don't touch mergeInValue as that makes everything unreachable. Instead, add check w/ getUnderlyingObjects to grab all the ptr derivatives, in tryToReplaceWithConstant

Author: choikwa

llvm/lib/Transforms/IPO/SCCP.cpp

@@ -176,7 +176,6 @@ static bool runIPSCCP(
       bool ReplacedPointerArg = false;
       for (Argument &Arg : F.args()) {
         if (!Arg.use_empty() && 
-            !Arg.hasNoAliasAttr() && //don't replace if arg is noalias
             Solver.tryToReplaceWithConstant(&Arg)) {
           ReplacedPointerArg |= Arg.getType()->isPointerTy();
           ++NumArgsElimed;

llvm/lib/Transforms/Utils/SCCPSolver.cpp

@@ -62,6 +62,21 @@ bool SCCPSolver::tryToReplaceWithConstant(Value *V) {
   Constant *Const = getConstantOrNull(V);
   if (!Const)
     return false;
+
+  // Don't replace noalias arg or derivatives
+  if (isa<PointerType>(V->getType())) {
+    SmallVector<const Value *, 4> Objects;
+    getUnderlyingObjects(V, Objects, nullptr);
+    
+    for (const auto Obj : Objects) {
+      if (const auto *Arg = dyn_cast<Argument>(Obj)) {
+        if (isa<PointerType>(Arg->getType()) &&
+            Arg->hasNoAliasAttr())
+          return false;
+      }
+    }
+  }
+
   // Replacing `musttail` instructions with constant breaks `musttail` invariant
   // unless the call itself can be removed.
   // Calls with "clang.arc.attachedcall" implicitly use the return value and
@@ -1168,11 +1183,7 @@ void SCCPInstVisitor::visitInstruction(Instruction &I) {
 bool SCCPInstVisitor::mergeInValue(ValueLatticeElement &IV, Value *V,
                                    ValueLatticeElement MergeWithV,
                                    ValueLatticeElement::MergeOptions Opts) {
-  if (const auto *Arg = dyn_cast<Argument>(V)) {
-    if (isa<PointerType>(Arg->getType()) &&
-        Arg->hasNoAliasAttr())
-      return false;  // do not merge w/ noalias ptr.
-  }
+
   if (IV.mergeIn(MergeWithV, Opts)) {
     pushUsersToWorkList(V);
     LLVM_DEBUG(dbgs() << "Merged " << MergeWithV << " into " << *V << " : "