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<title>Accurate Garbage Collection with LLVM</title>
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<p>The <tt>llvm.gcroot</tt> intrinsic is used to inform LLVM that a stack
variable references an object on the heap and is to be tracked for garbage
collection. The exact impact on generated code is specified by a <a
-href="#plugin">compiler plugin</a>.</p>
+href="#plugin">compiler plugin</a>. All calls to <tt>llvm.gcroot</tt> <b>must</b> reside
+ inside the first basic block.</p>
<p>A compiler which uses mem2reg to raise imperative code using <tt>alloca</tt>
into SSA form need only add a call to <tt>@llvm.gcroot</tt> for those variables
<p>It is also important to mark intermediate values with <tt>llvm.gcroot</tt>.
For example, consider <tt>h(f(), g())</tt>. Beware leaking the result of
-<tt>f()</tt> in the case that <tt>g()</tt> triggers a collection.</p>
+<tt>f()</tt> in the case that <tt>g()</tt> triggers a collection. Note, that
+stack variables must be initialized and marked with <tt>llvm.gcroot</tt> in
+function's prologue.</p>
<p>The first argument <b>must</b> be a value referring to an alloca instruction
or a bitcast of an alloca. The second contains a pointer to metadata that
;; Tell LLVM that the stack space is a stack root.
;; Java has type-tags on objects, so we pass null as metadata.
%tmp = bitcast %Object** %X to i8**
- call void @llvm.gcroot(i8** %X, i8* null)
+ call void @llvm.gcroot(i8** %tmp, i8* null)
...
;; "CodeBlock" is the block corresponding to the start
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