[ModRefInfo] Add must alias info to ModRefInfo.

Summary:
Add an additional bit to ModRefInfo, ModRefInfo::Must, to be cleared for known must aliases.
Shift existing Mod/Ref/ModRef values to include an additional most
significant bit. Update wrappers that modify ModRefInfo values to
reflect the change.

Notes:
* ModRefInfo::Must is almost entirely cleared in the AAResults methods, the remaining changes are trying to preserve it.
* Only some small changes to make custom AA passes set ModRefInfo::Must (BasicAA).
* GlobalsModRef already declares a bit, who's meaning overlaps with the most significant bit in ModRefInfo (MayReadAnyGlobal). No changes to shift the value of MayReadAnyGlobal (see AlignedMap). FunctionInfo.getModRef() ajusts most significant bit so correctness is preserved, but the Must info is lost.
* There are cases where the ModRefInfo::Must is not set, e.g. 2 calls that only read will return ModRefInfo::NoModRef, though they may read from exactly the same location.

Reviewers: dberlin, hfinkel, george.burgess.iv

Subscribers: llvm-commits, sanjoy

Differential Revision: https://reviews.llvm.org/D38862

llvm-svn: 321309

llvm/include/llvm/Analysis/AliasAnalysis.h

@@ -95,53 +95,92 @@ enum AliasResult {
 ///
 /// This is no access at all, a modification, a reference, or both
 /// a modification and a reference. These are specifically structured such that
-/// they form a two bit matrix and bit-tests for 'mod' or 'ref'
+/// they form a three bit matrix and bit-tests for 'mod' or 'ref' or 'must'
 /// work with any of the possible values.
-
 enum class ModRefInfo {
+  /// Must is provided for completeness, but no routines will return only
+  /// Must today. See definition of Must below.
+  Must = 0,
+  /// The access may reference the value stored in memory,
+  /// a mustAlias relation was found, and no mayAlias or partialAlias found.
+  MustRef = 1,
+  /// The access may modify the value stored in memory,
+  /// a mustAlias relation was found, and no mayAlias or partialAlias found.
+  MustMod = 2,
+  /// The access may reference, modify or both the value stored in memory,
+  /// a mustAlias relation was found, and no mayAlias or partialAlias found.
+  MustModRef = MustRef | MustMod,
   /// The access neither references nor modifies the value stored in memory.
-  NoModRef = 0,
+  NoModRef = 4,
   /// The access may reference the value stored in memory.
-  Ref = 1,
+  Ref = NoModRef | MustRef,
   /// The access may modify the value stored in memory.
-  Mod = 2,
+  Mod = NoModRef | MustMod,
   /// The access may reference and may modify the value stored in memory.
   ModRef = Ref | Mod,
+
+  /// About Must:
+  /// Must is set in a best effort manner.
+  /// We usually do not try our best to infer Must, instead it is merely
+  /// another piece of "free" information that is presented when available.
+  /// Must set means there was certainly a MustAlias found. For calls,
+  /// where multiple arguments are checked (argmemonly), this translates to
+  /// only MustAlias or NoAlias was found.
+  /// Must is not set for RAR accesses, even if the two locations must
+  /// alias. The reason is that two read accesses translate to an early return
+  /// of NoModRef. An additional alias check to set Must may be
+  /// expensive. Other cases may also not set Must(e.g. callCapturesBefore).
+  /// We refer to Must being *set* when the most significant bit is *cleared*.
+  /// Conversely we *clear* Must information by *setting* the Must bit to 1.
 };
 
 LLVM_NODISCARD inline bool isNoModRef(const ModRefInfo MRI) {
-  return MRI == ModRefInfo::NoModRef;
+  return (static_cast<int>(MRI) & static_cast<int>(ModRefInfo::MustModRef)) ==
+         static_cast<int>(ModRefInfo::Must);
 }
 LLVM_NODISCARD inline bool isModOrRefSet(const ModRefInfo MRI) {
-  return static_cast<int>(MRI) & static_cast<int>(ModRefInfo::ModRef);
+  return static_cast<int>(MRI) & static_cast<int>(ModRefInfo::MustModRef);
 }
 LLVM_NODISCARD inline bool isModAndRefSet(const ModRefInfo MRI) {
-  return (static_cast<int>(MRI) & static_cast<int>(ModRefInfo::ModRef)) ==
-         static_cast<int>(ModRefInfo::ModRef);
+  return (static_cast<int>(MRI) & static_cast<int>(ModRefInfo::MustModRef)) ==
+         static_cast<int>(ModRefInfo::MustModRef);
 }
 LLVM_NODISCARD inline bool isModSet(const ModRefInfo MRI) {
-  return static_cast<int>(MRI) & static_cast<int>(ModRefInfo::Mod);
+  return static_cast<int>(MRI) & static_cast<int>(ModRefInfo::MustMod);
 }
 LLVM_NODISCARD inline bool isRefSet(const ModRefInfo MRI) {
-  return static_cast<int>(MRI) & static_cast<int>(ModRefInfo::Ref);
+  return static_cast<int>(MRI) & static_cast<int>(ModRefInfo::MustRef);
+}
+LLVM_NODISCARD inline bool isMustSet(const ModRefInfo MRI) {
+  return !(static_cast<int>(MRI) & static_cast<int>(ModRefInfo::NoModRef));
 }
 
