[PATCH] D86233: [LangRef] Define mustprogress attribute

[Nikita Popov via Phabricator via llvm-commits]

nikic added inline comments.


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Comment at: llvm/docs/LangRef.rst:1963
+    Standard <http://eel.is/c++draft/intro.progress>`_. This attribute does
+    not apply transitively to callees. This is implied by `willreturn`.
 
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jdoerfert wrote:
> ```
> This attribute indicates that the function is required to return, unwind,
> or interact with the environment in an observable way e.g. via a
> volatile memory access, I/O, or other synchronization. 
> The ``mustprogress`` attribute is intended to model the
> requirements of the first section of `[intro.progress] of the C++
> Standard <http://eel.is/c++draft/intro.progress>`_. This attribute does
> not apply transitively to callees, that means a callee might not make progress.
> Note that `willreturn` implies `mustprogress`.
> ```
Some notes from my side:
 * I'm not sure how to interpret the "does not apply to callees" part. If mustprogress function A calls not-mustprogress function B and B goes into an infinite loop, then A will not return, unwind or interact with the environment, thus violating the mustprogress contract. Now, the callee does not necessarily have to be mustprogress in general, but I believe it needs to progress for any invocation from a mustprogress function -- or not?
 * I would explicitly state that violating the progress requirement results in undefined behavior.
 * It may be worthwhile to explicitly mention that a non-interacting loop inside a mustprogress function can thus be assumed to terminate and may be removed. That would help clarify the practical effect setting this attribute has.


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