[GVN] Look through select/phi when determining underlying object

This addresses an optimization regression in Rust we have observed
after #82458. We now only
perform pointer replacement if they have the same underlying object.
However, getUnderlyingObject() by default only looks through
linear chains, not selects/phis. In particular, this means that
we miss cases involving involving pointer induction variables.

This patch fixes this by introducing a new helper
getUnderlyingObjectThroughPhisAndSelects() (let me know if you
have a better name...) which basically does what getUnderlyingObjects()
does, just specialized to the case where we must arrive at a single
underlying object in the end, and with a limit on the number of
inspected values.

Doing this more expensive underlying object check has no measurable
compile-time impact on CTMark.

Author: nikic

llvm/include/llvm/Analysis/ValueTracking.h

@@ -736,6 +736,11 @@ inline Value *getUnderlyingObject(Value *V, unsigned MaxLookup = 6) {
   return const_cast<Value *>(getUnderlyingObject(VConst, MaxLookup));
 }
 
+/// Like getUnderlyingObject(), but will also look through phi/select nodes
+/// to establish a single underlying object.
+const Value *getUnderlyingObjectThroughPhisAndSelects(const Value *V,
+                                                      unsigned MaxLookup = 6);
+
 /// This method is similar to getUnderlyingObject except that it can
 /// look through phi and select instructions and return multiple objects.
 ///

llvm/lib/Analysis/Loads.cpp

@@ -743,9 +743,8 @@ static bool isPointerAlwaysReplaceable(const Value *From, const Value *To,
   if (isa<Constant>(To) &&
       isDereferenceablePointer(To, Type::getInt8Ty(To->getContext()), DL))
     return true;
-  if (getUnderlyingObject(From) == getUnderlyingObject(To))
-    return true;
-  return false;
+  return getUnderlyingObjectThroughPhisAndSelects(From) ==
+         getUnderlyingObjectThroughPhisAndSelects(To);
 }
 
 bool llvm::canReplacePointersInUseIfEqual(const Use &U, const Value *To,

llvm/lib/Analysis/ValueTracking.cpp

@@ -6609,6 +6609,48 @@ void llvm::getUnderlyingObjects(const Value *V,
   } while (!Worklist.empty());
 }
 
+const Value *
+llvm::getUnderlyingObjectThroughPhisAndSelects(const Value *V,
+                                               unsigned MaxLookup) {
+  const unsigned MaxVisited = 8;
+
+  SmallPtrSet<const Value *, 8> Visited;
+  SmallVector<const Value *, 8> Worklist;
+  Worklist.push_back(V);
+  const Value *Object = nullptr, *FirstObject = nullptr;
+  do {
+    const Value *P = Worklist.pop_back_val();
+    P = getUnderlyingObject(P, MaxLookup);
+
+    if (!FirstObject)
+      FirstObject = P;
+
+    if (!Visited.insert(P).second)
+      continue;
+
+    if (Visited.size() == MaxVisited)
+      return FirstObject;
+
+    if (auto *SI = dyn_cast<SelectInst>(P)) {
+      Worklist.push_back(SI->getTrueValue());
+      Worklist.push_back(SI->getFalseValue());
+      continue;
+    }
+
+    if (auto *PN = dyn_cast<PHINode>(P)) {
+      append_range(Worklist, PN->incoming_values());
+      continue;
+    }
+
+    if (!Object)
+      Object = P;
+    else if (Object != P)
+      return FirstObject;
+  } while (!Worklist.empty());
+
+  return Object;
+}
+
 /// This is the function that does the work of looking through basic
 /// ptrtoint+arithmetic+inttoptr sequences.
 static const Value *getUnderlyingObjectFromInt(const Value *V) {

llvm/test/Transforms/GVN/condprop.ll

@@ -813,7 +813,7 @@ define void @select_same_obj(i1 %c, ptr %p, i64 %x) {
 ; CHECK-NEXT:    br i1 [[CMP]], label [[IF:%.*]], label [[EXIT:%.*]]
 ; CHECK:       if:
 ; CHECK-NEXT:    call void @use_ptr(ptr [[P]])
-; CHECK-NEXT:    call void @use_ptr(ptr [[P3]])
+; CHECK-NEXT:    call void @use_ptr(ptr [[P]])
 ; CHECK-NEXT:    ret void
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret void
@@ -902,7 +902,7 @@ define void @phi_same_obj(i1 %c, ptr %p, i64 %x) {
 ; CHECK-NEXT:    br i1 [[CMP]], label [[IF2:%.*]], label [[EXIT:%.*]]
 ; CHECK:       if2:
 ; CHECK-NEXT:    call void @use_ptr(ptr [[P]])
-; CHECK-NEXT:    call void @use_ptr(ptr [[P3]])
+; CHECK-NEXT:    call void @use_ptr(ptr [[P]])
 ; CHECK-NEXT:    ret void
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret void
@@ -975,7 +975,7 @@ define void @phi_same_obj_cycle(i1 %c, ptr %p, i64 %x) {
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp eq ptr [[P_IV]], [[P]]
 ; CHECK-NEXT:    br i1 [[CMP]], label [[IF:%.*]], label [[LOOP_LATCH]]
 ; CHECK:       if:
-; CHECK-NEXT:    call void @use_ptr(ptr [[P_IV]])
+; CHECK-NEXT:    call void @use_ptr(ptr [[P]])
 ; CHECK-NEXT:    call void @use_ptr(ptr [[P]])
 ; CHECK-NEXT:    br label [[LOOP_LATCH]]
 ; CHECK:       loop.latch: