[PATCH] D104007: [BasicAA] Properly mark that coroutine suspension may modify parameters

[Xun Li via Phabricator via llvm-commits]

lxfind added a comment.

> From clang's perspective, what it needs to ensure is that every argument is stored in an allocation local to the callee (in other words, allocations corosplit can handle).  "allocations corosplit can handle" should be either allocas, or arguments marked byval.  These are pointers that don't exist in the caller, so corosplit can mess with the address in memory without worrying about code it can't see in the caller.

How do you prevent optimization passes (prior to CoroSplit) from messing up with those pointers, though?
Let's say we have an argument that's not marked with byval (due to target specifics) but is pass-by-value in C++. In Clang we could emit IR to copy the argument value to a local alloca. However latter optimization passes could very well optimize out that copy because the copy would appear to be useless without considering what coro_suspend does, which is why we are changing BasicAA to prevent such elimination.
Or are you suggesting we need to modify Clang such that every pass-by-value argument in C++ must be marked with byval?


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https://reviews.llvm.org/D104007