[PATCH] D107281: [SimpifyCFG] Speculate a store preceded by a local non-escaping load

[Momchil Velikov via Phabricator via llvm-commits]

chill added a comment.

In D107281#2922685 <https://reviews.llvm.org/D107281#2922685>, @lebedev.ri wrote:

> I'm afraid i'm not seeing why `@load_before_store_escape` must not be transformed.
> Alive2 says it's fine: https://alive2.llvm.org/ce/z/F7ctum
> I'm not seeing any syncronization primitives, so i'm not really seeing yet why we'd be introducing a race?

Once we've made the address of the local variable visible to other functions, we cannot discard the possibility that those other functions may spawn threads
that write to the variable. In the original program, a data race might have been prevented by the condition, but once we move the store outside the
condition, we must be sure a data race wasn't possible anyway, no matter what the condition evaluates to.

Seeing just a load does not guarantee absence of a data race (unlike if we see a store) - the load may still be a part of a race, just not being UB (cf. https://llvm.org/docs/Atomics.html#optimization-outside-atomic)


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