[llvm][CodeGen] Check for memory instructions when querying for alias status

Summary:
Add a check to make sure that MachineInstr::mayAlias returns prematurely if at least one of its instruction parameters does not access memory. This prevents calls to TargetInstrInfo::areMemAccessesTriviallyDisjoint with incompatible instructions.

A side effect of this change is to render the mayAlias helper in the AArch64 load/store optimizer obsolete. We can now directly call the MachineInstr::mayAlias member function.

Reviewers: hfinkel, t.p.northover, mcrosier, eli.friedman, efriedma

Reviewed By: efriedma

Subscribers: efriedma, kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78823

llvm/lib/CodeGen/MachineInstr.cpp

@@ -1220,6 +1220,10 @@ bool MachineInstr::mayAlias(AAResults *AA, const MachineInstr &Other,
   if (!mayStore() && !Other.mayStore())
     return false;
 
+  // Both instructions must be memory operations to be able to alias.
+  if (!mayLoadOrStore() || !Other.mayLoadOrStore())
+    return false;
+
   // Let the target decide if memory accesses cannot possibly overlap.
   if (TII->areMemAccessesTriviallyDisjoint(*this, Other))
     return false;

llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp

@@ -1144,24 +1144,11 @@ static int alignTo(int Num, int PowOf2) {
   return (Num + PowOf2 - 1) & ~(PowOf2 - 1);
 }
 
-static bool mayAlias(MachineInstr &MIa, MachineInstr &MIb,
-                     AliasAnalysis *AA) {
-  // One of the instructions must modify memory.
-  if (!MIa.mayStore() && !MIb.mayStore())
-    return false;
-
-  // Both instructions must be memory operations.
-  if (!MIa.mayLoadOrStore() && !MIb.mayLoadOrStore())
-    return false;
-
-  return MIa.mayAlias(AA, MIb, /*UseTBAA*/false);
-}
-
 static bool mayAlias(MachineInstr &MIa,
                      SmallVectorImpl<MachineInstr *> &MemInsns,
                      AliasAnalysis *AA) {
   for (MachineInstr *MIb : MemInsns)
-    if (mayAlias(MIa, *MIb, AA))
+    if (MIa.mayAlias(AA, *MIb, /*UseTBAA*/ false))
       return true;
 
   return false;
@@ -1219,7 +1206,7 @@ bool AArch64LoadStoreOpt::findMatchingStore(
       return false;
 
     // If we encounter a store aliased with the load, return early.
-    if (MI.mayStore() && mayAlias(LoadMI, MI, AA))
+    if (MI.mayStore() && LoadMI.mayAlias(AA, MI, /*UseTBAA*/ false))
       return false;
   } while (MBBI != B && Count < Limit);
   return false;