[bruce343434]

I looked at it closer. In C, it is a side effect to assign to a variable. For an extern function not annotated __attribute__((pure)) the compiler has to assume the function call generates side effects. This prevents it from reordering the assignment and function call. Since x86-64 ABI has caller saved registers, in the case where it calls eff() first, it has to save x and y, and after the call, restore them.

[moonchild]

The work on e-graphs I refer to is <https://egraphs-good.github.io/>—amortised rebuilds.

My example has nothing whatsoever to do with abi, and everything to do with ir. f and g are exactly semantically equivalent, and this equivalence is trivial to show; that the compilers generate different code for each demonstrates redundancies in their ir.

> it is a side effect to assign to a variable

But that variable is not aliased here.