[LoopRotate] Get and update MSSA only if available in legacy pass manager.

Summary:
Potential fix for: https://bugs.llvm.org/show_bug.cgi?id=44889 and https://bugs.llvm.org/show_bug.cgi?id=44408

In the legacy pass manager, loop rotate need not compute MemorySSA when not being in the same loop pass manager with other loop passes.
There isn't currently a way to differentiate between the two cases, so this attempts to limit the usage in LoopRotate to only update MemorySSA when the analysis is already available.
The side-effect of this is that it will split the Loop pipeline.

This issue does not apply to the new pass manager, where we have a flag specifying if all loop passes in that loop pass manager preserve MemorySSA.

Reviewers: dmgreen, fedor.sergeev, nikic

Subscribers: Prazek, hiraditya, george.burgess.iv, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74574

(cherry picked from commit 1326a5a4cfe004181f2ec8231d84ecda2b93cb25)

1 files changed, 6 insertions, 5 deletions

diff --git a/llvm/lib/Transforms/Scalar/LoopRotation.cpp b/llvm/lib/Transforms/Scalar/LoopRotation.cpp
index 0868e742f4e..67c20b2edae 100644
--- a/llvm/lib/Transforms/Scalar/LoopRotation.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopRotation.cpp
@@ -81,10 +81,8 @@ public:
   void getAnalysisUsage(AnalysisUsage &AU) const override {
     AU.addRequired<AssumptionCacheTracker>();
     AU.addRequired<TargetTransformInfoWrapperPass>();
-    if (EnableMSSALoopDependency) {
-      AU.addRequired<MemorySSAWrapperPass>();
+    if (EnableMSSALoopDependency)
       AU.addPreserved<MemorySSAWrapperPass>();
-    }
     getLoopAnalysisUsage(AU);
   }
 
@@ -101,8 +99,11 @@ public:
     const SimplifyQuery SQ = getBestSimplifyQuery(*this, F);
     Optional<MemorySSAUpdater> MSSAU;
     if (EnableMSSALoopDependency) {
-      MemorySSA *MSSA = &getAnalysis<MemorySSAWrapperPass>().getMSSA();
-      MSSAU = MemorySSAUpdater(MSSA);
+      // Not requiring MemorySSA and getting it only if available will split
+      // the loop pass pipeline when LoopRotate is being run first.
+      auto *MSSAA = getAnalysisIfAvailable<MemorySSAWrapperPass>();
+      if (MSSAA)
+        MSSAU = MemorySSAUpdater(&MSSAA->getMSSA());
     }
     return LoopRotation(L, LI, TTI, AC, &DT, &SE,
                         MSSAU.hasValue() ? MSSAU.getPointer() : nullptr, SQ,