[UnrollAndJam] Do not preserve loop nests if a loop was fully unrolled.

If UnJ completely unrolls a loop and removes it entirely, the loop remains in
the current loop nest. If the loop nest gets reused the loops will no longer be
valid. As there is no way to remove a loop from a LoopNest, this patch removes
the preserve of the LoopNestAnalysis so that it will be regenerated.

Fixes #124518

Author: davemgreen

llvm/lib/Transforms/Scalar/LoopUnrollAndJamPass.cpp

@@ -425,7 +425,7 @@ static bool tryToUnrollAndJamLoop(LoopNest &LN, DominatorTree &DT, LoopInfo &LI,
                                   const TargetTransformInfo &TTI,
                                   AssumptionCache &AC, DependenceInfo &DI,
                                   OptimizationRemarkEmitter &ORE, int OptLevel,
-                                  LPMUpdater &U) {
+                                  LPMUpdater &U, bool &AnyLoopRemoved) {
   bool DidSomething = false;
   ArrayRef<Loop *> Loops = LN.getLoops();
   Loop *OutmostLoop = &LN.getOutermostLoop();
@@ -441,8 +441,11 @@ static bool tryToUnrollAndJamLoop(LoopNest &LN, DominatorTree &DT, LoopInfo &LI,
         tryToUnrollAndJamLoop(L, DT, &LI, SE, TTI, AC, DI, ORE, OptLevel);
     if (Result != LoopUnrollResult::Unmodified)
       DidSomething = true;
-    if (L == OutmostLoop && Result == LoopUnrollResult::FullyUnrolled)
-      U.markLoopAsDeleted(*L, LoopName);
+    if (Result == LoopUnrollResult::FullyUnrolled) {
+      if (L == OutmostLoop)
+        U.markLoopAsDeleted(*L, LoopName);
+      AnyLoopRemoved = true;
+    }
   }
 
   return DidSomething;
@@ -457,11 +460,13 @@ PreservedAnalyses LoopUnrollAndJamPass::run(LoopNest &LN,
   DependenceInfo DI(&F, &AR.AA, &AR.SE, &AR.LI);
   OptimizationRemarkEmitter ORE(&F);
 
+  bool AnyLoopRemoved = false;
   if (!tryToUnrollAndJamLoop(LN, AR.DT, AR.LI, AR.SE, AR.TTI, AR.AC, DI, ORE,
-                             OptLevel, U))
+                             OptLevel, U, AnyLoopRemoved))
     return PreservedAnalyses::all();
 
   auto PA = getLoopPassPreservedAnalyses();
-  PA.preserve<LoopNestAnalysis>();
+  if (!AnyLoopRemoved)
+    PA.preserve<LoopNestAnalysis>();
   return PA;
 }

llvm/test/Transforms/LoopUnrollAndJam/delete_middle_loop.ll

@@ -0,0 +1,36 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -passes="loop(invalidate<all>,loop-unroll-and-jam,loop-unroll-and-jam)" -allow-unroll-and-jam -unroll-and-jam-count=4 < %s -S | FileCheck %s
+
+; This test completely unrolls the middle loop out of a 3-deep loop nest.
+
+define i16 @test_it() {
+; CHECK-LABEL: define i16 @test_it() {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    br label %[[FOR_COND:.*]]
+; CHECK:       [[FOR_COND_LOOPEXIT:.*]]:
+; CHECK-NEXT:    br label %[[FOR_COND]]
+; CHECK:       [[FOR_COND]]:
+; CHECK-NEXT:    br label %[[DO_BODY2:.*]]
+; CHECK:       [[DO_BODY2]]:
+; CHECK-NEXT:    br label %[[WHILE_COND3:.*]]
+; CHECK:       [[WHILE_COND3]]:
+; CHECK-NEXT:    br i1 true, label %[[DO_COND:.*]], label %[[WHILE_COND3]]
+; CHECK:       [[DO_COND]]:
+; CHECK-NEXT:    br label %[[FOR_COND_LOOPEXIT]]
+;
+entry:
+  br label %for.cond
+
+for.cond:                                         ; preds = %do.cond, %entry
+  br label %do.body2
+
+do.body2:                                         ; preds = %do.cond, %for.cond
+  br label %while.cond3
+
+while.cond3:                                      ; preds = %while.cond3, %do.body2
+  br i1 true, label %do.cond, label %while.cond3
+
+do.cond:                                          ; preds = %while.cond3
+  br i1 true, label %for.cond, label %do.body2
+}
+