[clang] [PAC] Implement function pointer re-signing (PR #98847)

[Daniil Kovalev via cfe-commits]

================
@@ -165,6 +166,88 @@ CGPointerAuthInfo CodeGenModule::getPointerAuthInfoForType(QualType T) {
   return ::getPointerAuthInfoForType(*this, T);
 }
 
+static bool isZeroConstant(llvm::Value *value) {
+  if (auto ci = dyn_cast<llvm::ConstantInt>(value))
+    return ci->isZero();
+  return false;
+}
+
+static bool equalAuthPolicies(const CGPointerAuthInfo &left,
+                              const CGPointerAuthInfo &right) {
+  if (left.isSigned() != right.isSigned())
+    return false;
+  assert(left.isSigned() && right.isSigned() &&
+         "should only be called with non-null auth policies");
+  return left.getKey() == right.getKey() &&
+         left.getAuthenticationMode() == right.getAuthenticationMode();
+}
+
+llvm::Value *CodeGenFunction::EmitPointerAuthResign(
+    llvm::Value *value, QualType type, const CGPointerAuthInfo &curAuthInfo,
+    const CGPointerAuthInfo &newAuthInfo, bool isKnownNonNull) {
+  // Fast path: if neither schema wants a signature, we're done.
+  if (!curAuthInfo && !newAuthInfo)
+    return value;
+
+  llvm::Value *null = nullptr;
+  // If the value is obviously null, we're done.
+  if (auto pointerValue = dyn_cast<llvm::PointerType>(value->getType())) {
+    null = CGM.getNullPointer(pointerValue, type);
+  } else {
+    assert(value->getType()->isIntegerTy());
+    null = llvm::ConstantInt::get(IntPtrTy, 0);
+  }
+  if (value == null) {
+    return value;
+  }
+
+  // If both schemas sign the same way, we're done.
+  if (equalAuthPolicies(curAuthInfo, newAuthInfo)) {
----------------
kovdan01 wrote:

It looks like this is the only place where `equalAuthPolicies`, and checks against discriminator can probably be moved to that function from the code below, so the resulting if statement would be smth like

```
if (equalAuthPolicies(curAuthInfo, newAuthInfo))
  return value;
```

Feel free to ignore

https://github.com/llvm/llvm-project/pull/98847