[MemorySSA] LCSSA preserves MemorySSA.

Summary:
Enabling MemorySSA in the old pass manager leads to MemorySSA being run
twice due to the fact that LCSSA and LoopSimplify do not preserve
MemorySSA. This is the first step to address that: target LCSSA.

LCSSA does not make any changes that invalidate MemorySSA, so it
preserves it by design. It must preserve AA as well, for this to hold.

After this patch, MemorySSA is still run twice in the old pass manager.
Step two follows: target LoopSimplify.

Subscribers: mehdi_amini, jlebar, Prazek, llvm-commits, george.burgess.iv, chandlerc

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60832

llvm-svn: 359032

include/llvm/Analysis/LoopAnalysisManager.h

@@ -61,9 +61,6 @@ struct LoopStandardAnalysisResults {
   MemorySSA *MSSA;
 };
 
-/// Enables memory ssa as a dependency for loop passes.
-extern cl::opt<bool> EnableMSSALoopDependency;
-
 /// Extern template declaration for the analysis set for this IR unit.
 extern template class AllAnalysesOn<Loop>;
 

include/llvm/Analysis/MemorySSA.h

@@ -104,6 +104,9 @@
 
 namespace llvm {
 
+/// Enables memory ssa as a dependency for loop passes.
+extern cl::opt<bool> EnableMSSALoopDependency;
+
 class Function;
 class Instruction;
 class MemoryAccess;

lib/Analysis/LoopAnalysisManager.cpp

@@ -18,11 +18,6 @@
 using namespace llvm;
 
 namespace llvm {
-/// Enables memory ssa as a dependency for loop passes in legacy pass manager.
-cl::opt<bool> EnableMSSALoopDependency(
-    "enable-mssa-loop-dependency", cl::Hidden, cl::init(false),
-    cl::desc("Enable MemorySSA dependency for loop pass manager"));
-
 // Explicit template instantiations and specialization definitions for core
 // template typedefs.
 template class AllAnalysesOn<Loop>;

lib/Analysis/MemorySSA.cpp

@@ -81,6 +81,11 @@ bool llvm::VerifyMemorySSA = true;
 #else
 bool llvm::VerifyMemorySSA = false;
 #endif
+/// Enables memory ssa as a dependency for loop passes in legacy pass manager.
+cl::opt<bool> llvm::EnableMSSALoopDependency(
+    "enable-mssa-loop-dependency", cl::Hidden, cl::init(false),
+    cl::desc("Enable MemorySSA dependency for loop pass manager"));
+
 static cl::opt<bool, true>
     VerifyMemorySSAX("verify-memoryssa", cl::location(VerifyMemorySSA),
                      cl::Hidden, cl::desc("Enable verification of MemorySSA."));

lib/Transforms/Utils/LCSSA.cpp

@@ -34,9 +34,9 @@
 #include "llvm/Analysis/BranchProbabilityInfo.h"
 #include "llvm/Analysis/GlobalsModRef.h"
 #include "llvm/Analysis/LoopPass.h"
+#include "llvm/Analysis/MemorySSA.h"
 #include "llvm/Analysis/ScalarEvolution.h"
 #include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
-#include "llvm/Transforms/Utils/Local.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/Function.h"
@@ -45,6 +45,7 @@
 #include "llvm/IR/PredIteratorCache.h"
 #include "llvm/Pass.h"
 #include "llvm/Transforms/Utils.h"
+#include "llvm/Transforms/Utils/Local.h"
 #include "llvm/Transforms/Utils/LoopUtils.h"
 #include "llvm/Transforms/Utils/SSAUpdater.h"
 using namespace llvm;
@@ -444,6 +445,7 @@ struct LCSSAWrapperPass : public FunctionPass {
     AU.addPreserved<ScalarEvolutionWrapperPass>();
     AU.addPreserved<SCEVAAWrapperPass>();
     AU.addPreserved<BranchProbabilityInfoWrapperPass>();
+    AU.addPreserved<MemorySSAWrapperPass>();
 
     // This is needed to perform LCSSA verification inside LPPassManager
     AU.addRequired<LCSSAVerificationPass>();
@@ -490,5 +492,6 @@ PreservedAnalyses LCSSAPass::run(Function &F, FunctionAnalysisManager &AM) {
   // BPI maps terminators to probabilities, since we don't modify the CFG, no
   // updates are needed to preserve it.
   PA.preserve<BranchProbabilityAnalysis>();
+  PA.preserve<MemorySSAAnalysis>();
   return PA;
 }