Add a compat note about how Clang doesn't zero-initialize __block local variables.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@118641 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/www/compatibility.html b/www/compatibility.html
index 68fd025929c07f3ebaf1a8b8875032e11579309f..c25ea8a4aa07eee58a2528f7a1be5f265abc8f75 100644 (file)
--- a/www/compatibility.html
+++ b/www/compatibility.html
@@ -35,6 +35,7 @@
       <li><a href="#vector_builtins">"missing" vector __builtin functions</a></li>
       <li><a href="#lvalue-cast">Lvalue casts</a></li>
       <li><a href="#blocks-in-protected-scope">Jumps to within <tt>__block</tt> variable scope</a></li>
+      <li><a href="#block-variable-initialization">Non-initialization of <tt>__block</tt> variables</a></li>
     </ul>
   </li>
   <li><a href="#objective-c">Objective-C compatibility</a>
@@ -222,6 +223,29 @@ uses of the variable, only the scopes in which it is visible. You should rewrite
 your code to put the <tt>__block</tt> variables in a scope which is only visible
 where they are used.</p>
 
+<!-- ======================================================================= -->
+<h3 id="block-variable-initialization">Non-initialization of <tt>__block</tt>
+variables</h3>
+<!-- ======================================================================= -->
+
+<p>In the following example code, the <tt>x</tt> variable is used before it is
+defined:</p>
+<pre>
+int f0() {
+  __block int x;
+  return ^(){ return x; }();
+}
+</pre>
+
+<p>By an accident of implementation, GCC and llvm-gcc unintentionally always
+zero initialized <tt>__block</tt> variables. However, any program which depends
+on this behavior is relying on unspecified compiler behavior. Programs must
+explicitly initialize all local block variables before they are used, as with
+other local variables.</p>
+
+<p>Clang does not zero initialize local block variables, and programs which rely
+on such behavior will most likely break when built with Clang.</p>
+
 <!-- ======================================================================= -->
 <h2 id="objective-c">Objective-C compatibility</h3>
 <!-- ======================================================================= -->