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I don't understand this fixed bug: https://llvm.org/bugs/show_bug.cgi?id=26774

How could foo() had ever printed "X"?

Both t0 and t1 loads are atomic, meaning you'll always get the whole object. But there's no synchronization between them and another thread can preempt the execution and change value of %ptr right after the first load.

and what is %ptr ? I'm having troubles googling it.

I assume %ptr is a generic "variable" name[0] for a pointer reference, i.e. the address of the value being loaded atomically. You can substitute %ptr in your head for any atomic address you might want to load from.

The code itself isn't well-formed because %ptr isn't actually defined anywhere—just pretend that it was. ;)

[0] Technically, LLVM does not have variables (as in, names you can assign to more than once), since it's in SSA form, but I don't know what a better name would be here. :)

Aight, I'll need to read their docs exhaustively, I had to anyway. Thanks nonetheless.

Then the CSE optimization that removed the printf would be just plain wrong. Why create a more complicated scenario just to point that out?

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