[llvm] [GVN] Restrict equality propagation for pointers (PR #82458)

[Nikita Popov via llvm-commits]

================
@@ -710,22 +710,63 @@ Value *llvm::FindAvailableLoadedValue(LoadInst *Load, BatchAAResults &AA,
   return Available;
 }
 
-bool llvm::canReplacePointersIfEqual(Value *A, Value *B, const DataLayout &DL,
-                                     Instruction *CtxI) {
-  Type *Ty = A->getType();
-  assert(Ty == B->getType() && Ty->isPointerTy() &&
-         "values must have matching pointer types");
-
-  // NOTE: The checks in the function are incomplete and currently miss illegal
-  // cases! The current implementation is a starting point and the
-  // implementation should be made stricter over time.
-  if (auto *C = dyn_cast<Constant>(B)) {
-    // Do not allow replacing a pointer with a constant pointer, unless it is
-    // either null or at least one byte is dereferenceable.
-    APInt OneByte(DL.getPointerTypeSizeInBits(Ty), 1);
-    return C->isNullValue() ||
-           isDereferenceableAndAlignedPointer(B, Align(1), OneByte, DL, CtxI);
+// Returns true if a use is either in an ICmp/PtrToInt or a Phi/Select that only
+// feeds into them.
+static bool isPointerUseReplacable(const Use &U) {
+  unsigned Limit = 40;
+  SmallVector<const User *> Worklist({U.getUser()});
+  SmallPtrSet<const User *, 8> Visited;
+
+  while (!Worklist.empty() && --Limit) {
+    auto *User = Worklist.pop_back_val();
+    Visited.insert(User);
+    if (isa<ICmpInst, PtrToIntInst>(User))
+      continue;
+    if (isa<PHINode, SelectInst>(User)) {
----------------
nikic wrote:

What I meant here is to write:
```
if (!isa<PHINode, SelectInst>(User))
  return false;
```
But now that the if block has become so small it doesn't really matter anymore.

https://github.com/llvm/llvm-project/pull/82458