[MemoryLocation] Move DSE's logic to new MemLoc::getForDest helper (NFC).

[fhahn]

Summary

DSE has some extra logic to determine the write location of library
calls like str*cpy and str*cat. This patch moves the logic to a new
MemoryLocation:getForDest variant, which takes a call and TLI.

This patch should be NFC, because no other places take advantage of the
new helper yet.

Suggested by @reames post-commit 7eec832def571.

[nikic]

LG

llvm/lib/Analysis/MemoryLocation.cpp

149

nit: return None;

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

1039

Move these two over as well?

[reames]

llvm/lib/Analysis/MemoryLocation.cpp

143

I might be missing something, but it looks like these are only handled generically inside MemoryLocation::getForArgument at the moment. Given the original code specialized for the constant case, and only returned getAfter (not also before), I think this move is not NFC.

[nikic]

llvm/lib/Analysis/MemoryLocation.cpp

143

The strncpy logic is added in the parent patch. You're right though that we lose the getAfter/getBeforeOrAfter distinction (though it probably doesn't matter much).

[fhahn]

llvm/lib/Analysis/MemoryLocation.cpp

143

Yeah, I guess we could add the various LibFunc_str* variants to the switch in getForArgument to use getAfter for them.

llvm/include/llvm/Analysis/MemoryLocation.h

public:
   static MemoryLocation getForSource(const AtomicMemTransferInst *MTI);
   static MemoryLocation getForSource(const AnyMemTransferInst *MTI);
 
   /// Return a location representing the destination of a memory set or
   /// transfer.
   static MemoryLocation getForDest(const MemIntrinsic *MI);
   static MemoryLocation getForDest(const AtomicMemIntrinsic *MI);
   static MemoryLocation getForDest(const AnyMemIntrinsic *MI);
+  static Optional<MemoryLocation> getForDest(const CallBase *CI,
+                                             const TargetLibraryInfo &TLI);
 
   /// Return a location representing a particular argument of a call.
   static MemoryLocation getForArgument(const CallBase *Call, unsigned ArgIdx,
                                        const TargetLibraryInfo *TLI);
   static MemoryLocation getForArgument(const CallBase *Call, unsigned ArgIdx,
                                        const TargetLibraryInfo &TLI) {
     return getForArgument(Call, ArgIdx, &TLI);
   }

llvm/lib/Analysis/MemoryLocation.cpp

MemoryLocation MemoryLocation::getForDest(const AnyMemIntrinsic *MI) {
   if (ConstantInt *C = dyn_cast<ConstantInt>(MI->getLength()))
     Size = LocationSize::precise(C->getValue().getZExtValue());
 
   // memcpy/memmove can have AA tags. For memcpy, they apply
   // to both the source and the destination.
   return MemoryLocation(MI->getRawDest(), Size, MI->getAAMetadata());
 }
 
+Optional<MemoryLocation>
+MemoryLocation::getForDest(const CallBase *CB, const TargetLibraryInfo &TLI) {
+  if (auto *MemInst = dyn_cast<AnyMemIntrinsic>(CB))
+    return getForDest(MemInst);
+
+  LibFunc LF;
+  if (TLI.getLibFunc(*CB, LF) && TLI.has(LF)) {
+    switch (LF) {
+    case LibFunc_strncpy:
+    case LibFunc_strcpy:
+    case LibFunc_strcat:
+    case LibFunc_strncat:
+      return getForArgument(CB, 0, &TLI);
+    default:
+      break;
+    }
+  }
+
+  return {};
+}
+
 MemoryLocation MemoryLocation::getForArgument(const CallBase *Call,
                                               unsigned ArgIdx,
                                               const TargetLibraryInfo *TLI) {
   AAMDNodes AATags = Call->getAAMetadata();
   const Value *Arg = Call->getArgOperand(ArgIdx);
 
   // We may be able to produce an exact size for known intrinsics.
   if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(Call)) {

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

bool isInvisibleToCallerBeforeRet(const Value *V) {
     }
     return I.first->second;
   }
 
   Optional<MemoryLocation> getLocForWriteEx(Instruction *I) const {
     if (!I->mayWriteToMemory())
       return None;
 
-    if (auto *MTI = dyn_cast<AnyMemIntrinsic>(I))
-      return {MemoryLocation::getForDest(MTI)};
-
     if (auto *CB = dyn_cast<CallBase>(I)) {
       // If the functions may write to memory we do not know about, bail out.
       if (!CB->onlyAccessesArgMemory() &&
           !CB->onlyAccessesInaccessibleMemOrArgMem())
         return None;
 
-      LibFunc LF;
-      if (TLI.getLibFunc(*CB, LF) && TLI.has(LF)) {
-        switch (LF) {
-        case LibFunc_strncpy:
-          if (const auto *Len = dyn_cast<ConstantInt>(CB->getArgOperand(2)))
-            return MemoryLocation(CB->getArgOperand(0),
-                                  LocationSize::precise(Len->getZExtValue()),
-                                  CB->getAAMetadata());
-          LLVM_FALLTHROUGH;
-        case LibFunc_strcpy:
-        case LibFunc_strcat:
-        case LibFunc_strncat:
-          return {MemoryLocation::getAfter(CB->getArgOperand(0))};
-        default:
-          break;
-        }
-      }
       switch (CB->getIntrinsicID()) {
       case Intrinsic::init_trampoline:
         return {MemoryLocation::getAfter(CB->getArgOperand(0))};
       case Intrinsic::masked_store:
         return {MemoryLocation::getForArgument(CB, 1, TLI)};
       default:
         break;
       }
-      return None;
+
+      return MemoryLocation::getForDest(CB, TLI);
     }
 
     return MemoryLocation::getOrNone(I);
   }
 
   /// Returns true if \p UseInst completely overwrites \p DefLoc
   /// (stored by \p DefInst).
   bool isCompleteOverwrite(const MemoryLocation &DefLoc, Instruction *DefInst,