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

[asbirlea]

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.

[dmgreen]

Sounds like a good idea to me.

[fhahn]

This workaround seems fine. I don't think there should be much interaction between LoopRotate and LICM/LoopUnswitch and running LoopRotate on all loops first should not really pessimism optimisations.

llvm/lib/Transforms/Scalar/LoopRotation.cpp

84

I think it would be good to add a comment that this will split the loop pipeline, to delay computing MemorySSA/skipping relatively costly updates?

llvm/lib/Transforms/Scalar/LoopRotation.cpp

LoopRotateLegacyPass(int SpecifiedMaxHeaderSize = -1) : LoopPass(ID) {
     else
       MaxHeaderSize = unsigned(SpecifiedMaxHeaderSize);
   }
 
   // LCSSA form makes instruction renaming easier.
   void getAnalysisUsage(AnalysisUsage &AU) const override {
     AU.addRequired<AssumptionCacheTracker>();
     AU.addRequired<TargetTransformInfoWrapperPass>();
-    if (EnableMSSALoopDependency) {
-      AU.addRequired<MemorySSAWrapperPass>();
+    if (EnableMSSALoopDependency)
       AU.addPreserved<MemorySSAWrapperPass>();
-    }
     getLoopAnalysisUsage(AU);
   }
 
   bool runOnLoop(Loop *L, LPPassManager &LPM) override {
     if (skipLoop(L))
       return false;
     Function &F = *L->getHeader()->getParent();
 
     auto *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
     const auto *TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
     auto *AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
     auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
     auto &SE = getAnalysis<ScalarEvolutionWrapperPass>().getSE();
     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,
                         false, MaxHeaderSize, false);
   }
 };
 } // end namespace
 

llvm/test/Other/opt-O2-pipeline.ll

 ; CHECK-NEXT:         Natural Loop Information
 ; CHECK-NEXT:         Lazy Branch Probability Analysis
 ; CHECK-NEXT:         Lazy Block Frequency Analysis
 ; CHECK-NEXT:         Optimization Remark Emitter
 ; CHECK-NEXT:         Tail Call Elimination
 ; CHECK-NEXT:         Simplify the CFG
 ; CHECK-NEXT:         Reassociate expressions
 ; CHECK-NEXT:         Dominator Tree Construction
-; CHECK-NEXT:         Basic Alias Analysis (stateless AA impl)
-; CHECK-NEXT:         Function Alias Analysis Results
-; CHECK-NEXT:         Memory SSA
 ; CHECK-NEXT:         Natural Loop Information
 ; CHECK-NEXT:         Canonicalize natural loops
 ; CHECK-NEXT:         LCSSA Verifier
 ; CHECK-NEXT:         Loop-Closed SSA Form Pass
+; CHECK-NEXT:         Basic Alias Analysis (stateless AA impl)
+; CHECK-NEXT:         Function Alias Analysis Results
 ; CHECK-NEXT:         Scalar Evolution Analysis
 ; CHECK-NEXT:         Loop Pass Manager
 ; CHECK-NEXT:           Rotate Loops
+; CHECK-NEXT:         Memory SSA
+; CHECK-NEXT:         Loop Pass Manager
 ; CHECK-NEXT:           Loop Invariant Code Motion
 ; CHECK-NEXT:           Unswitch loops
 ; CHECK-NEXT:         Simplify the CFG
 ; CHECK-NEXT:         Dominator Tree Construction
 ; CHECK-NEXT:         Basic Alias Analysis (stateless AA impl)
 ; CHECK-NEXT:         Function Alias Analysis Results
 ; CHECK-NEXT:         Natural Loop Information
 ; CHECK-NEXT:         Lazy Branch Probability Analysis

 ; CHECK-NEXT:     Dead Global Elimination
 ; CHECK-NEXT:     CallGraph Construction
 ; CHECK-NEXT:     Globals Alias Analysis
 ; CHECK-NEXT:     FunctionPass Manager
 ; CHECK-NEXT:       Dominator Tree Construction
 ; CHECK-NEXT:       Float to int
 ; CHECK-NEXT:       Lower constant intrinsics
 ; CHECK-NEXT:       Dominator Tree Construction
-; CHECK-NEXT:       Basic Alias Analysis (stateless AA impl)
-; CHECK-NEXT:       Function Alias Analysis Results
-; CHECK-NEXT:       Memory SSA
 ; CHECK-NEXT:       Natural Loop Information
 ; CHECK-NEXT:       Canonicalize natural loops
 ; CHECK-NEXT:       LCSSA Verifier
 ; CHECK-NEXT:       Loop-Closed SSA Form Pass
+; CHECK-NEXT:       Basic Alias Analysis (stateless AA impl)
+; CHECK-NEXT:       Function Alias Analysis Results
 ; CHECK-NEXT:       Scalar Evolution Analysis
 ; CHECK-NEXT:       Loop Pass Manager
 ; CHECK-NEXT:         Rotate Loops
 ; CHECK-NEXT:       Loop Access Analysis
 ; CHECK-NEXT:       Lazy Branch Probability Analysis
 ; CHECK-NEXT:       Lazy Block Frequency Analysis
 ; CHECK-NEXT:       Optimization Remark Emitter
 ; CHECK-NEXT:       Loop Distribution

