[LoopUtils] Updated deleteDeadLoop() to handle loop nest.

Reviewer: kariddi, sanjoy, reames, Meinersbur, bmahjour, etiotto,
kbarton
Reviewed By: Meinersbur
Subscribers: mgorny, hiraditya, llvm-commits
Tag: LLVM
Differential Revision: https://reviews.llvm.org/D70939

llvm/lib/Transforms/Utils/LoopUtils.cpp

@@ -673,7 +673,18 @@ void llvm::deleteDeadLoop(Loop *L, DominatorTree *DT = nullptr,
       LI->removeBlock(BB);
 
     // The last step is to update LoopInfo now that we've eliminated this loop.
-    LI->erase(L);
+    // Note: LoopInfo::erase remove the given loop and relink its subloops with
+    // its parent. While removeLoop/removeChildLoop remove the given loop but
+    // not relink its subloops, which is what we want.
+    if (Loop *ParentLoop = L->getParentLoop()) {
+      Loop::iterator I = find(ParentLoop->begin(), ParentLoop->end(), L);
+      assert(I != ParentLoop->end() && "Couldn't find loop");
+      ParentLoop->removeChildLoop(I);
+    } else {
+      Loop::iterator I = find(LI->begin(), LI->end(), L);
+      assert(I != LI->end() && "Couldn't find loop");
+      LI->removeLoop(I);
+    }
   }
 }
 

llvm/unittests/Transforms/Utils/CMakeLists.txt

@@ -15,6 +15,7 @@ add_llvm_unittest(UtilsTests
   FunctionComparatorTest.cpp
   IntegerDivisionTest.cpp
   LocalTest.cpp
+  LoopUtilsTest.cpp
   SizeOptsTest.cpp
   SSAUpdaterBulkTest.cpp
   UnrollLoopTest.cpp