[PATCH] D50608: [Alt] Silence GCC warning about broken strict aliasing rules

[Kim Gräsman via Phabricator via llvm-commits]

kimgr created this revision.
kimgr added reviewers: sammccall, xiangzhai, HaoLiu, llvm-commits.

The as<T>() method would trigger the following warning on GCC <7:

  warning: dereferencing type-punned pointer will break
      strict-aliasing rules [-Wstrict-aliasing]
  
    return *reinterpret_cast<T *>(Union.buffer);
                                              ^

Union.buffer is guaranteed to be aligned to whatever types it contains,
and json::Value maintains the invariant that it only calls as<T>() for a
T it has previously placement-newed into Union.buffer. This should
follow the rules for strict aliasing.

Using two static_cast<> via void * instead of reinterpret_cast<>
silences the warning and presumably makes GCC understand that no
strict-aliasing violation is happening.

No functional change intended.

See also https://reviews.llvm.org/D50607.


Repository:
  rL LLVM

https://reviews.llvm.org/D50608

Files:
  include/llvm/Support/JSON.h


Index: include/llvm/Support/JSON.h
===================================================================
--- include/llvm/Support/JSON.h
+++ include/llvm/Support/JSON.h
@@ -452,7 +452,10 @@
     new (reinterpret_cast<T *>(Union.buffer)) T(std::forward<U>(V)...);
   }
   template <typename T> T &as() const {
-    return *reinterpret_cast<T *>(Union.buffer);
+    // Using this double static_cast<> via void * instead of reinterpret_cast<>
+    // silences a -Wstrict-aliasing warning from GCC.
+    void *p = static_cast<void *>(Union.buffer);
+    return *static_cast<T *>(p);
   }
 
   template <typename Indenter>


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