[clang] Thread Safety Analysis: Add test for alias reassignment through pointer-to-pointer (PR #179028)

[Marco Elver via cfe-commits]

https://github.com/melver created https://github.com/llvm/llvm-project/pull/179028

Alias reassignment through pointer-to-pointer (nor ptr-to-ptr-to-ptr...) does not invalidate an alias for now. While this may result in either false positives or negatives, there's rarely a good reason not to just do direct assignment within the same scope.

For the time being, we retain this as a deliberate "escape hatch": specifically, this may be used to help the alias analysis to "see through" complex helper macros that e.g. read a value (such as a pointer) via inline assembly or other opaque helpers [1].

Add a test to document the current behaviour.

NFC.

[1] https://lore.kernel.org/all/20260130132951.2714396-1-elver@google.com/

>From 84669ac5fa993c4b1d1835bb1a548ae2643459b9 Mon Sep 17 00:00:00 2001
From: Marco Elver <elver at google.com>
Date: Sat, 31 Jan 2026 03:37:14 +0100
Subject: [PATCH] Thread Safety Analysis: Add test for alias reassignment
 through pointer-to-pointer

Alias reassignment through pointer-to-pointer (nor ptr-to-ptr-to-ptr...)
does not invalidate an alias for now. While this may result in either
false positives or negatives, there's rarely a good reason not to just
do direct assignment within the same scope.

For the time being, we retain this as a deliberate "escape hatch":
specifically, this may be used to help the alias analysis to "see
through" complex helper macros that e.g. read a value (such as a
pointer) via inline assembly or other opaque helpers [1].

Add a test to document the current behaviour.

NFC.

[1] https://lore.kernel.org/all/20260130132951.2714396-1-elver@google.com/
---
 .../SemaCXX/warn-thread-safety-analysis.cpp   | 21 +++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/clang/test/SemaCXX/warn-thread-safety-analysis.cpp b/clang/test/SemaCXX/warn-thread-safety-analysis.cpp
index d9efa745b7d59..466135a1d9cef 100644
--- a/clang/test/SemaCXX/warn-thread-safety-analysis.cpp
+++ b/clang/test/SemaCXX/warn-thread-safety-analysis.cpp
@@ -7498,6 +7498,27 @@ void testReassignment() {
   ptr->mu.Unlock();
 }
 
+// Alias reassignment through pointer-to-pointer (nor ptr-to-ptr-to-ptr...) does
+// not invalidate aliases for now.
+//
+// While this may result in either false positives or negatives, there's rarely
+// a good reason not to just do direct assignment within the same scope. For the
+// time being, we retain this as a deliberate "escape hatch": specifically, this
+// may be used to help the alias analysis to "see through" complex helper macros
+// that e.g. read a value (such as a pointer) via inline assembly or other
+// opaque helpers.
+void testReassignmentPointerToAlias(Foo *f) {
+  Mutex *mu = &f->mu;
+  // Escape hatch.
+  Mutex **mu_p = μ
+  Mutex *actual_mu = [&] { /* ... */ return mu; }();
+  *mu_p = actual_mu;
+
+  mu->Lock();
+  f->data = 42;
+  mu->Unlock();
+}
+
 // Nested field access through pointer
 struct Container {
   Foo foo;