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[IR] Introduce dead_on_return attribute

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Add `dead_on_return` attribute, which is meant to be taken advantage
by the frontend, and states that the memory pointed to by the argument
is dead upon function return. As with `byval`, it is supposed to be
used for passing aggregates by value. The difference lies in the ABI:
`byval` implies that the pointer is explicitly passed as argument to
the callee (during codegen the copy is emitted as per byval contract),
whereas a `dead_on_return`-marked argument implies that the copy
already exists in the IR, is located at a specific stack offset within
the caller, and this memory will not be read further by the caller upon
callee return – or otherwise poison, if read before being written.

RFC: https://discourse.llvm.org/t/rfc-add-dead-on-return-attribute/86871.
This commit is contained in:
Antonio Frighetto
2025-07-02 09:23:22 +02:00
parent d5608d6751
commit f1cc0b607b
13 changed files with 86 additions and 5 deletions
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llvm/docs/LangRef.rst
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@@ -1741,6 +1741,23 @@ Currently, only the following parameter attributes are defined:
This attribute cannot be applied to return values.
``dead_on_return``
This attribute indicates that the memory pointed to by the argument is dead
upon function return, both upon normal return and if the calls unwinds, meaning
that the caller will not depend on its contents. Stores that would be observable
either on the return path or on the unwind path may be elided.
Specifically, the behavior is as-if any memory written through the pointer
during the execution of the function is overwritten with a poison value
upon function return. The caller may access the memory, but any load
not preceded by a store will return poison.
This attribute does not imply aliasing properties. For pointer arguments that
do not alias other memory locations, ``noalias`` attribute may be used in
conjunction. Conversely, this attribute always implies ``dead_on_unwind``.
This attribute cannot be applied to return values.
``range(<ty> <a>, <b>)``
This attribute expresses the possible range of the parameter or return value.
If the value is not in the specified range, it is converted to poison.