 LLVM_NODISCARD inline ModRefInfo setMod(const ModRefInfo MRI) {
-  return ModRefInfo(static_cast<int>(MRI) | static_cast<int>(ModRefInfo::Mod));
+  return ModRefInfo(static_cast<int>(MRI) |
+                    static_cast<int>(ModRefInfo::MustMod));
 }
 LLVM_NODISCARD inline ModRefInfo setRef(const ModRefInfo MRI) {
-  return ModRefInfo(static_cast<int>(MRI) | static_cast<int>(ModRefInfo::Ref));
+  return ModRefInfo(static_cast<int>(MRI) |
+                    static_cast<int>(ModRefInfo::MustRef));
+}
+LLVM_NODISCARD inline ModRefInfo setMust(const ModRefInfo MRI) {
+  return ModRefInfo(static_cast<int>(MRI) &
+                    static_cast<int>(ModRefInfo::MustModRef));
 }
 LLVM_NODISCARD inline ModRefInfo setModAndRef(const ModRefInfo MRI) {
   return ModRefInfo(static_cast<int>(MRI) |
-                    static_cast<int>(ModRefInfo::ModRef));
+                    static_cast<int>(ModRefInfo::MustModRef));
 }
 LLVM_NODISCARD inline ModRefInfo clearMod(const ModRefInfo MRI) {
   return ModRefInfo(static_cast<int>(MRI) & static_cast<int>(ModRefInfo::Ref));
 }
 LLVM_NODISCARD inline ModRefInfo clearRef(const ModRefInfo MRI) {
   return ModRefInfo(static_cast<int>(MRI) & static_cast<int>(ModRefInfo::Mod));
 }
+LLVM_NODISCARD inline ModRefInfo clearMust(const ModRefInfo MRI) {
+  return ModRefInfo(static_cast<int>(MRI) |
+                    static_cast<int>(ModRefInfo::NoModRef));
+}
 LLVM_NODISCARD inline ModRefInfo unionModRef(const ModRefInfo MRI1,
                                              const ModRefInfo MRI2) {
   return ModRefInfo(static_cast<int>(MRI1) | static_cast<int>(MRI2));
@@ -160,11 +199,11 @@ enum FunctionModRefLocation {
   /// Base case is no access to memory.
   FMRL_Nowhere = 0,
   /// Access to memory via argument pointers.
-  FMRL_ArgumentPointees = 4,
+  FMRL_ArgumentPointees = 8,
   /// Memory that is inaccessible via LLVM IR.
-  FMRL_InaccessibleMem = 8,
+  FMRL_InaccessibleMem = 16,
   /// Access to any memory.
-  FMRL_Anywhere = 16 | FMRL_InaccessibleMem | FMRL_ArgumentPointees
+  FMRL_Anywhere = 32 | FMRL_InaccessibleMem | FMRL_ArgumentPointees
 };
 
 /// Summary of how a function affects memory in the program.
@@ -344,7 +383,7 @@ class AAResults {
   /// result's bits are set to indicate the allowed aliasing ModRef kinds. Note
   /// that these bits do not necessarily account for the overall behavior of
   /// the function, but rather only provide additional per-argument
-  /// information.
+  /// information. This never sets ModRefInfo::Must.
   ModRefInfo getArgModRefInfo(ImmutableCallSite CS, unsigned ArgIdx);
 
   /// Return the behavior of the given call site.
@@ -624,6 +663,8 @@ class AAResults {
   /// or reads the specified memory location \p MemLoc before instruction \p I
   /// in a BasicBlock. An ordered basic block \p OBB can be used to speed up
   /// instruction ordering queries inside the BasicBlock containing \p I.
+  /// Early exits in callCapturesBefore may lead to ModRefInfo::Must not being
+  /// set.
   ModRefInfo callCapturesBefore(const Instruction *I,
                                 const MemoryLocation &MemLoc, DominatorTree *DT,
                                 OrderedBasicBlock *OBB = nullptr);

llvm/include/llvm/Analysis/AliasAnalysisEvaluator.h

@@ -35,19 +35,23 @@ class AAEvaluator : public PassInfoMixin<AAEvaluator> {
   int64_t FunctionCount;
   int64_t NoAliasCount, MayAliasCount, PartialAliasCount, MustAliasCount;
   int64_t NoModRefCount, ModCount, RefCount, ModRefCount;
+  int64_t MustCount, MustRefCount, MustModCount, MustModRefCount;
 
 public:
   AAEvaluator()
       : FunctionCount(), NoAliasCount(), MayAliasCount(), PartialAliasCount(),
         MustAliasCount(), NoModRefCount(), ModCount(), RefCount(),
-        ModRefCount() {}
+        ModRefCount(), MustCount(), MustRefCount(), MustModCount(),
+        MustModRefCount() {}
   AAEvaluator(AAEvaluator &&Arg)
       : FunctionCount(Arg.FunctionCount), NoAliasCount(Arg.NoAliasCount),
         MayAliasCount(Arg.MayAliasCount),
         PartialAliasCount(Arg.PartialAliasCount),
         MustAliasCount(Arg.MustAliasCount), NoModRefCount(Arg.NoModRefCount),
         ModCount(Arg.ModCount), RefCount(Arg.RefCount),
-        ModRefCount(Arg.ModRefCount) {
+        ModRefCount(Arg.ModRefCount), MustCount(Arg.MustCount),
+        MustRefCount(Arg.MustRefCount), MustModCount(Arg.MustModCount),
+        MustModRefCount(Arg.MustModRefCount) {
     Arg.FunctionCount = 0;
   }
   ~AAEvaluator();