llvm/test/Other/opt-O3-pipeline.ll

 ; CHECK-NEXT:         Natural Loop Information
 ; CHECK-NEXT:         Lazy Branch Probability Analysis
 ; CHECK-NEXT:         Lazy Block Frequency Analysis
 ; CHECK-NEXT:         Optimization Remark Emitter
 ; CHECK-NEXT:         Tail Call Elimination
 ; CHECK-NEXT:         Simplify the CFG
 ; CHECK-NEXT:         Reassociate expressions
 ; CHECK-NEXT:         Dominator Tree Construction
-; CHECK-NEXT:         Basic Alias Analysis (stateless AA impl)
-; CHECK-NEXT:         Function Alias Analysis Results
-; CHECK-NEXT:         Memory SSA
 ; CHECK-NEXT:         Natural Loop Information
 ; CHECK-NEXT:         Canonicalize natural loops
 ; CHECK-NEXT:         LCSSA Verifier
 ; CHECK-NEXT:         Loop-Closed SSA Form Pass
+; CHECK-NEXT:         Basic Alias Analysis (stateless AA impl)
+; CHECK-NEXT:         Function Alias Analysis Results
 ; CHECK-NEXT:         Scalar Evolution Analysis
 ; CHECK-NEXT:         Loop Pass Manager
 ; CHECK-NEXT:           Rotate Loops
+; CHECK-NEXT:         Memory SSA
+; CHECK-NEXT:         Loop Pass Manager
 ; CHECK-NEXT:           Loop Invariant Code Motion
 ; CHECK-NEXT:           Unswitch loops
 ; CHECK-NEXT:         Simplify the CFG
 ; CHECK-NEXT:         Dominator Tree Construction
 ; CHECK-NEXT:         Basic Alias Analysis (stateless AA impl)
 ; CHECK-NEXT:         Function Alias Analysis Results
 ; CHECK-NEXT:         Natural Loop Information
 ; CHECK-NEXT:         Lazy Branch Probability Analysis

 ; CHECK-NEXT:     Dead Global Elimination
 ; CHECK-NEXT:     CallGraph Construction
 ; CHECK-NEXT:     Globals Alias Analysis
 ; CHECK-NEXT:     FunctionPass Manager
 ; CHECK-NEXT:       Dominator Tree Construction
 ; CHECK-NEXT:       Float to int
 ; CHECK-NEXT:       Lower constant intrinsics
 ; CHECK-NEXT:       Dominator Tree Construction
-; CHECK-NEXT:       Basic Alias Analysis (stateless AA impl)
-; CHECK-NEXT:       Function Alias Analysis Results
-; CHECK-NEXT:       Memory SSA
 ; CHECK-NEXT:       Natural Loop Information
 ; CHECK-NEXT:       Canonicalize natural loops
 ; CHECK-NEXT:       LCSSA Verifier
 ; CHECK-NEXT:       Loop-Closed SSA Form Pass
+; CHECK-NEXT:       Basic Alias Analysis (stateless AA impl)
+; CHECK-NEXT:       Function Alias Analysis Results
 ; CHECK-NEXT:       Scalar Evolution Analysis
 ; CHECK-NEXT:       Loop Pass Manager
 ; CHECK-NEXT:         Rotate Loops
 ; CHECK-NEXT:       Loop Access Analysis
 ; CHECK-NEXT:       Lazy Branch Probability Analysis
 ; CHECK-NEXT:       Lazy Block Frequency Analysis
 ; CHECK-NEXT:       Optimization Remark Emitter
 ; CHECK-NEXT:       Loop Distribution

