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[LangRef][AliasAnalysis] Clarify
noalias
affects only modified objects
We already mention that `noalias` is modeled after the C99 `restrict` qualifier but we did omit one important requirement in the description. For the restrict guarantees the object affected has to be modified during the execution of the function, in any way (see 6.7.3.1.4 in [0]). There are two reasons we want this restriction as well: 1) To match the `restrict` semantics when we lower it to `noalias`. 2) To allow the reasoning that the object pointed to by a `noalias` pointer is not modified through means not derived from this pointer. Hence, following the uses of that pointer is sufficient to determine potential modifications. The discussion on this came up as part of D73428. In that patch the Attributor is taught to derive `noalias` for call site arguments based on alias queries against objects that are accessed in the callee. This is possible even if the pointer passed at the call site was "not-`noalias`". To simplify the logic there *and* to allow the use of `noalias` as described in 2) above, it is beneficial to follow the C `restrict` semantics in cases where there might be "read-read-aliases". Note that AliasAnalysis* queries for read only objects already result in `NoAlias` even if the pointers might "alias". * From this point of view our Alias Analysis is basically a Dependence Analysis. [0] http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf Reviewed By: efriedma Differential Revision: https://reviews.llvm.org/D74935
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