[llvm] [LangRef] Update initializes definition (PR #134370)

[Nikita Popov via llvm-commits]

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@@ -1690,10 +1690,24 @@ Currently, only the following parameter attributes are defined:
 
 ``initializes((Lo1, Hi1), ...)``
     This attribute indicates that the function initializes the ranges of the
-    pointer parameter's memory, ``[%p+LoN, %p+HiN)``. Initialization of memory
-    means the first memory access is a non-volatile, non-atomic write. The
-    write must happen before the function returns. If the function unwinds,
-    the write may not happen.
+    pointer parameter's memory ``[%p+LoN, %p+HiN)``. Colloquially, this means
+    that all bytes in the specified range are written before the function
+    returns, and not read prior to the initializing write. If the function
+    unwinds, the write may not happen.
+
+    Formally, this is specified in terms of an "initialized" shadow state for
+    all bytes in the range, which is set to "not initialized" at function entry.
+    If a memory access is performed through a pointer based on the argument,
+    and an accessed byte has not been marked as "initialized" yet, then:
+
+     * If the byte is stored with a non-volatile, non-atomic write, mark it as
+       "initialized".
+     * If the byte is stored with a volatile or atomic write, the behavior is
+       undefined.
+     * If the byte is loaded, return an undef value.
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nikic wrote:

Returning poison is a bit problematic due to load poison widening, but I would be okay with specifying poison here. (As we currently just ignore the whole load poison widening issue everywhere anyway.)

https://github.com/llvm/llvm-project/pull/134370