llvm/test/Other/opt-Os-pipeline.ll

 ; CHECK-NEXT:         Lazy Block Frequency Analysis
 ; CHECK-NEXT:         Optimization Remark Emitter
 ; CHECK-NEXT:         Combine redundant instructions
 ; CHECK-NEXT:         Optimization Remark Emitter
 ; CHECK-NEXT:         Tail Call Elimination
 ; CHECK-NEXT:         Simplify the CFG
 ; CHECK-NEXT:         Reassociate expressions
 ; CHECK-NEXT:         Dominator Tree Construction
-; CHECK-NEXT:         Basic Alias Analysis (stateless AA impl)
-; CHECK-NEXT:         Function Alias Analysis Results
-; CHECK-NEXT:         Memory SSA
 ; CHECK-NEXT:         Natural Loop Information
 ; CHECK-NEXT:         Canonicalize natural loops
 ; CHECK-NEXT:         LCSSA Verifier
 ; CHECK-NEXT:         Loop-Closed SSA Form Pass
+; CHECK-NEXT:         Basic Alias Analysis (stateless AA impl)
+; CHECK-NEXT:         Function Alias Analysis Results
 ; CHECK-NEXT:         Scalar Evolution Analysis
 ; CHECK-NEXT:         Loop Pass Manager
 ; CHECK-NEXT:           Rotate Loops
+; CHECK-NEXT:         Memory SSA
+; CHECK-NEXT:         Loop Pass Manager
 ; CHECK-NEXT:           Loop Invariant Code Motion
 ; CHECK-NEXT:           Unswitch loops
 ; CHECK-NEXT:         Simplify the CFG
 ; CHECK-NEXT:         Dominator Tree Construction
 ; CHECK-NEXT:         Basic Alias Analysis (stateless AA impl)
 ; CHECK-NEXT:         Function Alias Analysis Results
 ; CHECK-NEXT:         Natural Loop Information
 ; CHECK-NEXT:         Lazy Branch Probability Analysis

 ; CHECK-NEXT:     Dead Global Elimination
 ; CHECK-NEXT:     CallGraph Construction
 ; CHECK-NEXT:     Globals Alias Analysis
 ; CHECK-NEXT:     FunctionPass Manager
 ; CHECK-NEXT:       Dominator Tree Construction
 ; CHECK-NEXT:       Float to int
 ; CHECK-NEXT:       Lower constant intrinsics
 ; CHECK-NEXT:       Dominator Tree Construction
-; CHECK-NEXT:       Basic Alias Analysis (stateless AA impl)
-; CHECK-NEXT:       Function Alias Analysis Results
-; CHECK-NEXT:       Memory SSA
 ; CHECK-NEXT:       Natural Loop Information
 ; CHECK-NEXT:       Canonicalize natural loops
 ; CHECK-NEXT:       LCSSA Verifier
 ; CHECK-NEXT:       Loop-Closed SSA Form Pass
+; CHECK-NEXT:       Basic Alias Analysis (stateless AA impl)
+; CHECK-NEXT:       Function Alias Analysis Results
 ; CHECK-NEXT:       Scalar Evolution Analysis
 ; CHECK-NEXT:       Loop Pass Manager
 ; CHECK-NEXT:         Rotate Loops
 ; CHECK-NEXT:       Loop Access Analysis
 ; CHECK-NEXT:       Lazy Branch Probability Analysis
 ; CHECK-NEXT:       Lazy Block Frequency Analysis
 ; CHECK-NEXT:       Optimization Remark Emitter
 ; CHECK-NEXT:       Loop Distribution

llvm/test/Other/pass-pipelines.ll

 ; CHECK-O2-NEXT: Deduce and propagate attributes (CGSCC pass)
 ; CHECK-O2-NEXT: OpenMP specific optimizations
 ; CHECK-O2-NEXT: Deduce function attributes
 ; Next up is the main function pass pipeline. It shouldn't be split up and
 ; should contain the main loop pass pipeline as well.
 ; CHECK-O2-NEXT: FunctionPass Manager
 ; CHECK-O2-NOT: Manager
 ; CHECK-O2: Loop Pass Manager
+; CHECK-O2: Loop Pass Manager
 ; CHECK-O2-NOT: Manager
 ; FIXME: We shouldn't be pulling out to simplify-cfg and instcombine and
 ; causing new loop pass managers.
 ; CHECK-O2: Simplify the CFG
 ; CHECK-O2-NOT: Manager
 ; CHECK-O2: Combine redundant instructions
 ; CHECK-O2-NOT: Manager
 ; CHECK-O2: Loop Pass Manager