refactoring (MachineLoop::getTopBlock is correct BTW)

[kachkov98]

In D136434#3873894, @fhahn wrote:

Could you add a test case for the issue?

Sure, I'll add unit test for this method.

[kachkov98]

Add unit test

[kachkov98]

Ping (not critical for upstream because this method is never used, but we are using it in downstream compiler)

llvm/lib/CodeGen/MachineLoopInfo.cpp

 
 MachineBasicBlock *MachineLoop::getBottomBlock() {
   MachineBasicBlock *BotMBB = getHeader();
   MachineFunction::iterator End = BotMBB->getParent()->end();
   if (BotMBB->getIterator() != std::prev(End)) {
     MachineBasicBlock *NextMBB = &*std::next(BotMBB->getIterator());
     while (contains(NextMBB)) {
       BotMBB = NextMBB;
-      if (BotMBB == &*std::next(BotMBB->getIterator()))
+      if (BotMBB->getIterator() == std::prev(End))
         break;
       NextMBB = &*std::next(BotMBB->getIterator());
     }
   }
   return BotMBB;
 }
 
 MachineBasicBlock *MachineLoop::findLoopControlBlock() {

llvm/unittests/CodeGen/CMakeLists.txt

add_llvm_unittest(CodeGenTests
   DIEHashTest.cpp
   DIETest.cpp
   DwarfStringPoolEntryRefTest.cpp
   InstrRefLDVTest.cpp
   LowLevelTypeTest.cpp
   LexicalScopesTest.cpp
   MachineInstrBundleIteratorTest.cpp
   MachineInstrTest.cpp
+  MachineLoopInfoTest.cpp
   MachineOperandTest.cpp
   RegAllocScoreTest.cpp
   PassManagerTest.cpp
   ScalableVectorMVTsTest.cpp
   SelectionDAGAddressAnalysisTest.cpp
   TypeTraitsTest.cpp
   TargetOptionsTest.cpp
   TestAsmPrinter.cpp
   MLRegallocDevelopmentFeatures.cpp
   )
 
 add_subdirectory(GlobalISel)
 
 target_link_libraries(CodeGenTests PRIVATE LLVMTestingSupport)

llvm/unittests/CodeGen/MachineLoopInfoTest.cpp

+//===- MachineLoopInfoTest.cpp --------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/CodeGen/MachineLoopInfo.h"
+#include "llvm/CodeGen/MachineBasicBlock.h"
+#include "llvm/CodeGen/MachineFunction.h"
+#include "llvm/CodeGen/MachineModuleInfo.h"
+#include "llvm/CodeGen/TargetFrameLowering.h"
+#include "llvm/CodeGen/TargetInstrInfo.h"
+#include "llvm/CodeGen/TargetLowering.h"
+#include "llvm/CodeGen/TargetSubtargetInfo.h"
+#include "llvm/MC/TargetRegistry.h"
+#include "llvm/Target/TargetMachine.h"
+#include "gtest/gtest.h"
+
+using namespace llvm;
+
+namespace {
+// Include helper functions to ease the manipulation of MachineFunctions.
+#include "MFCommon.inc"
+
+TEST(MachineLoopInfoTest, TopBottomBlocks) {
+  LLVMContext Ctx;
+  Module Mod("Module", Ctx);
+  auto MF = createMachineFunction(Ctx, Mod);
+  auto *TopBlock = MF->CreateMachineBasicBlock();
+  auto *BottomBlock = MF->CreateMachineBasicBlock();
+
+  MF->push_back(TopBlock);
+  MF->push_back(BottomBlock);
+
+  LoopInfoBase<MachineBasicBlock, MachineLoop> LI;
+  MachineLoop *Loop = LI.AllocateLoop();
+  Loop->addBasicBlockToLoop(TopBlock, LI);
+  Loop->addBasicBlockToLoop(BottomBlock, LI);
+
+  ASSERT_EQ(TopBlock, Loop->getTopBlock());
+  ASSERT_EQ(BottomBlock, Loop->getBottomBlock());
+}
+} // end namespace